A character, working in the military, is suddenly forced into duties that by all rights ought to belong to someone of much higher rank; everyone who ought to be doing those duties has unexpectedly found themselves dead, indisposed, or unavailable.
This is most likely to occur in the navy or its Space Operaticequivalent—it requires that the plot be isolated enough from the rest of the military that they can't just respond by immediately sending a replacement of the appropriate rank. Trapped Behind Enemy Lines is another possibility, as is the characters becoming prisoners of war, where under The Laws and Customs of War, the senior-most commands, even if none of them belonged to the same unit prior to capture.
In Real Life, if there are a number of survivors of the same rank, the most senior of them holds command (unless otherwise designated, ie, the XO is always second-in-command and the OPS officer is always third, regardless of rank, and doctors, lawyers and clergy cannot be placed in command over combat units even if they are the only officers present- in which case the most senior NCO would assume command, or the most senior private). In fiction, the situation is often adequately chaotic that the one that actually gives orders may find himself pressed into command and leadership. (In really chaotic situations, it may dawn on him that he is giving orders to superiors — at which point, the highest-ranking superior generally tells everyone to follow their plan. Contrast With Due Respect.)
Often this requires them to press juniors and otherwise unsuitable people into roles as their subordinates. Occasionally, the promotee subsequently becomes Drunk with Power.
If the dead commander was A Father to His Men, the new one may find his troops are Losing the Team Spirit over his death — though he can issue a Rousing Speech reminding them that the dead commander would be So Proud of You if they soldier on.
Usually the "promotion" involved is strictly temporary. Either the people who are supposed to do the job will return from whatever made them unavailable in the first place, or a replacement will eventually appear. On the other hand, in a continuing series, a stint of You Are in Command Now is not exactly a hindrance to promotion, since it shows Leadership.
Compare Unexpected Successor, Falling into the Cockpit, Take Up My Sword, Time to Step Up, Commander.
If the promotion occurs because the promoted deliberately offed the person occupying the spot in the first place, and this is considered a legitimate situation, then you have a case of Klingon Promotion.
Note that if the character does not command, it falls under Field Promotion, since it is handled out by the commander, even if he would not normally be the commander.
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In a 1990s radio campaign for a Vancouver RV dealership called Fraser Way, the owner of a competing dealership needed ideas on how to beat Fraser Way's deals, so every employee became the Sales Manager for a few seconds.
This is the premise of the original Mobile Suit Gundam: Bright Noa, a mere officer trainee with no battle experience whatsoever, is forced to become The Captain of the Cool StarshipWhite Base when nearly all of his superior officers are killed in the first episode surprise attack by the Principality of Zeon, and the one survivor (the captain) succumbing to his wounds shortly afterward. This also nicely mirrors the protagonist Amuro Ray Falling into the Cockpit of the eponymous battle robot that he knew only the bare minimum about.
In Mobile Suit Gundam SEED, the same thing happens to Lieutenant Murrue Ramius, who becomes The Captain of the Archangel after all higher-ranking officers are killed in a surprise attack. She offers command to Lieutenant Mu La Flaga, who has seniority (but identical rank) and greater experience, but he insists on deferring to her because he is the only available pilot other than Kira. Since the ship has two functional mecha at the time, both Mu and Kira are needed in the cockpits, leaving Murrue the only choice.
Played straight at the beginning of the series when Yang Wenli is given command of the 2nd Fleet by his severely wounded commanding officer during the height of the Battle of Astarte as he's the highest-ranking officer left who's not incapacitated. He then proceeds to save the fleet from complete destruction.
Julian Mintz, a mere Lieutenant, becomes the military leader of the remaining Free Planets Alliance military. It is a multiple subversion actually: Officers higher than him are still around, but they just cannot choose whom among them will takes Yang's mantle (first subversion), so they chooses Yang's foster child to act both as a figurehead (second subversion) and to arbitrate between them when they don't agree with each other, so while he seems to be a powerless puppet, he IS giving orders to people higher than him in the hierarchy and they willingly obey such orders. (But then again, It's the Yang Team, and they ALWAYS put competence above hierarchy.)
In the movie The Diamond Dust Rebellion, Rangiku has to take temporary command of Squad 10 after Captain Hitsugaya turns up missing. She does a damn good job of it, too.
In canon, Lieutenants Izuru Kira and Shuuhei Hisagi take command of their squads after their captains, Gin Ichimaru and Kaname Tousen, turn traitor. Momo Hinamori would've done the same due to her squad after her own captain, Sousuke Aizen, was also revealed as a traitor, but she was both physically and emotionally unfit for duty at the time.
After Yamamoto's death, Shunsui Kyoraku becomes the new Captain-Commander.
Invoked by Commander Erwin in Attack on Titan to avoid being executed by saying that the Military Police will then have to deal with the two Titan shifters attacking each other, amidst all the other problems involved, calmly telling them who is in charge of what as the guns were pointed at him.
A non-army example in Retro Chill - after being the second-in-command to Calvin in the Five-Man Band, he suddenly gets forced to lead the group after Calvin is kidnapped by the villains.
Now-Captain Kanril Eleya of Bait and Switch got her first command after the Borg killed or assimilated the USS Kagoshima's entire command staff, leaving her the seniormost lieutenant junior grade left on the ship.
Corporal Hicks in Aliens becomes able to authorize a nuclear attack thanks to alien-caused attrition in the higher ranks, though the company lieutenant was only wounded, not killed, in an incident precipitated by his own ineptitude. When he's up and around again, Lt. Gorman seems to acknowledge his failure of command, allowing Hicks and a civilian Ripley to continue calling the shots. (Granted, the situation had devolved to the point that any attempt to re-assert command had a good chance of being ignored anyway.)
Toyed with repeatedly in The Empire Strikes Back (the Trope Namer): Darth Vader, as Supreme Commander, holds no formal rank in the Imperial system save as an agent of the Emperor himself (meaning that any order he gives is treated as no different from an order by the Emperor), allowing him to hand out nice little impromptu promotions by Force-choking the incompetent officers. In one scene, he chokes Admiral Ozzel for screwing up, and immediately addresses Ozzel's XO Captain Piett as Admiral Piett, putting him in command right then and there (before Ozzel's body has even hit the floor) Piett's first order of business is then removing Ozzel's body. In the Star Wars Expanded Universe, the novel Heir To The Empire reveals that while it was risky, serving aboard Vader's flagship, the Executor, was also seen as the fast track to promotion for the sufficiently quick-thinking and competent—and that Vader's tactics worked, weeding out the officers who couldn't keep up, so that only the very good or the very lucky were left. This meant, though, that when Executor was destroyed at Endor, the Empire lost more than just a powerful warship...
In the first movie, when Luke's commander is killed in the assault on the Death Star, this huge, meaningful music cue plays to signify that Luke is now in command of the mission. (What's left of it; just him and his wingmen.)
There's another, more subtle instance of this a few minutes earlier. After Darth Vader shoots down two of Gold Squadron's Y-wings during their trench run, the lone survivor—Gold Five—radios Red Leader to report their destruction. Red Leader responds with "I copy, Gold Leader," acknowledging that as the sole surviving member of the squadron he became its leader. At least, until Vader shot him down too a few seconds later.
The 2009 Star Trek movie has this happen to every single character. At least five different characters inherit command of at least two different ships during the movie. Russian whiz kid Chekov is the only one of the main characters who ends up in his position without someone else getting sick or flat out killed, and he is briefly given command of the Enterprise when everyone else is away.
Dr. McCoy technically gets this too. He doesn't take command of Enterprise, but he becomes its chief doctor after the old one is killed.
Invoked by James Kirk, who at Spock Prime's suggestion goads Spock into breaking in rage, causing Spock to relieve himself from command and Kirk to be put in charge.
