At the end of Code Geass, Prisoners are freed and people rejoice when the evil overlord, in the middle of his global reign of terror, is killed by Zero, the supposedly dead freedom fighter. The kicker? The evil lord is actually the protagonist, and wasn't actually evil. Maybe.
It helps that, in this case, Lelouch's army is literally incapable of self-determination. He geassed them all into mindless slaves. They have no will other than obeying his orders or that of his immediate subordinates, and they were all in on it.
This actually happens to the Black Knights at the end of the first season as well. Given the series "Chess Motif" it's actually symbolic. When Lelouch, serving as the king and the group's strategist, is eliminated from the board, the rest of the group falls to pieces.
Fighting Britanian Empire with only a handful of forces hoping that this pitiful struggle will somehow ignite the rebellions all over the world was some wacky plan from the very beginning.
Played straight and defied in Fullmetal Alchemist. The coup specifically targeted Central command's top officers to disorganize and rout the enemy forces. It worked, at least until Fuhrer Bradley revealed himself to survive the assassination attempt. Defied by the Northern Force of Briggs, where their general was taken as hostage.
General Olivier Armstrong: It seems you do not know me well, gentlemen. One of our contingency is "If the situation calls for it, leave me behind." Survival of the fittest is the iron law of Briggs, and if I bite the dust here, it just means I am not fit to live. Do not take the notion of "I have raised them" as superficial. They can work with, or without me, and that's what makes a Briggs soldier.
The Mahora Festival arc in Mahou Sensei Negima!, where thanks to an added condition in the ritual the heroes were rushing to stop, all that was needed to attain complete victory was for that arc's Big Bad to be defeated by Negi. This really pissed off Chisame, since it meant that all her efforts didn't matter in the end.
Averted in Jack Rakan's Pensieve Flashback, where killing Lifemaker didn't stop the end-of-the-world ritual, and it took the entire fleet of three world superpowers to block it.
Subverted in Naruto, where after Gato dies, his thugs plan on looting the Land of Waves since he can't pay them. While the heroes don't have enough strength left to fight them off, the arrival of the citizens of the Land of Waves combined with Naruto and Kakashi making Shadow Clones forces them to retreat. Which makes it a double subversion.
Apparently played straight with Orochimaru and the Sound. Within days of his death, discipline and security collapsed in at least two high-security facilities.
One Piece does the "inch of his life" variety in every major arc. It often works because these are not armies, but pirate crews that work on Asskicking Equals Authority. By winning, Luffy almost immediately proves that the rank and file aren't coming out on top. Specific examples:
Captain Axe-Hand Morgan's defeat caused his Marines to stop fighting immediately. He was an extremely Bad Boss and they all hated him, enough to have him arrested after the fact.
Luffy had to defeat Arlong and destroy Arlong Park in order to set the village people free. It helps that, in just a couple of attacks, the Straw Hats put his Mooks out of commission and were left with his foremen.
The only way to stop the war in Alabasta was for Luffy to beat Crocodile and prove he's been manipulating the country this entire time. Played for Drama since the rebels/soldiers/Baroque Work agents weren't going to stop for anything.
Enel had to be defeated because, aside from his high-ranking soldiers, nobody liked him anyway.
Rob Lucci, while only being The Dragon to Spandam's Big Bad, had to be defeated by Luffy, or else he would just zoom on ahead and kill Robin. And even when Rob Lucci went down, the Straw Hats still had to make a grand escape from the Buster Call.
More justified with Gecko Moria, who was holding all the shadows from people around the world. Without those shadows, people die when exposed to sunlight. Once he was beat up enough, he forcefully let go all of the shadows he had collected.
Averted with Impel Down and Marineford. Luffy isn't strong enough to beat Magellan, and nobody too high up in the Marine chain is actually defeated. The war only ends because Whitebeard, whom the Marines were really after, was killed, and Shanks popped up, who agreed he'd take on all of the Marines and pirates if things didn't stop.
Averted in the Fishman Island arc. When Hody Jones goes down, his senior crew members go insane and just start killing everyone. They reasoned that even if they died, they could kill enough people for their legacy of hatred to persist. The Straw Hats had to beat the entire lineup and a vast portion of their 100,000 strong army to settle things. Incidentally, most of said army was human slaves and fearfully-loyal Fishmen, so they at least gave up when Hody went down.
Utawarerumono repeatedly uses this trope. In the anime version all the wars, except that against Kunnekamun, are won that way.
In Yu-Gi-Oh! GX, when the supreme king is defeated, his massive demon army immediately scatters.
In Dragon Ball ZMovie 12, a fairly large group of old villains are wreaking havoc after escaping Hell. Son Gohan takes out their de facto leader, Frieza, in one punch. Immediately, the rest of the army scatters and flees.
Subverted in Maoyuu Maou Yuusha. The Hero attempted to assassinate the Demon King, believing he could end the war immediately. After finding that the demon ruler is really a DemonQueen, she informs him that if he did kill her, the human rulers would cover it up because the war benefits them and the demons would simply chose a new ruler in no time. After finding that she is not evil, Hero realizes the better option for ending the war is to work with her.
Averted in Marvel Comics' "The Siege," when Norman Osborn is defeated but the heroes still have to deal with the rest of his invasion force, especially the Sentry. However, played straight in the Marvel Universe in general when pretty much everything Osborn had a hand in is entirely dismantled, dismissed, or overhauled the moment he's in jail.
Subverted in the Bone comic series. After the Hooded One is defeated, the Rat Creatures and Pawans initially act as though they are defeated and retreat, but then regroup and attack again a minute later.
In Halo: Halos in Space, it is suggested that killing the "boss alien" will make all the others go back to Alien Town.
The former Trope Namer comes from a famous Crowd Song in the 1939 film version of The Wizard of Oz; however, there it was justified, as the Munchkins were oppressed by the Witch of the East, and the "evil army" of the Witch of the West (the Winkies) turned around and cheered the destruction of their oppressor.
In Star Wars: Return of the Jedi, Emperor Palpatine's death seems to reduce the entire Imperial starfleet to virtual catatonia. They aren't even shown retreating or regrouping; they just suddenly are gone. They had also just lost Darth Vader, the second in command, and Admiral Piet, the commander of the fleet, along with their Death Star and flagship Star Destroyer, so this was more of a complete rout.
This was picked up on by Timothy Zahn, who worked a Hand Wave for it into the plot The Thrawn Trilogy. The Emperor was bolstering his forces morale via the Dark Side, and when he died the shock forced the Imperials into retreat. It also didn't help that the Executor, the most powerful vessel of the fleet and carrying all the leading personnel, was destroyed a few minutes prior.
Also occurs in The Phantom Menace when the droid control ship is destroyed, causing their droid forces to deactivate on the spot.
Averted in Revenge of the Sith. Early in the film General Grevious immediately takes command after Dooku is slain by Anakin and the Republic is fully aware Dooku's death isn't enough to end the war. Furthermore, the Separatist forces on Utapau keep fighting even after Obi-Wan kills Grievous. Afterwards Palpatine goes as far to have Vader execute all the remaining Separatist leaders to ensure no loose ends.
In The Chronicles of Riddick, Riddick kills the Big Bad and the war stops, because whoever kills the guy takes his place. Played with because the immediate battle was already over at that point, and had been for days. The army curb-stomped Helion Prime in a single night, and the only thing Riddick actually stopped was the subsequent genocide of everyone left on the surface. Nor do the Necromongers stop their crusade for the Underverse altogether; Riddick has simply become their new leader. Riddick reveals that his brief rule is not met with approval by the Necromongers, as he has stopped virtually all conquest and refuses to make a pilgrimage to the Underverse to become a true Lord Marshall (e.g. have the Super Speed ability). Finally, he departs to find his lost homeworld, leaving Vaako in charge.
In The Last Starfighter, when the titular Gunstar has to defeat an entire armada, the aim is to destroy the command ship's communications turret. Doing so will throw the large array of fighters into disarray. In the video game, it is the Instant-Win Condition.
The battle at the start of the movie Gangs of New York seems to invert this—the Dead Rabbits stop fighting almost instantly.
Nicely subverted in the film version of Prince Caspian, when both sides agree that the entire battle shall come down to a duel between High King Peter of Narnia and King Miraz of Telmar. Peter wins, but there's a Starscream handy to start the battle up anyway. It's okay, though, because that's when a literal Deus ex Machina saves the day.
In Army of Darkness, the skeletons run screaming and the good guys declare victory the instant Semi-Good Ash disposes of Bad Ash.
Averted in Black Hawk Down; when Mike Durant is captured, a militia man explains to him that even if General Aidid is captured, they will not suddenly put down their arms and adopt democracy.