Captain Robau actually closes out his scene on-board the U.S.S. Kelvin by invoking the tradition (mentioned above by Chief O'Brien) with his First Officer.
Captain Robau: If I don't report in 15 minutes, evacuate the crew.
George Kirk: Sir, we could issue...
Captain Robau: There is no help for us out here. Use autopilot... and get off this ship.
George Kirk: Aye, Captain.
Captain Robau: ...You're captain now, Mr. Kirk.
Repeated later by Pike as he's leaving with Kirk, Sulu, and Olsen on a shuttle. As he's walking, he randomly promotes Kirk (a cadet about to wash out of the Academy) to first officer, which is Spock's position. When Spock complains, Pike tells him that he's the captain now not the first officer.
In U-571, the main character is an American submarine officer whose captain is killed in action. Being second-in-command, he then has to take charge of the crew to complete the mission, evading depth charge attacks, engaging in an underwater battle, and being forced to make horrible decisions that cost members of his crew their lives....all while manning a German-built sub whose controls they are utterly unfamiliar with. At one point, a crewman says that the Chief Petty Officer should assume command rather than Lt. Tyler. The Chief shuts him down very quickly.
In fact, the Chief gives Lt. Tyler an epic ass chewing in private for letting the men know that he didn't know what to do. His men needed to have confidence in their new commander's leadership, even if it meant boldly making the wrong decisions until they figured out what to do.
In Zulu, Lieutenant Chard of the Royal Engineers finds himself in command of an obscure supply depot that has lost its major and come under attack by Zulus. Lieutenant Bromhead, despite being an infantryman, must defer to Chard because he received his commission three months later. In reality, Chard had three years' seniority over Bromhead and was explicitly left in command by the Major in charge of the post. Seniority aside, Bromhead was deaf, which was the reason he was posted to Rorke's Drift, where no action was expected.
In The Warriors, the gang's leader Cleon is killed in the opening of the film. Second-in-command Swan takes his place, though the bruiser of the group, Ajax, tries to dispute this.
The Guns of Navarone. Major Franklin starts off as mission leader thanks to his rank, but Mallory becomes the de facto leader after Franklin is incapacitated by injury.
In the opening scenes of Saving Private Ryan (on Omaha Beach), Captain Miller receives more or less this exact response from his sergeant after asking who's in command of their area of the beach. He's already in command of most of the men around him, though, he was merely confirming that nobody higher-up was around.
Full Metal Jacket gives us this line: "You're senior NCO, Cowboy. You're in charge. Continue on with the patrol, and call in at the next checkpoint." Cowboy's response to this lies somewhere at the end of the sliding scale of Oh Crap.
On several occassions in Downfall, Hitler randomly promotes officers to higher military positions. General Weidling is ordered to defend Berlin when he only came in to attest that he didn't move his command post and therefore shouldn't be executed. Ritter von Greim is an even better example however: he was also already a general, but when he makes it to the bunker he is put in command of the entire German air force (which is all but completely defunct by this point in time), and told that he has to rebuild it from the ground up. When Hitler starts claiming that he'll be able to give Greim a thousand jet aircraft on short notice, it's become obvious that reality and him don't see eye to eye anymore.
All of these happened in Real Life - ironically, von Greim's You Are in Command Now moment nearly led to another You Are in Command Now - Hitler could very easily have conferred the promotion by phone or telegram, but he just had to give von Greim the command in person, with the result that von Greim's pitiful little aeroplane was nearly shot down over the Tiergarten and von Greim injured so badly he couldn't fly it. If his lover, top display pilot Hanna Reitsch hadn't been there to fly the plane from the backseat, he would certainly have been killed.
In Terminator 3, John Connor and his future wife Kate reach the West Coast command bunker, but none of the civilian leadership did because of SkyNet's interference. So John seizes control of the confused and scattered resistance by default.
Depending on your view of how well they fit together, sequel Terminator Salvation shows the logical outcome of this: John Connor is an able NCO, seemingly in command of his own "flying column" of loyal troopers and pilots, but still just another grunt in the armies of mankind. When the rest of the Resistance High Command is killed, John uses his position as an influential Voice Over The Radio to rally the rest of the Resistance around him.
With much foreshadowing, this happens to Sergeant Savage in We Were Soldiers, leaving him in command of a platoon cut off from the rest of their battallion overnight. In Real Life, there were higher-ranking men present, but they were all pinned down, and he happened to be next to the radio operator, meaning he was the senior man able to communicate with the Battallion HQ.
In the Babylon 5 TV movie In the Beginning, Sheridan's greatest victory during the Earth-Minbari War turns out to be the result of an ambush by the Minbari fleet that destroys all ships but his. Sheridan is the XO on the ship until he notices that his captain has been crushed by a piece of bulkhead. He takes command and manages to destroy the Minbari flagship by luring it into a minefield. Despite not being a full captain at the time, everyone always refers to him being a captain at the time because of this trope. Even Delenn states that only one human captain has ever survived a battle with the Minbari fleet, referring to him.
Earthforce appears to follow the naval tradition that the commander of a ship is always the Captain, regardless of actual grade.
Jules Verne wrote a novel called Dick Sand, A Captain at Fifteen, where all the crew of a ship is killed while hunting a whale, except for the eponymous character.
Elizabeth Moon has a good Space Is an Ocean example, in Winning Colors: Esmay Suiza takes command of a small spaceship, despite being a junior-grade lieutenant who's about twelfth in the chain of command, because everyone ahead of her is either in the pay of the enemy or killed by those who are.
Midshipman's Hope by David Feintuch is all about this trope. Deconstructed and subverted as well.
Robert A. Heinlein, being an ex-navy officer, brought this up a number of times in his books.
Starship Troopers has two examples. First he gives a historical example: during the capture of the Chesapeake by the Shannon, a midshipman was technically in command when it surrendered, but got cashiered for desertion because he had gone below deck. Then he explains a possible case where a third lieutenant (cadet/ensign) could end up commanding a division, and notes that "you'd buy the farm" if it happened.
The actual historical situation was a bit more complicated, He wasn't actually cashiered for desertion; the charge was brought against him, but the evidence basically came down to the accused and another officer making contradictory claims, and both of them had a vested interest in not being found to be the person in command when the vessel was lost. The charge that stuck was something along the lines of "failure to properly encourage his men", which by the standards of the time basically meant that he didn't kill a few of them to convince the others to stop retreating so that he actually could get back above decks (and he was, eventually and posthumously, exonerated of even that).
Also, someone mentions an offstage example where a junior officer was in command of a brigade.
He also wrote a book around this trope, Starman Jones. The eponymous character signs aboard the passenger liner Asgard as a steward (and has to forge papers to get that position). He gets a position as apprentice astrogator because of his ability and because the ship is badly short-handed in astrogation. At the end of the book he winds up as captain because the original captain, astrogator and assistant astrogator have all died and only an astrogator can hold command of a spaceship that is underway.
In Heinlein's If This Goes On, John Lyle, a junior officer in the rebel forces attacking the Prophet's capital of New Jerusalem, is thrust into the position of commanding the whole force, when his commanding officer is wounded and his tank-analogue seems to be the only one in effective communication with all the others. Lyle isn't technically next in the chain of command (he's a staff officer/aide de camp for the commander, doubling up as tank commander). However, he doesn't trust the next in line to handle the situation (too cautious) so he continues to issue orders in the commander's name, even though the commander is out of contact. Once things reach a "safe" state, he tells the real next-in-line "you are in command now".
And in The Moon Is a Harsh Mistress the Everyman narrator, Mannie Davis, ends up in control of the Lunar Revolution's government and wins the Revolutionary War when the rest of the Lunar government are killed by Earth's bombing. This is a subversion, because he then finds out that the government actually survived - but their communications with his military base were cut off by the bombing. Understandably though, in view of his success, they wholeheartedly endorse his actions.