Inglourious Basterds revolves around several plots to kill Hitler, Goebbels, Goering, and Bormann. The film heavily implies that fulfilling this condition will win the war for the Allies. (Not that it would have worked out so neatly in Real Life...)
The 2010 Alice in Wonderland subverts this trope in favor of an Instant-Win Condition; it's not the Red Queen that Alice has to kill, it's the Jabberwock. The massive, otherwise invulnerable dragon was the only reason anyone obeyed her, and her entire army rebelled the second it was dead.
In the movie Prince Valiant the titular hero fights with the throne usurper while the palace guards fight his supporters. Once he kills the usurper, guards stop fighting and swear allegiance to him. It Makes Sense in Context, but not fully.
Subverted in Sahara. The heroes rationalize that if they take out Kasiim, his army should surrender. When they succeed, both immediately note that it should never have worked. Then the camera pans out to reveal that the rebel army showed up and surrounded Kasiim's army while they weren't paying attention.
The Movie of V for Vendetta has shades of this: V's plot to take down the government hinges on the assumption that the rank-and-file will become ineffective after he kills off the head of the government... and kills off all second-in-commands, and blows up a building for good measure. Not so in the original comic book, where V not only killed off the heads of government, he took over, then destroyed, their primary means of communication. No leaders, no way to keep order, and a whole lot of pissed-off people being told by the One-Man Army that did it all that they can now either choose true freedom or doom themselves again.
In the Marvel Cinematic Universe, HYDRA is an aversion, as befits an organization named after the Hydra Problem. After the Red Skull was killed(?), several HYDRA agents were recruited into SHIELD and re-formed the group under their enemies' noses. Seventy years later when this was discovered by Captain America, he and his allies couldn't just kill the leader but had to dismantle SHIELD entirely in order to cripple HYDRA's activities. And even then, multiple splinter cells remain like those led by Baron Von Strucker and John Garrett.
Justified in In The Name Of The King, where the death of Gallian causes all the Krug to stop, turn around, and wander off, even though they were winning the battle. The Krug are actually mindless beasts, who were only turned into an army by Gallian's sorcery. After his death, they're beasts again.
Averted with King Konreid's death, but only because an Unexpected Successor is found. In fact, the trope almost worked because Konreid's nephew Duke Fallow (who is in league with Gallian) would have otherwise taken the throne and ended the fight against Gallian (all the generals are sword to obey the king, whoever it is).
The ultimate example would be the destruction of Sauron in J. R. R. Tolkien's The Lord of the Rings. Not only did his death end the war, but the frontal attack on his forces by Gandalf, Aragorn, and the others was just a diversion so Frodo could kill him. Saruman does cause trouble after his death (this was left out of the movie) but he certainly isn't a threat to all Middle-Earth, just the Shire.
A popular joke mentions that Aragorn granted the king's forgiveness to all of Sauron's forces, since fighting them would be suicide.
In The Movie, Sauron helpfully knocks out his entire army with a pressure wave from his explosive death. It also has the after-effect of causing a massive earthquake that shatters the ground, collapsing his land in a neat semi-circle around the heroes and killing off about two-thirds of his army (the rest ran like hell).
And note that this is averted in The Hobbit, where the Orc/Warg army at the Battle of Five Armies is certainly somewhat discomforted when Beorn manages to kill their leader Bolg, but does not immediately crumble. This is because those Orcs were acting under their own will and not that of Sauron. This is also emphasised in the early chapters of The Two Towers, when other Northern Orcs do not get along with the Mordor-orcs under Sauron's direct control (and the Uruk-hai under Saruman's control don't get along with either side).
In The Silmarillion, the Dwarven contigent of the Union of Maedhros abruptly leaves the battle after their King is killed injuring The Dragon.
That is a downplayed example. They make a fighting retreat and the wounding of Glaurang impresses Morgoth's army so much that no one dares interfere.
Harry Potter partially has this. When Voldemort was defeated the first time, his organisation immediately crumbled. However, it was an Oddly Small Organization and it is implied that there was a lot of chaos, what with arresting Death Eaters and figuring out who was forced into it or enchanted or what. Also, Voldemort had been on his own and operating under his own agenda, so his supporters would most likely have been separated and taken by surprise. The end of Deathly Hallows averts this, as pretty much every Death Eater was either dead, incapacitated or had done a Heel-Face Turn by the time Voldemort dies.
Voldemort tries the trope himself late in Deathly Hallows, where he thinks "killing" Harry will stop the Hogwarts rebellion. It doesn't work, even well before Harry reveals that he's alive.