A much less extreme version occurs in Space Cadet where Cadet Jensen, the senior cadet, takes command of an expedition to an unexplored region of Venus to investigate a native uprising when the Lieutenant in charge is comatosed in an accident. Aside from Jensen and the Lieutenant, the expedition consists of two other cadets. However, they are there because no one else was in a position to respond so Jensen is acting well above his pay grade.
The book Spaceship Medic by Harry Harrison deals with this scenario. The eponymous medic ends up in charge after all the rest of the ship's officers are wiped out by a meteorite hitting the bridge (then has some adventures, saves the day, and goes back to being a doctor).
Happens at least once a book in Honor Harrington: the flagship of a formation is destroyed, or battle damage kills the captain of a spaceship. Sometimes things work out fine; usually they don't. In each example, David Weber spends at least a paragraph explaining why it is that this person is now in command, instead of the higher ranking/more qualified personnel on the next ship over (or even in the next room). The results are not always pleasant, but the logic is always sound at the time.
He also uses this with the short story "Ms. Midshipwoman Harrington". The eponymous character gains command due to the battle, controls the situation, and emerges victorious. She is later given a promotion earlier than normal, but only to Ensign instead of several or more ranks.
Most plot-relevant to the entire series is Honor's assumption of command at First Hancock in The Short Victorious War — already flag captain, she commands the task group when the admiral is incapacitated, despite not being the senior remaining officernote The circumstances of Honor's assuming command are not entirely illegal, however — while she was not the senior remaining officer of the task force, the senior remaining officer was on another ship without access to the flag bridge's command datalinks, it would have required a nontrivial amount of time simply to brief him on the details of the current situation, and the task force was under fire and a command decision needed to be made immediately.. She's vindicated (winning the personal approval of the Queen, no less), but the fallout from that one action sets the stage for the following novels Field of Dishonor and Flag in Exile.
And subverted as well in The Short Victorious War: A communications officer on a dreadnought is taking the night watch because of his lack of seniority, essentially there to keep the Captain's chair warm and call him if there is an emergency. The dreadnought suddenly finds itself in an engagement with a group of Havenite battlecruisers, much to the surprise of both sides, and not having time to wait for the Captain, ends up winning the engagement through sheer disproportionate weight of firepower by reading down a checklist.
Not just in the Honorverse; examples abound in David Weber's other books.
In the stand-alone novel In Fury Born the protagonist Alicia DeVries gets one of these when her unit is shattered during the Shallingsport operation. She begins as a sergeant first class, leading a squad, and less than ten minutes after the battle starts ends up in command of her entire company. Then again, her company in the same span of time goes from a full complement of 275 troops to 63 survivors (they drop into what was supposed to be a clear LZ, in what was supposed to have been a surprise raid... and land right on top of a dug-in enemy battalion with heavy weapons that knows they're coming, and has been waiting for them).
In his Empire from the Ashes trilogy, main character Colin MacIntyre goes from mid-21st century space survey pilot to captain of Dahak, a super-advanced moon-sized Cool Starship from a long-dead space-faring empire, to Governor of Earth, to Emperor of the 5th Imperium of Man. Talk about a serious promotion!
In To Say Nothing of the Dog by Connie Willis, the overworked Oxford time-travel department ends up being run in large part by T.J., an undergraduate with the wrong major, while everyone else is out on assignment in the past (he's the only black person on the staff, and all the parts of the past they're interested in are too dangerous for him). Mr. Dunworthy, the head of the department, tells T.J. the (entirely fictitious)* The closest real case to this from the battle would be the Kaga, where the after-action report was written by her aviation control officer (a bomb hit the bridge). He was so far down the chain of command that he apparently didn't realize he was in charge until after the ship sank. story of Ensign Klepperman, who ended up in command of a ship during the Battle of Midway when the entire bridge crew was killed, sunk two destroyers and a cruiser, and was eventually killed in the line of duty.
In Diane Duane's Star Trek: TOS novel Doctor's Orders, Dr. McCoy is given the con during a First Contact mission as part of a joke by Kirk. Then Kirk disappears into a temporal anomaly, a Klingon warship shows up, and Starfleet regulations won't allow McCoy to hand over command to any line officer until relieved by Kirk or Federation brass. Which means he's stuck in command in the middle of a major crisis with everyone's lives depending on his command training (which he does not have) and military ability. He does well enough that upon learning that that he was the ship's physician, the Klingon captain remarks that he needs to kill his ship's physician if they were all like McCoy — Klingon Promotion being what it is. McCoy then tells him there's more than one reason; based on McCoy's diagnosis of the Klingon's health either his physician is outright incompetent, or is already trying to gain command of the ship by giving him substandard care.
Invoked in John Hemry's Paul Sinclair novel A Just Determination; a limited duty officer points out to an ensign that the ensign might find himself in command while just an ensign to persuade him that his testimony would be taken seriously at trial.
Bill Mauldin drew a cartoon during World War II on this topic: two privates are using crates as desks in a lean-to "orderly room." One is talking on a field phone, saying, "Yes, we've sent our quota to the rest camp.... This is the company commander speaking!"
The Lieutenant in L. Ron Hubbard's Final Blackout is the embodiment of this, stubbornly refusing to relinquish command of what is left of his brigade in World War III (which is regarded by those who care as a later "phase" of World War II), and goes on to take over England.
In On the Beach, we are told that, following nuclear exchanges in World War III, "command got down to very junior officers indeed". Mentioned is Major Chan Sze Lin as being in command of what was left of the Chinese strategic forces when the Australian Prime Minister contacted him in a last ditch attempt to salvage some of the world.
A central element of the Destroyermen series by Taylor Anderson is how people are forced to accept new responsibilities. The most literal example to this trope is how the destroyer Mahan takes a 10 inch shell to the bridge, killing all of its officers. When Captain Reddy makes contact with the ship, an engineering officer is the captain.
However, Reddy is not willing to let a "Snipe" command a ship, so he sends his XO to take temporary command. Unfortunately, the crew doesn't quite warm up to the XO and instead prefers to trust an insane pilot.
In Sandy Mitchell's Ciaphas Cain novels, Jenit Sulla is promoted to captain because when another company lost its captain, none of the lieutenants took the initiative to lead them all in his place, whereas Sulla had taken command of her company after Captain Detoi had been wounded in the same action.
This was fairly common in the 597th Valhallan since they were originally two units algamated into one after severe combat losses. Colonel Kasteen was a company commander in a garrison regiment but was given command after every other senior officer was eaten by tyrannids. Major Broklaw, a soldier with far more seniority and combat experience, became her XO because the senior officers in his crack planetary assault regiment were not eaten as quickly. Understandably he was not a happy camper initially.
Titanicus. Cally Samstag finds herself in charge of the remnant forces of activated 26th, a third line unit reduced from over 60 members to 18 in one engagement, with every officer dead. Erik Varco represents something of an inversion to the trope: he leads his battered remnant armour squadron because he was already in command when a traitor Warlord Titan annihilated them.
Brutally crushed in Sabbat Martyr, in Ensign Valdemeer, an eager young junior officer on a grand Imperial warship. When most of the bridge crew is slain and the captain cut off from his usual empathic command of the ship, he asks for Valdemeer to take the helm. Valdemeer tries to fight back, but the ship is already near-destroyed, and he and the captain are killed without being able to inflict telling damage on their killers.
In Ghostmaker, when a small group of Ghosts were protecting some wounded Volpone blue bloods, Culcis, one of the wounded, got up despite his injuries and got some of his fellows to follow him, and volunteered that they could shoot. When he appears again in Necropolis, he has risen in rank, and tells Corbec that the doctor was kind enough to mention his part in the defense, which had gotten his officers' eyes on him.