The Redwall series. Pick a book, any book. Somewhat justified in that the mooks of choice, the rats, are uniformly idiotic to the point of being comic relief save for the occasional brutal, charismatic leader who turns them into a threat.
Subverted in the Discworld novel Small Gods. Big Bad Vorbis, leader of The Empire (to whom the protagonists belong), starts a war with pretty much every other nation on the planet. He gets killed by a turtle (it's a long story) and the hero brings his body to the battlefield to try and convince the other nations not to attack. Their response? They've put too much effort into preparing for war to stop and not attack. After all, they've come all this way. So, long story short, they attack anyway. The overall message is that war is bigger than any one man, even if that one man was mostly responsible for all the crap that happened in the first place. (Luckily the Great God Om bullies everyone else's gods into telling them to call the war off, and that gets their attention.)
In Jingo, Vimes encounters a desert tribe and attempts to capture their leader to force surrender... which makes them charge even faster, since they believe that any leader stupid enough to be captured so easily isn't worth following.
Explored in Night Watch. Vimes and Carcer are whisked back out of time, leaving their armies behind. In the words of Havelock Vetinari, who was there and caught up the long way around, Carcer's men quailed when he was gone. Vimes's men? They tore the enemy apart when they saw he was down.
In the Warhammer 40,000Blood Angels novel Deus Sanguinius, when Rafen proclaims Arkio's death and shows them the body it doesn't end the battle, but the Blood Angels who had fought on Arkio's side switch to supporting their brothers.
One of Stephen Lawhead's books in the Pendragon Cycle averted this in regards to the Saecsens. The narrator mentions that killing the leader is a bad idea, as his subordinates will then fight to the last man to avenge him. Capturing him alive, however, kills their will to fight.
In the Warhammer 40,000Ultramarines novel Dead Sky, Black Sun, Honsou's killing one of the opposing generals gives him the chance to talk. He has to point out that they have fought and died while the other general lurked behind to win his followers' support.
Justified at the end of Eragon: Durza tricked and mentally enslaved the Urgals, which was why they were fighting. In reality, the Urgals were not particularly good at fighting together when not mind-controlled, because Durza had enslaved Urgals from a number of different clans, a good number of which were at war or otherwise on bad terms with the others. So when Durza dies they are freed, and promptly panic or turn on each other.
In Wicked, immediately after Nessarose is killed, apparently all of munchkinland seemed to be driven into total disarray, and pure chaos in the streets.
Subverted in the Ciaphas Cain novel Duty Calls. The antagonist of the book is a Chaos Warmaster with an army of encharmed slaves. Cain tricks him into a duel and kills him. He then half-hopes that this will cause all the enslaved people to come to their senses but in his heart he knows that "this only happens in fairy tales". And indeed Chaos forces are just enraged by the loss of their idol and attack ferociously.
Played straight earlier in Death or Glory, where Cain killing the Ork Warboss Korbul causes the Orks to fracture into competing warbands and scatter within hours.
Though, as pointed out further down in the Table Top section, when the old Ork Warlord dies the Orks often begin to fight each other in order to establish a new leader.
Nob's that accompanied the warboss tried this, even ignoring the Cain himself. Fortunately, the reinforcements arrived and killed all of them.
It's also mentioned on the good guys side in The Traitor's Hand where it's mentioned that protocol states that the two senior officers of an army can never take the same dropship down during a deployment (unless something was extremely wrong) as the enemy would only need one lucky shot in order to invoke this trope.
"Traitor's Gambit" defies and plays this trope straight at the same time. While talking with enemies Cain himself ridicules the assassination attempt since the chain of command means that if general will be killed the next senior officer will replace him without any loss in combat effectiveness. Amberley notes that while IG task force could remain operational despite general's demise, it surely will inflict heavy blow on the troops morale. Not to mention the fact that lord general was gifted tactician.
Cain is rather surprised when said lord general claims that Cain's death would have a similar effect. Cain is just a Commissar after all. However, his heroic reputation is so great that his death would crush the troops' morale even more than the general's death.
In Beyond the Black River, the Picts retreat when Zogar Sag dies.
In The Scarlet Citadel, Conan's own troops turn to other strong nobles when rumored that their king is dead. Conan wins them back exactly the same way. The Aquilonians even have a proverb that lampshades this trope:
'The sword that slays the king cuts the cords of the empire.'