Milo takes the initiative after a sergeant was killed. Gaunt wrestles with putting him in the position but decides that being the youngster trooper, it would be too much (Rawne considers this a mistake on his part).
Later in the battle, Gaunt is temporarily "promoted" to be the supreme commander of all Imperial forces in Vervunhive, despite the fact that he's only a Colonel-Commissar. This is because the other ranking officers are either dead, disgraced, or members of a Tank regiment (and apparently an Infantry commander always takes precedence over a Tank officer). However, after the battle, he is restored to his regular rank, though he keeps the city's ceremonial power sword.
In Traitor General, Uexkull kills a local commander for not searching vigorously enough to find infiltrators, and then disables his second in command for not answering promptly enough when he asked. The third-in-command is asked about the infiltrators and leaps to search — whereupon Uexhkull has him kill the second-in-command and go to the search.
"Who is," Uexkull asked, his voice like a slither of dry scale, "second in command? Say, for instance, if the garrison commander is suddenly deprived of brain activity?"
William King's Space Wolf novel Grey Hunters. Ragnor finds himself in a position to attack some enemies from behind, and relieve some hard-pressed Space Marines. He launches the attack and orders them to follow his flare to escape. They obey, and then he is told that the force includes Berek, the commander of their entire mission. Fortunately, while Berek is a bit of a Glory Hound, he is just; he praises Ragnor's quick thinking and says he is grateful (in fact, it leads directly to his Field Promotion).
He put every ounce of command he could into his voice, hoping that whoever was down there would have the sense to respond.
In Graham McNeill's novel Storm of Iron, Leonid finds himself in command. When he looks at the men coming to his first briefing, he is still trying to assimilate that it is his regiment.
Ben Counter's Soul Drinkers novel Chapter War. After Varr's death, Kullek takes command of the 901st Regiment. He tells the Howling Griffons that the chain of command was none too clear.
Tavi in Jim Butcher's Codex Alera series finds himself in command of the First Aleran Legion when an enemy attack leaves him the sole surviving officer who can command. He goes from being the Third Sub-Tribune to the Tribune Logistica (Third assistant to the Quartermaster, about as low in the chain of command as an officer can get), whose duties to that point consisted mostly of latrine-digging (a position he never even earned, being appointed there for intelligence-gathering purposes) to commander of a legion. He does far better than anyone else could ever have hoped to.
In Dudley Pope's Ramage, at the very start of the book the main character finds out that every senior officer has been killed, making him the captain of a frigate. Which is badly damaged, sinking, and under attack by a superior enemy vessel.
In an interesting use, John Geary finds himself in command of the Alliance space fleet by virtue of being the most senior captain after the higher ups are all killed. Interesting because he got the promotion 100 years ago and, it was believed at the time, posthumously. The fact that he was even there to receive command was dumb luck when the fleet picked up his stasis pod on the way to battle.
Colonel Carabali also gets this when the Marine general is killed with the rest of the leadership.
In F. M. Busby's Zelde M'Tana, the eponymous character comes on board a starship as part of a slave cargo. Due to a mid-voyage mutiny, a relationship with the new captain, some dramatically convenient casualties, and, to be fair, a whole lot of work on her part, she winds up in command. And then they have to send her away as soon as the XO is well enough to replace her, because while she's done a good job, there would be too much resentment from the ranks to keep her on.
In The Wheel of Time: "When we Shienarans ride, every man knows who is next in line if the man in command falls. A chain unbroken right down to the last man left, even if he's nothing but a horseholder. That way, you see, even if he is the last man, he is not just a straggler running and trying to stay alive. He has the command, and duty calls him to do what must be done."
In the Dragonlance Chronicles this happens several times to the young elven princess Laurana. Sturm Brightblade puts her in command of the Solamnic Knights defending the High Clerist's Tower right before he goes off to make his Heroic Sacrifice. And after Laurana saves the tower, Lord Gunthar Uth Wistan gives her command of his entire army. This is especially remarkable since the Solamnic Knights believe women should Stay in the Kitchen, but as most of the Solamnic leaders were killed at the High Clerist's Tower, Gunthar realizes that Laurana is the only person left with the brains, charisma, and guts to successfully lead his troops.
In The Thrawn Trilogy, Pellaeon is revealed to not have been a full captain at Endor, receiving a You Are in Command Now promotion when his captain was killed...and that he, as the survivor with the highest rank, was the one who sounded the retreat. Due to various retcons, Pellaeon now was a full captain at Endor...it's just that his ship was commanded by an admiral, so he was still second in command until said admiral was killed.
At the end of the trilogy, when Thrawn dies, Pellaeon takes command of the fleet, and several books set later have him reporting to or working with various high-ranking Imperial villains of the book; he always survives their inevitable deaths, and since he's actually very competent he becomes Supreme Commander of the Imperial Fleet by the Hand of Thrawn duology. That's somewhere between Thrawn's and Vader's level of authority. That's right, Pellaeon becomes the most powerful man in the Empire, or what's left of it, by being unspectacularly good at his job and surviving everyone who outranked him. It does take years and years of work — he's in his 60's in the Thrawn trilogy.
In The Fifth Elephant, Sgt. Colon finds himself in command of The Watch after ALL his superior officers head to Überwaldnote Commander Vimes is there on a diplomatic issue. Capt. Carrot goes on leave to pursue Lt. Angua, who has run off to Überwald for personal reasons. It rapidly goes pear-shaped, to the point where Colon provokes the first Watchman's strike.
In Valiant, Michael Jan Friedman has a certain Second Officer Jean-Luc Picard (who would have held the rank of Lt. Commander at the time) on board the Stargazer. Captain Daithan Ruhalter and First Officer Stephen Leach are killed and critically injured, respectively, suddenly forcing Picard into the position of temporary CO. He doesn't exactly have an easy time of it, but since future canon has him as the Captain of the Stargazer, he eventually gets a two-grade jump to Captain.
Subsequent novels reveal that not everybody was happy about the promotion. Specifically, Admiral Arlen McAteer has his own vision of Starfleet, and it doesn't include young upstart captains like Picard. While McAteer can't demote Picard himself, as Picard reports to Admiral Mehdi, he can set tasks for him that would reveal Picard's incompetence or lack of experience, forcing Mehdi to reverse his decision. This is ironic, considering the books go into great detail to show McAteer himself as overly-ambitious, seeing nothing wrong with ambition himself (to the point of disliking Shakespeare for his criticism of it).
In Michael Flynn's The January Dancer, Hugh finds himself The O'Carroll, and leading the loyalist side, because he happened to be out of town during the coup.
You're in Command Now, Mr Fog by J.T. Edson tells how, as a 17 year old freshly-promoted first lieutenant, Dusty Fog found himself in command of a company during the The American Civil War after the commanding officer is killed by an enemy sniper.
In John Birmingham's Axis of Time, a multinational fleet from the 21st century is transported to 1942 right next to the US Pacific Fleet battlegroup heading for Midway. The JDS Siranui is recognized as a Japanese ship, and the Pacific Fleet opens fire. One of the shells destroys the Siranui's bridge, killing most of the command crew. Sub-Lieutenant Maseo Miyazaki wakes up to be told by the ship's Combat Intelligence that he is the ranking officer and urges him to authorize the CI to defend the ship by any means necessary. Unfortunately, Miyazaki is killed later by a group of racist marines.
In Red Storm Rising, fleet staff officer LTCDR Toland ends up conning the USS Nimitz after two missile hits kill the admiral, the captain, the rest of the staff and everyone in the CIC, despite being a reservist who hadn't commanded anything bigger than a Boston Whaler in nearly ten years. He was alive, he was on the bridge, and the XO was busy with damage control.