In Malevil, this is used as the plan of attack near the end of the novel. The Bigger Bad is marching his army toward the hero's castle, he rules his conscripts with fear, and bad luck has cost him his two best lieutenants. If they can kill him and his last second-in-command then his army should disband. They have to succeed because while he can't take the castle in a single battle, they won't be able to win a prolonged guerrilla war against him.
The Gaunt's Ghosts novel Necropolis ends with Gaunt killing Heritor Asphodel and crippling his forces in the process. Justified by the Zoicans being controlled by a signal he was continually broadcasting.
Julie Sims manages to wound Wallenstein in 1632, causing confusion that leads to his army being defeated. This is a point of Deliberate Values Dissonance as down-timers consider it bad manners to mark leaders whereas up-timers say they are responsible for the war in the first place and should be sought out.
Zig-zagged in Anabasis. When the prince the Greek mercenaries are fighting for is dead the war is over and there is nothing to do but march home. However during the negotiations with the Greek mercenaries the Persian king assassinates the Greek officers. The Greeks simply elect new ones and march home.
Averted in The Elfstones of Shannara. Near the end of the book, Allanon faces down the Dagda Mor, the leader of the invading Demon armies, in a spectacular Wizard Duel and defeats him. However, while the Demons are briefly given pause- none of them is particularly eager to tangle with the guy who killed their leader, who was also the strongest of their race as Demon society works on Asskicking Equals Authority- they are motivated primarily by hatred for all non-Demon life rather than any sort of loyalty to the Dagda Mor, and as such the invasion is fully capable of continuing without him, albeit with less strategy and overall coordination. Only a last minute re-creation of the can from which the Demons escaped- a full chapter later- saves the world.
Justified in Freeglader. The Shryke army that attacks the fleeing Undertowners is a newly hatched flock. Shrykes, especially young ones, tend to go into a frenzyupon tasting blood, becoming practically uncontrollable - the only one who can keep them in line being their Roost-Mother. When Mother Muleclaw the Third is killed, her army, lacking directions and vicious with bloodthirst, turns on itself, all the Shrykes devouring each other.
Downplayed but still played mostly straight in Red Storm Rising during the Soviet invasion of Iceland. A direct missile hit on the command centre didn't stop local forces from fighting, but with nobody giving orders above company and platoon level the defence turned into a disorganised mess that the Russians quickly overcame.
Jaime Lannister sees that he's losing the Battle of the Whispering Wood, and rallies his guard and charges straight for Robb. While this wouldn't have entirely solved the problem, it would have put Brandon, a crippled child, as the current leader of the Starks, significantly reducing their threat. Robb's bodyguards manage to stop him.
While besieging Riverrun, it's suggested that they try to kill Brynden Tully with poisoned arrows, assuming that whoever replaces him won't be so stubborn and will yield the castle. Jaime instead releases their prisoner Edmure Tully. Edmure outranks Brynden, and once he's in charge he yields it.
In the battle for Astapor the Astapori break once their leader goes down. Their leader was a corpse, so you can see their starting position wasn't very strong.
The many tribes that compose wildling army are kept together only by Mance Rayder, and several suggestions are made on ways for the grossly outnumbered Night's Watch to kill him. Ultimately Stannis comes to their aid, defeats routs army, and Mance is taken prisoner.
Live Action TV
In the last episode of Life, Charlie Crews is in a car with Roman and his hired goons, being driven off to an uncertain, but likely unpleasant, fate. In his ultimate Crowning Moment Of Awesome, Charlie takes Roman out with a jab to the throat, and the goons barely react as Roman chokes to death. Then Charlie informs them that whatever hold Roman had over them is gone now, and they can just go their separate ways— and awesomely ends his whole speech with, "Do you guys understand English?" (They apparently do.)
Played with in Kaizoku Sentai Gokaiger; by the end of the series, the Zangyack Empire has lost not only its Emperor, his son and several of their top lieutenants and scientists, but a massive fleet drawn from across the universe. A couple of months after the final battle, the Gokaigers read about the Empire's descent into factionalism and decide their next adventure will be on the Zangyack's home turf.
Invoked and planned by Phil Leotardo in The Sopranos; if the New Jersey mafia loses its leaders, it can then be easily assimilated by the New York one.
Supernatural uses this in the season 7 finale: when the Leviathans' leader Dick Roman is killed by Dean and Castiel, Crowley comments that they won't be a Big Bad-level threat anymore, since Roman's been their leader since basically the dawn of time and losing him will throw them into confusion. The remaining Levis are quickly dispatched by Crowley's own demon army.