In The Independent Command Commander Peter Raeder, an Ace Pilot-turned-engineering duty officer after a Career-Ending Injury, is made acting captain of the light carrier Invincible when the captain and XO are badly injured in an enemy hit-and-run attack on the base.
In Star Carrier: Deep Space the Confederation invokes its Military First Right law to take command of the America battlegroup without approval from the USNA chain of command. A Pan-European admiral is placed in command of the fleet made up of the USNA and Pan-European battlegroups. As the America is the largest carrier in this fleet, the Pan-European admiral moves his flag to the America and the USNA admiral is forced to be his staff member instead. Later, the fleet is ambushed by several Slan ships, and one of the Slan beams destroys the flag bridge. America's Captain Grey attempts to continue the mission but comes into conflict with the captain of the Pan-European battleship from whence the slain admiral transferred, whose intention is to return to Sol for new orders. The ship's AI claims that there is equal amount of precedent for both command claims, and Grey ends up commanding only the USNA ships while the Pan-Europeans leave.
Duncan fits this trope in the Dragon Age novel The Calling. After King Maric finally allows the Grey Wardens back into Ferelden, a small group of Orlesian Wardens led by commander Genevieve arrives to investigate the Deep Roads for Darkspawn activity. Duncan is the most recently recruited member of the group. By the end of the novel, only Duncan and Fiona are left. He contacts the Grey Warden HQ in the Weisshaupt Fortress and is told to assume command of all the Wardens in Ferelden by virtue of being the only one left (Fiona doesn't count as she's an elf and a mage, both working against her).
In An Instinct for War's story Human Rain, every other officer in Motojiro's company is dead or wounded after the battle at Nanshan, so he gets promoted and put into command of the unit for the attack on Port Arthur.
The very premise of Star Trek: Voyager is that the original Starfleet crew is blended with the absorbed Maquis, and three members of the main cast receive their positions following the catastrophe that created that situation; the EMH arguably gets promoted to the status of sentient being as the result of this trope. This also occurs in several individual episodes:
The episode "Displaced" has the crew being abducted one by one, thus forcing everyone else to pick up the slack. Suddenly a VERY young ensign is the Chief of Security. In a bit of gallows humor Chakotay jokes, "Who said there was no room for advancement on this ship?"
The Doctor commanding Voyager all by himself in "Workforce" when the crew has to evacuate, although this was actually the point of him being upgraded to the Emergency Command Hologram that temporarily replaces his medical database with a tactical one. This is how he is able to pull off a tricky Shoot the Bullet maneuver that disables two enemy ships.
And "One", where the Doctor and Seven of Nine are the only crew members not in stasis.
"Course: Oblivion" has the crew being afflicted by a strange disease one by one, which causes this trope when the higher-ups get affected. They were a duplicate crew from a previous episode. Then they blow up.
"Disaster" puts Troi in command, as the most senior officer left on the bridge during the eponymous disaster (despite the fact that she has no command training, which gets addressed in a later episode). This is one of the areas where Starfleet breaks from current-day practices. Troi would most likely have been a commissioned officer and would not have taken command. Ensign Ro would have, especially since she was a command division (line) officer.
"Descent" leaves Dr. Crusher in command of a skeleton crew while everyone senior to her had off-ship duties. Unlike Troi, however, Crusher had taken and passed the Bridge Officers' Test, and actually enjoys command, as she would later get command of the medical ship USS Pasteur (in an Alternate Timeline).
"The Arsenal of Freedom" has Geordi in command, and puts him in conflict with the more senior Chief Engineer Logan. When Logan tries to pull rank, Geordi states that he is in command until he is relieved by either Captain Picard or Commander Riker. This is Truth in Television, as Geordi would be considered the Officer of the Deck as delegated by (and thus may only be relived by) the captain or XO.
"Remember Me" has Dr. Crusher quickly ascend the ranks as everyone else on the ship ceased to have ever existed in the local universe.
While the episode itself does not feature it, Tapestry reveals that this is how Picard got his first command — he was serving as a bridge officer on the Stargazer when the captain was killed and the first officer was injured, leading to Picard taking command and salvaging the situation (in the aftermath, Starfleet Command was impressed enough with Picard's actions that they promoted him directly to captain and made him the new commanding officer of the Stargazer).
In the Deep Space Nine episode "Valiant", during a training mission, all the training officers leading a group of cadets are killed. The cadets assume command of the ship and the mission.
Also in the same episode was the call forward of the page quote: by the time Ensign Nog receives command of Valiant, a majority of the bridge crew and cadets were dead, forcing Nog to tell everyone to abandon ship within a minute. Only him, Jake, and one other cadet survived.
The Star Trek: The Original Series episode "Catspaw" left Assistant Chief Engineer Lt. DeSalle in charge of the Enterprise while all the higher-ranking main characters (Kirk, Spock, Scotty, and Sulu) explored the surface. Over the course of the series, DeSalle appeared in a grand total of 3 episodes. Robert Bloch's original script had everyone senior to Uhura off the ship, and left her in command, but Executive Meddling wouldn't allow for a black woman being put in command of the Enterprise.
Near the end of Season 1, King Robert dies. The boy he believed to be his son, Joffrey, takes the throne. However, on the (correct) belief that Joffrey is not actually Robert's son, Robert's younger brothers Stannis and Renly both proclaim themselves King.
Ned Stark, Lord of Winterfell, leaves Winterfell to take up the post of Hand of the King, elevating his eldest son Robb to Acting Lord of Winterfell. Later, Robb leaves too, and his younger brother Bran becomes Acting Lord. Winterfell is then invaded by Theon Greyjoy, who declares himself to be Lord.
King Joffrey names his grandfather Tywin Lannister as Hand of the King. However, Tywin isn't actually present as he's fighting a war, and his eldest son Jaime is held captive by their enemies, so Tywin appoints his younger son Tyrion as Acting Hand.
When the King is away from the throne, the Hand exercises all the King's powers, to the extent of sitting on the Iron Throne.
Castiel from Supernatural goes from an angelic foot soldier (albeit a high-ranking one) in Season 4 to the "sheriff of Heaven" in Season 6 when all the archangels have been killed and imprisoned. He then battles with Raphael, the last remaining archangel, for control of Heaven. He wins the battle and becomes God.
In the episode where the brothers are sent back in time to the Wild West, Dean and the innkeeper find the sheriff's smoldering corpse in one of the rooms. Dean asks who the new sheriff is, and the innkeeper pulls the badge off the corpse and pins it to Dean.
Hawkeye has to take command of the eponymous unit in M*A*S*H several times during his stay in Korea. It always endsbadly. Then there's Frank Burns — which is worse. The only temporary commander of the 4077th that is tolerable is Major Winchester, and that's because he uses his times in command to loaf (and doesn't interfere with day-to-day operations).
In the re-imagined series of Battlestar Galactica, virtually the entire military sees this trope in action.
Bill Adama, old warhorse ship's captain about to be put out to pasture, finds himself in command of the entire Colonial military, what's left of it. All sorts of people who previously filled fairly minor roles are suddenly thrust into positions of great authority... in many cases, they're doing the same jobs they did before, but those jobs are now of critical importance because there's nobody else doing them. For that matter, the eponymous Battlestar is itself promoted from "antique vessel about to be mothballed" to "flagship of the Colonial battle fleet." (Also only ship of the Colonial battle fleet, but that's par for the course with this trope.)
His son, Lee, is appointed CAG by Colonel Tigh as he is the highest ranking pilot on board, despite background material saying that Lee is a reservist.
After the deaths of Admiral Cain and Colonel Fisk, there's no-one to take command of the battlestar Pegasus, so Admiral Adama appoints Barry Garner, an engineer, as its commanding officer. This doesn't work out very well, as Garner treats the crew more like parts of a machine than as human beings and jumps straight into a Cylon trap.