Averted during the Normandy invasion. The plane carrying the commander of Easy Company is shot down but despite the chaotic conditions of the airdrop the paratroopers quickly start assembling into makeshift units and soon after Lt Winters assumes command of the company.
The situation is inverted during the battle of Foyles. Winters has been promoted and Lt Dike, the new commander of Easy Company, is incompetent and paralyzed with indecision. This results in the attack bogging down and the soldiers are about to rout when Dike is dismissed from command. With a replacement commander the soldiers rally and take the town.
Played straight in an episode of Burn Notice where Weston asks the smuggler leader he was trying to save at the time what he thinks his men will do when the person cutting their paychecks is killed off.
Averted in Stargate SG-1. The Ori are all killed partway through the Post Script Season, but their followers are unaware and continue the war in the Ori's name, and must be dealt with separately. It is mentioned that the "Flames of Celestis" (the Doci's connection to the Ori) went out when the Ori died and this has the Priors concerned, but they've elected not to share this with the masses.
In Lucha Libre, the easiest way to win a trios or atomicos match is to score the pinfall or submission on the designated captain of that team.
In the 5th Edition of Warhammer, killing the general not only cost your opponent a powerful hero and all the General-related benefits, but actually caused every enemy unit to test for Panic, possibly causing the entire enemy army to run for the hills. Nowadays, this only affects Undead armies, who start to fall apart because the necromancer is no longer animating them and because they have fewer General-related benefits, though killing the enemy general is still a victory condition in certain scenarios.
Warhammer 40,000 uses this more in the background material than in game mechanics:
Orks (and Warhammer's Orcs) are normally in a state of Enemy Civil War until some powerful Warboss gathers them into a WAAAGH!, and if he dies his army tends to dissolve into its component tribes and clans as his would-be heirs compete to take over. This doesn't mean that the Orks will stop fighting their original enemy, though, it just means that enemy is dealing with dozens of competing warbands instead of a united horde.
Followers of Chaos can be even more fractious than Greenskins, so killing their leader may simply result in a brief pause as a new leader asserts themselves, drive them to eat each other (maybe literally), or fight on even harder.
Played mostly straight with the Tau Ethereals, the members of the guiding caste who, it is implied, use a form of Mind Control to make sure Tau society works the way it does. If they die, the rest of the Tau army can undergo a Heroic BSOD and bug out... or, without the Ethereal's guidance, go into an Unstoppable Rage and massacre the enemy with massed pulse rifle fire.
Going straight for the enemy command structure was a trademark tactic of Warmaster Horus Lupercal, Primarch of the Sons of Horus and leader of the Horus Heresy. When the Siege of Terra was dragging on and loyalist reinforcements were getting closer, Horus lowered the shields of his capital ship to tempt the Emperor to try the same approach with a teleportation attack. Ironically, Horus was killed in the duel and his own legion fell victim to this trope, fleeing with his body and leading the retreat.
Averted in a spectacular way when the Ultramarines went after the Alpha Legion's Primarch. After (possibly) killing Alpharius and his top commanders, the Ultramarines found that this in no way impacted the Alpha Legion's performance, and after taking a drubbing to their foe's superior coordination, maneuvering, and "dishonorable" tactics, the Ultramarines retreated and resorted to an orbital bombardment.
Narrowly averted, then played straight in the Old World of Darkness. In the Demon The Fallen back-story, had Michael defeated Lucifer before the War of Wrath began, the Rebellion would have been stopped. He couldn't. Much later, when the Fallen were cast into the Abyss, God made damn sure Lucifer wasn't with them, going as far as to let him go free (even if depowered), because in his presence, other Demons would have endured ANY imprisonment.
The almost universally known example is, of course, Chess. If you can checkmate the enemy king, it doesn't matter how horribly outnumbered your pieces are or what strong position your enemy has achieved, they simply lose.
Macbeth. Although the battle itself is happening offstage and we are led to believe that it's going poorly for Macbeth's loyalists, once Macduff emerges from the castle with Macbeth's head on a pike the fighting stops. Everybody immediately hails Prince Malcolm as the new King of Scotland.
Also happens earlier, where Macbeth and Banquo defeat Macdonwald's forces by killing him and displaying his head on a pike. In this case, Macdonwald's army is formed mostly of mercenaries, who realize that since their payer is dead, they have no more reason to fight.
Battle for Wesnoth turns this into a game element. Each force is led by a commander, whose death means defeat no matter how many minions he has left. These units can still act if there are allied leaders however, but defeating all leaders is the most common way to win a scenario.
One of the PVP battlegrounds in World of Warcraft allows an instant win for killing the other faction's NPC leader.
In the classic dungeon Dire Maul, after the party kills King Gordok all the surviving ogres declare them the new rulers of Dire Maul. If the party does so without killing the other bosses, they will get extra rewards in the tribute chest that they receive.
Killing some raid bosses causes the trash near them to disappear until the instance resets next week. Many bosses' adds disappear when they're defeated, but there are some subversions, such as Sartharion (whose adds often kill the players after they defeat him with the drakes still alive), Herod (who causes an army of very weak Scarlet Trainees to rush the party after he dies), and Instructor Razuvious (whose Understudies are still alive, but get a damage increasing debuff that enables you to kill them very quickly).
Wrath of the Lich King is a subversion and a justified use of this trope. When Arthas Menethil is killed, the heroes learn that without a Lich King the undead Scourge will run out of control, becoming more dangerous than before. Bolvar Fordragon then takes up the Helm of Domination to become the new Lich King and order the Scourge to stand down.
Total Annihilation and Supreme Commander have similar conditions as the standard, with the added caveat that a destroyed Commander/ACU goes up like a nuclear bomb, typically taking out the rest of their army/base anyway. This is justified by the fact that they're the literal brains of the operation, and serve as the Avatar for the player. Every other unit is just a machine controlled by the commander, and have no direction upon his death. Skirmish and multiplayer games can be set to allow the player to continue if they still have other forces remaining.
In Total Annihilation, this sometimes leads to 'combombing', where a player will deliberately position their commander in the centre of an opponent's base- typically early in a game, when the resulting explosion will do more damage than anything else currently available could, wiping out most or all of the opponent's forces and infrastructure. Naturally, this only works if the option to fight on is enabled, otherwise this will result in either a Mutual Kill or an instant loss.
On the other end of the morality spectrum was the other version of 'combombing', which involved flying in a rapidly built transport plane to pick up the enemy commander, and then self-destruct the plane. Units without hilariously massive amounts of health would not survive. If the option not to fight on is enabled, it's an instant win. If it is, the bomber would usually try to position the commander in the most harmful position possible, to win quickly. Naturally, players frowned on this. Fortunately, the AI was just as vulnerable to it.
In the Total War games, killing the enemy general causes his troops to take a morale hit, making it much easier to mop them up. Of course, the same can happen to your general, too. In the Medieval games peasant uprisings are particularly prone to this, being mostly made up of peasants with terrible morale. So long as the general unit isn't made of spearmen, 20 cavalry can rout an army of a thousand.
On the strategic level, killing the faction leader and all of his heirs causes the faction in question to dissolve into leaderless rebels.
Averted to a degree from Empire onwards. Killing generals on the battlefield would not necessarily stop enemies (especially veteran units) from retreating that easily while in Shogun 2, assassinating the daimyo and his heirs only serves to hand power over to said daimyo's wife.
Subverted in Starcraft, twice. First in Protoss Mission 2, "Into the Flames", Tassadar destroys a Cerebrate in hopes it will disable (or at least disorganize) the forces it controls. It does neither. Then in the finale, the Protoss with help from the Terrans go after the Overmind itself, figuring this should render the Zerg swarm helpless. Cue the expansion: Brood War.
Played straight whenever a vital Zerg controller is successfully destroyed; Any Zerg that entity was controlling will go berserk and massacre anything near it, including fellow Zerg. This is a BIG DEAL for the Zerg if said entity was responsible for their entire species. To their credit, both the original Zerg Overmind and later Kerrigan were Genre Savvy enough to create "backup controllers", that would salvage small portions of the swarm in case that happened.
Cleaning up the devastating aftermath of this trope is a major theme in the Heart Of The Swarm expansion.
In several battles in the Shining Force series, killing the resident (minor) boss enemy will instantly cause every other enemy on the field to drop dead. Also applicable to your team in most games, where the death of the leader's character is an automatic total defeat.
An example of this is seen frequently in Beat Em Ups, where if you defeat the final boss of a stage while he still has Mooks on screen, all of them spontaneously and simultaneously die.
The Age of Empires series has the Regicide game mode, where the death of a player's king would cause them to lose, and all their units surrender.