This again happens when Garner leaves the CIC during an intense battle to fix the FTL drives, telling Lee that "you have the conn". Lee gets promoted to full commander at the end of the episode.
In the finale, Lt. Hoshi is later promoted to Admiral in the wake of a mutiny led by the man who otherwise would have got the job had he not snapped, and everyone else that would have stood between him and the position had also been killed off by that point. Not a bad finish for the guy who entered the series as the guy standing in the corner looking utterly mortified while Admiral Cain orders an attack on Galactica.
Hoshi is only really in command of the civilian ships and whatever military units are left when the Galactica goes off on its final mission. There are still plenty of more senior officers but they are all going on the mission. His new rank will not become effective until they are all killed (very likely at that point). At this point the military rank structure is shot anyway and being competent and loyal gets you the job.
Invoked in "Blood on the Scales." Gaeta and Zarek launch a mutiny and attempt to oust Adama from command. Roslin manages to escape Galactica, and Adama is brought back to the CIC. After Gaeta orders him to tell Roslin to stand down, still addressing him as "Admiral," Adama scoffs at this and rips off his insignia, tossing them to Gaeta. "You're the Admiral now. So you call up Roslin. Make her laugh."
Laura Roslin falls under the civilian version of this trope, Unexpected Successor. Please don't list her here.
On 3rd Rock From The Sun, Sally was next in command after Dick. In one episode, both Dick and Sally were incapacitated and Tommy assumed the role of High Commander (with only Harry left to command).
Stargate Atlantis has had several promotions by death of senior officers, including Major Sheppard becoming the military commander of the expedition, and Dr. Keller becoming the chief medical officer.
In the prelude of Firefly, Malcolm Reynolds, a sergeant, ends up in command of all the ground forces in the Battle of Serenity Valley. When one of his men says they need an officer's authorization to get air support, he tears a patch off a dead lieutenant and hands it to the trooper, congratulating him on his promotion.
By the beginning of season five, in the episode "Holoship", Red Dwarf's Rimmer states that he has been in command of Red Dwarf since Lister was revived.
This is true, as Rimmer and Lister were the only surviving crew members and Rimmer outranked Lister. Lister needed only to pass the chef's exam in order to assume command, but he couldn't manage that. However, as Lister and Cat are able to fly Starbug and Kryten was the most knowledgeable of the bunch, you have to wonder in what way Rimmer was really in charge from season 5 onwards.
JAG: At various points throughout the show Harm, Mac and Singer gets the chance to be Acting JAG for short whiles. Sturgis takes over as Acting JAG for an extended period of time between Admiral Chegwidden's retirement and General Cresswell's appointment.
Now the Captain called me to his bed He fumbled for my hand "Take these silver bars," he said "I'm giving you command." "Command of what, there's no one here There's only you and me — All the rest are dead or in retreat Or with the enemy."
If you choose the Lawful story arc in Neverwinter Nights 2, your character goes from new recruit city watchman, to lieutenant, to squire, to Captain of Crossroad Keep, to Knight-Captain of Crossroad Keep in under a week, due to the death by burnt-down-headquarters of the former holder of the first post and the suddenly increased importance of the fortress in the second post. It also helps that by that point you practically single-handedly rein in crime in the city and win Neverwinter a war.
Gears of War has the soldier Marcus, convicted of abandoning this post and branded a traitor, getting a field promotion when the commanding officer dies. This is a case of practicality: Marcus is chosen because, despite his treason, he is still hands down the best, most qualified soldier in the squad, despite being absolutely hated by the Gear commander. Dominic, Baird and Cole all follow Marcus not because he's in charge, but because he knows what he's doing. Baird initially puts up some resistance, but even he comes to see the wisdom of having Marcus in command.
Greil of Fire Emblem: Path of Radiance begins putting his son Ike in command of their mercenary force during missions as soon as Daein invades Crimea near the beginning of the game. The implication is that Greil anticipated his own death in the war that was sure to come, and wants his son to be prepared to take command for real in that event (he's completely right in this). A dialogue between Titania and other members of the Greil mercenaries tells us later that Greil always wanted his son to become The Leader of his mercenary unit: had the war not happened, Ike would have become their leader eventually, just later.
Temporarily averted and subverted in Ace Combat 5. The player character is originally just nugget second lieutenant (fresh out of training pilot), but then Captain Bartlett gets shot down. The next mission, Nagase is assigned command of the squadron, as she's currently the most experienced in the squad. She turns the command down, giving it to the player. The mission after that, a new squad commander is sent by the Osean Air Force. Too bad he gets shot down as he's trying to land. Player character Blaze eventually becomes such a respected flight lead that a man with years more experience is happy to follow his lead, along with ridiculously fast promotion to Captain.
Subverted in Ace Combat 6: One of the antagonists, Victor Vocheck, took part in the initial invasion and takeover of the player's capital city, but gets hit and crippled for the rest of the war. Some of his subordinates assume command of his squadron until they got shot down by the player character. The situation becomes so bad, that the squadron had to consolidate with another squadron who is put under the command of Vocheck's protege, Lt. Commander Ilya Pasternak.
The final mission of Shattered Skies gives us this exchange from the pretender Yellow Squadron pilots:
Yellow Squadron pilot A: Oh! Jean-Louis' been hit!
Yellow Squadron pilot B: Gene, get a grip! You've got to take over the command!
Also, if you manage to down the Yellow Squadron pilots in the right order (something nearly impossible to do, as there is no way of differentiating them), the dialogue continues as the pilots' command structure disintegrates and pilot B gets command, then C...
In the 2008 Turok video game, thanks to rapid character attrition, the Redshirt Army is soon reduced to several grunts and a sergeant. Said sergeant promptly takes command, which turns out pretty badly for everyone else, as he's slowly going bonkers due to a head injury.
In squad-focused games like Battlefield 2, fickle internet connections can cause the most senior squad member (by global score/rank) to assume leadership should the original leader be dropped from the game. Sometimes results in a Hot Potato among the remaining squad, who likely joined so they wouldn't have to command.
Happens a few times in Command & Conquer, notably with Nod. The best example would be in the original; Seth is (heavily implied to be) trying to get rid of both the player, a Nod commander, and Kane, Nod's leader. Kane puts a bullet through his head, recalls the player's troops, gives you a new assignment, and ends with "Oh—And congratulations on your promotion." as he looks down at Seth's corpse.
In Final Fantasy VIII, Squall Leonhart finds himself summarily shoved into command of the entirety of Balamb Garden, though in this case it's because of a combination of Garden's role changing from "military academy" to "active fighting force" and a case of Because Destiny Says So: Headmaster Cid is acting on advance knowledge of the Stable Time Loop that includes Squall's defeat of Ultimecia. Considering that Squall is an emotionally stunted seventeen-year-old and wants to be in charge about as much as he wants a hole drilled in his head, it goes pretty well.
Mostly justified in that he tends to be the calmest person in any given situation and can quickly analyse a situation and come up with a plan.
In Wing Commander IV, Colonel Blair finds himself in this position aboard the Intrepid when Captain Eisen leaves for Earth to reveal Tolwyn's plan, even though he's Space Force and not Navy. Somewhat subverted in the novelization, in that he was assigned a Lieutenant from the Navy who was too junior to promote to Captain (rank, not position) to give the actual commands for carrying out Blair's orders. In the real world, aircraft carrier captains must themselves be part of the aviation community (i.e. have been a naval aviator themselves).
Even in video games, Star Trek ventures into this territory:
In Star Trek Online, this is how the Starfleet player becomes captain of their own ship, starting off the game as an ensign. The ship in question is a small one with a crew of only 200, and it's specifically noted that the Borg who boarded it while the player was assisting another ship wiped out the officers first, leaving the player as the only surviving officer of that crew. (This is also noted as unusual behavior by the Borg and eventually explored later on in a high level mission.) Once the player reports in to Admiral Quinn to get his battlefield commission formalized, the Admiral outright admits that they've been reduced to this by how pressed Starfleet is to deal with their various border conflicts.
This is mentioned twice in the backstories for the teachers in Star Trek: Klingon Academy. One of them was due to a vacancy that the man created himself via Klingon Promotion. The other was briefly Captain of his ship on the grounds that he was the seniormost officer on the bridge when the original Captain died, and then surprised the first officer (who was busy elsewhere when the Captain died) by turning command over to him at the battle's end instead of attempting a Klingon Promotion.
Happens also with Star Trek: Bridge Commander, though the situation seems a bit silly if you think about it. Your character has been a commander for only 3 months when the Captain is killed by a supernova. Starfleet sees fit to advance you to rank of captain, give you your own first officer, and loan you Captain Picard for advice for a few missions. A possible justification for this is only implied and not outright said: Starfleet is sorely undermanned in the area of the galaxy where the game takes place. To wit: you only see about a dozen other starfleet ships, but fight hundreds of Cardassian ships.
A few missions later, your entire crew is transferred to a Sovereign-class ship (i.e. the same class as the Enterprise-E). Apparently, your accomplishments are enough to command a flagship-level ship.
In the cutscene of the first World in Conflict mission, we see our soon-to-be-least-favorite commander driving through the streets of Seattle which is being attacked by the Soviets, trying to reach the commanding officer in charge of the defense on the radio. "Well, who is in command then? What do you mean, 'I am'?"
In Shin Megami Tensei: Strange Journey, you are summarily promoted to mission commander when Commander Gore is killed in action. His last act is to entrust completion of the mission to you and, from that point on, all crewmen will follow your lead. Even Arthur, your ship's artificial intelligence, will only issue missions to you and advise you as best as he can based on his role as mission analyst, but doesn't actually have any authority over you. It gets to the point when Gore is resurrected, and he briefly returns to your ship, but he only challenges your decisions (on an unofficial level) if you, personally, prove to be a threat to humanity.
Happens repeatedly with several of the longer sidequests in Skyrim. In the Winterhold College, you go from greenhorn student to arch-mage after the previous arch-mage is killed by his deceitful advisor, you singlehandedly ends a world threat, and the Psijic Monks back you. Among the Companions, you start as a fresh Shieldbrother/sister and end as Harbinger after avenging the previous Harbinger's death (which you indirectly caused), reforging their prize artifact, and ending a curse. The Thief Guild sees you join as a simple cutpurse and end as Guildmaster by killing the previous Guildmaster after revealing his treachery, joining the Nightingales, returning an incredible Daedric artifact, and restoring the guild to its full glory. And you join the Dark Brotherhood by inadvertently stealing one of their hits and end up leading them after legitimately assassinating the previous Matron, proving that you are the Listener, performing the greatest and most elaborate assassination of the era, and bringing the Brotherhood back to its glory days. Unsurprisingly almost all of these involve the deaths of the original leaders of the groups.
In the backstory of Fallout: New Vegas, during the battle between the Brotherhood of Steel and the NCR at Helios One, the Brotherhood Elder Elijah deserted in the midst of the battle. As such, the Head Paladin Nolan McNamara was forced to take the position of Elder.
Occurs in Mech Warrior 2: Mercenaries when your commander is killed during the intro. Of course, you're taking command of a mercenary outfit that consists of yourself at that point.
Mech Warrior 4: Black Knight has you promoted after the 2nd campaign during a Wham Episode where the party contracting your services betrays you and attempts to destroy the unit so they don't have to pay up.
In Dragon Age: Origins, the prologue ends with the death of every Grey Warden in Ferelden at the ill-fated Battle of Ostagar, except for the Warden and Alistair. Despite Alistair being the Senior Warden (by six months), he's reluctant to assume leadership due to both personal issues and having fallen into a depression after the loss of Duncan, his surrogate father-figure. As a result, the Warden is forced to take command mere hours after their Joining.
Rogue Galaxy: Like with Squall above, whenever the party goes on a mission to another planet, Jaster is always deferred to as leader even by people more experienced adventurers than him (basically every member save Lilika and Jupis) mostly due to his calm and collected nature.
If Arc Villain Balak survived the "Bring Down the Sky" DLC in Mass Effect 1, in Mass Effect 3 he finds himself the highest-ranking batarian naval officer left alive after the Reapers curbstomp the Hegemony. It's possible for Shepard to talk him into an Enemy Mine and add The Remnant of the batarian military to the war effort.
Don't forget Commander Shepard him/herself, who is unexpectedly given command of the Normandy SR-1 after Captain Anderson is Kicked Upstairs. And Mass Effect 3 gives us Primarch Victus, who actually hates suddenly becoming the leader of all turians; he's a military man, he hates politicians, and would rather be on the frontlines where he can make a difference than at a negotiations table.
Assuming that an intelligence service (with an openly active branch serving as elite bodyguards) can count as 'the military', and that there is an allowance for the one telling you that you are in command now to note that it doesn't actually mean much, The Elder Scrolls III: Morrowind features this in the course of the main quest — your superior in the Blades, Caius Cosades, promotes you to Operative (a mid-range rank, although the confirmation of that is only found if one looks in the editor) and then explains that he is being withdrawn to Cyrodiil for consultations and to investigate reports of his cover (a drug-addict) being more true than it should be, leaving you the highest ranking Blades agent on Vvardenfell (to Caius' knowledge, but as the Grand Spymaster for Vvardenfell he ought to know) for the time being and thus technically in charge. Of course, since the other Blades agents mostly do their own things and any orders from Cyrodiil to the contrary are liable to come with an actual superior, Caius basically tells you to ignore the big picture and just keep doing what you're already doing.
From The Order of the Stick: In Cliffport City, when the CPPD Chief is beheaded by Nale, the Rookie spontaneously take the lead and order around his fellow cops. A few senior ones shortly ponder about this, before deciding to just roll with it.
In Drowtales, Ariel is a little lost when her squad leader is missing and they need to figure out what to do. Ini'riia makes it clear that being Blue Blood makes her in command by default... that is, unless she wishes to let a senior member of a different clan take command...
On an episode of The Simpsons, Homer ends up in command of a nuclear sub after the Captain tells him that he is in command until he gets back. The Captain is then accidentally launched out of a torpedo tube. By Homer.
In the Star Trek: The Animated Series episode "The Lorelei Signal", all the male members of the crew are incapacitated. Uhura takes command of the ship and leads a landing party to rescue the male senior officers with Nurse Chapel as Chief Medical Officer and second-in-command. Uhura is never actually given command, and her commandeering the ship is technically mutinous (and she justifies this in the ship's log), but her superiors commend her actions once she rescues them. She also gets left in command in "Bem" when Kirk, Spock, Scotty and Sulu are all part of a landing party.
The Magic School Bus episode "Out of This World" had Dorothy Ann take command of the USS Enterprise-Shout-Out bus and both rescue Ms. Frizzle and Carlos from an asteroid and prevent said asteroid from hitting Earth.
The first episode of The Mighty Ducks combines this with Take Up My Sword. The original team leader Canard sacrificed himself to save the others, but not before giving his magic mask to Wildwing, albeit it wasn't until the next episode where Wildwing embraced his role.
In Jonny Quest Versus The Cyber Insects, each time Dr. Zin kills one of his numberedMooks, he promotes the one whose number is next, for example when he kills 425 he promotes 426. He even emphasizes "You are in charge now, [minion number]!" every time.
In My Little Pony: Friendship Is Magic this is basically what the Royal Guards tell Twilight when they reveal that with Princesses Celestia and Luna missing and her sister in law ruling a distant kingdom, she is now in charge as the last available Princess.
When the Battle of the Bulge begins all the higher ranking officers of the division are absent. When one of the officers starts complaining about this, the ever-competent Major Winters quietly tells him to concentrate on organising his men and finding ammunition and winter clothing for them, instead of wasting time waiting for superiors who might never turn up.
Dick Winters does this several times throughout the war. He drops into Normandy as a 1st lieutenant in command of a platoon, but ends up commanding the company because his CO is MIA (turns out, the CO never even set foot in Normandy - his plane blew up on the way). Later, when Major Horton is killed during Market Garden, he becomes battalion XO.
Some time after he becomes Battalion XO and while he's still a captain, Winters acts as 2nd Battalion CO while Strayer (the actual CO) performs the duty of an absent Regimental Officer. This results in Winters having to contend with going up against the 1st and 3rd battalion COs, who are both lieutenant colonels, when it comes to acquiring resources for both the missions to which his battalion gets assigned as well as provisions for his men.
Notably, with few exceptions, Easy Company had little problem with sudden loss of leadership: if someone of high rank was injured or otherwise unable to continue fighting, the company would quickly adjust for the loss and continue the mission. The one time leadership became a problem ( When Lt. Dike froze with indecision during the attack on Foy), Winters turned to Lieutenant Speirs, told him the trope and sent him out to lead Easy Company, who followed him with no problems.
At Pearl Harbor:
The USS Aylwin (DD-355) had four officers on board when the first Japanese aircraft appeared, all ensigns. Ensign Stanley B. Caplan, the senior one, had been at sea for only eight months and was in command of it for thirty-six hours (ironically enough, as he was obeying orders from the destroyer squadron commander to put out to sea, several men onboard saw their senior officers on a motor launch, but they could not stop to take them onboard until after the attack was over).
The same thing happened to USS Blue; again only 4 ensigns were on board (must have been the standard officer duty section) and they got underway and operated for 30 hours with an ensign in command, attacking two submarines and shooting down 5 Japanese airplanes.
Well it makes sense, a ship in harbor during peace time is unlikely to face any truly dangerous situations so it's a good chance to give the junior officers a bit of practice at the responsibility of standing watch without risking them sinking the ship and meanwhile the senior officers can get a break and take some shore leave.
It was also Sunday, so most of the crew was on liberty at the time.
The USS Nevada (BB-36) spent most of the attack under the command of Lieutenant Commander Francis J. Thomas, who not only coordinated the defense of the battleship, but also got her underway and attempted a dash for the open sea. When it became apparent that Nevada would sink before she could clear the channel, he ordered her run aground to avoid blocking the entrance to the harbor. At the same time, Ensign J.K. Taussig assumed command of the ship's Anti-Air batteries when the communications lines between them and the ship's command center were damaged. He continued to coordinate the air defense of Nevada even after losing a leg in an explosion, until being evacuated by Navy Corpsmen that had been called for by his gun crews.
During the Japanese occupation of the Philippines, American soldiers who escaped capture took command of local guerrilla groups and promoted themselves to the rank appropriate for the size of their band. For the most part, the US military honored these self-made ranks after the war.
In the Age of Sail this trope could often apply to members of the British Royal Marines. Lieutenants often ended up commanding landing forces whose size would call for people several ranks above them despite the presence of naval officers of suitable rank joining their force. They were trained for it, unlike the naval officers, and a lieutenant was the highest-ranked marine officer you'd normally find in the fleet.
Older Than Feudalism: The Ten Thousand: in 401 BC a force of Greek mercenaries is hired by Cyrus the Younger to supplement his Persian troops to fight against his brother Artaxerxes II for the throne of the Persian empire. They meet at the battle of Cunaxa. While the Greek force suffers no significant casualties during the battle, their employer, Cyrus, is killed during the battle.They are then left stranded in the middle of the Persian empire. They are asked by the Persians to lay down their weapons but they refuse. The leaders of the Greek are invited to a feast to negotiate a solution but are instead executed. The Greek elect new leaders, among them Xenophon, who chronicled their fighting retreat through Persia, which lasted around 2 years.
In the Soviet Red Army, especially during World War II, this happened frequently. Interestingly, their system was set up so an officer usually had to serve a given number of years in a rank before being promoted to the next one and they saw no particular reason to change the rule even if the officer in question was, through attrition, leading a much larger unit than he'd started with. If the officer was competent he was allowed to retain command and sometimes even promoted to a higher one while retaining the rank, thus leading to captains leading companies reporting to a lieutenant who led the battalion, while the regiment might be commanded by a captain who had several majors as his subordinates.
On the first day of the ANZAC invasion at Gallipoli, Turkey, lack of planning meant a lot of the soldiers just lost track of their commanding officers, if they were "lucky" enough to even get to the beaches, so this trope happened all over the place, for the unprepared Australian and New Zealand soldiers.
During The American Revolution and the War of 1812, the British wore, like everyone else, Bling of War, especially their officers (even though the Americans were no different in this regard). The Americans, used to hunting, could aim (even with muskets, even though they had to get very close to do so), and would chose the most visible targets. As a consequence, after the Battle of Bunker Hill and the Battle of New Orleans, certain British regiments were under the command of the senior private.
This is referenced in The Patriot, when the British commander asks the militia leader to stop targeting officers as a courtesy of war (and also so that the officers could maintain control of the enlisted men). Benjamin bluntly replies that they were soldiers in the field and as such acceptable targets. During the Final Battle, he is shown shooting an officer just before leading a bayonet charge.
Between World War I and II, the German Reichswehr was severely limited by the peace treaty in the number of soldiers it could recruit. To make up for this, every soldier was trained in the job of his commanding officer, specifically so that the Reichswehr could invoke this trope to increase their number as soon as the treaty would no longer apply.
In the last days of World War II, Hitler's most powerful underlings who were not in Berlin, Hermann Göring and Heinrich Himmler, attempted to take control of the crumbling Third Reich and negotiate peace with the Allies respectively. Outraged at the supposed betrayal of his own designated successor (Göring) and his most loyal follower (Himmler), Hitler banished both of them from the Nazi Party and prepared for his suicide. In his last will and testament, he named Admiral Karl Dönitz, the commander-in-chief of the Kriegsmarine, as President and Joseph Goebbels as Chancellor. Shortly after Hitler's suicide, Goebbels killed himself as well. The reasoning behind selecting Dönitz was because, in Hitler's mind, the Kriegsmarine had failed him the least.
Martin Bormann also got a promotion as a result: he became head of the Nazi Party. He, too, committed suicide by cyanide capsule, likely to avoid capture by the Red Army.
An even more egregious example: The new Reichfuehrer-SS was Karl Hanke, Gauleiter of Silesia. A fine choice, where it not for the fact that he was trapped in Wroclaw, nearly 200 miles into Soviet territory, surrounded by the Soviet 6th Army.
This is the origin of a French army tradition; Adjutants (a senior enlisted rank) are called "My Lieutenant" due to an incident during the Napoleonic wars. A unit that was being led by its senior NCO after losing much of its officers in battle impressed Napoleon, who mistook the uniform for a Lieutenant's. When he was informed that the "brave Lieutenant" was "merely" an Adjutant, he told his staff that from now on they would be addressed as Lieutenant.
During the Battle of Guadalcanal in World War II, Lt. Cmdr. Bruce McCandless was wounded by Japanese gunfire on the bridge of the USS San Francisco. He regained consciousness and realized that every other officer on the bridge had been killed, so he promptly assumed command and led the ship through the rest of the battle. He was rewarded with the Medal of Honor and a promotion to Commander.