In Mount & Blade and its variants, this is both averted and played straight. Averted when it comes to the enemy leader and played straight when it comes to your army.
Both played straight and averted in Fire Emblem. Some missions end immediately after you defeat the opposing army's commander, while in others the enemy will continue fighting even after you kill the boss until you complete another objective, like wiping out the enemy to a man, surviving for a given number of turns, or occupying the tile the boss was sitting on. It's entirely possible (and annoying) in one of these missions for your lord unit to kill the boss only to be finished off by a bunch ofMooksnext turn.
Warcraft is good with this trope. Played straight in the first where the orcs win after King Llane is murdered by Garona. Invoked by Orgrim Doomhammer in the second when he killed Lothar, but subverted as it only makes the Alliance angrier and the Horde lost the battle. Averted in the third when Arthas killed Mal'Ganis (because he was not the real leader of the Scourge) but played straight with the death of Archimonde.
Llane himself frequently stated that "as long as a strong heart was on the throne, the men's morale would not falter." It's possible that Stormwind would have held if Llane survived.
The RamboLight Gun game uses this in a minor way: whenever there's an "officer" among the enemies, killing him first will startle some of the soldiers on-screen, which will not attack for a few seconds.
Iji subverts this twice. Killing off Elite Krotera doesn't stop the Tasen's dominion as they're on the brink of extinction and they'll fight back at anything to the last man. Near the end, Komato General Tor discusses this trope with Iji as he reminds her he's just one general in their immense army and that if he goes down, another general will give the command to obliterate Earth. And anyway, he already gave the order and only a word from him will stop the final attack. Only beating the crap out of him convinces him to call it off.
Diablo II does this in Act 3. When you kill the high council the Zakarumites will no longer attack you and flee in fear.
Played pretty much straight in Soul Calibur III: Raphael Sorel's story mode starts during the attack of his mansion/castle by disproportionate forces: an army (complete with some catapults and trebuchets) against him, AmySorel and his too-good-to-be-shown-onscreen servants. What do you have to do? Defeat the general, of course!
Played straight out of the book in the Dynasty Warriors series. Killing the enemy commander results in instant victory for your side, even if you and your commander are the only ones still alive.
Simultaneously subverted. When you kill an enemy officer, the soldiers in his unit keep fighting... unless they're too busy running away in fear.
In Resistance: Fall of Man, the deaths of the Angels at the end of the game causes the rest of the Chimera to die off once they lose their psychic contact with their Angel controllers. However, they come back in the sequel after the emergence of a new Angel/human hybrid, Daedalus.
Scarface: The World is Yours handles this in an odd way: you can kill all the gangsters you want, but unless you mow down the leader, the gang in question will be back for more. While killing the leader in of itself does not kill all the rest, it does prevent that gang from returning, and the leader is usually one of the last to spawn anyway.
Also, all of the Tau's units in the game will suffer massive morale damage if their Ethereal is killed.
The Instant-Win Condition for the Tau and Ork strongholds in the Dark Crusade campaign is killing their leader, which causes the army to fracture and retreat. Deliciously subverted with the Orks however, as Gorgutz not only manages to escape but detonates his entire stronghold as he leaves, just because.
This seems to occur at the end of Half-Life, with the death of the Nihilanth ending the threat of its army. The ending of the expansion Opposing Force however shows that it was really a thermonuclear warhead obliterating the entire facility that ended the invasion, the Xen army not able to teleport to any other location on Earth. (The game's relevance to continuity is dubious, but Word of God has confirmed the validity of the ending.)
In Fallout: New Vegas, defeating Legate Lanius either by either killing him or convincing him to retreat will officially win the battle for the NCR (and if the player is fighting for Mr. House or an Independent Vegas, all that's left to do is to take care of General Oliver via the same means). However, this is also simultaneously averted: Lanius is second-in-command of the Legion after Caesar. Regardless of whether or not you're gunning for the Legion, killing Caesar himself has basically no effect on the end battle, although it is stated that Caesar himself is essential to the Legion's survival as a whole, as he's the personality the Legion is built around and without him it will fracture.
After the Master's death in Fallout 1, most of the surviving first-generation Super Mutants emigrated east, settling in Broken Hills, and later Jacobstown. Since they and most other remnants of the Master's Army have no interest in continuing their former mission, this trope is pretty much played straight.
Subverted by the Enclave from Fallout 2 through Fallout 3. In spite of losing at least four major headquarters and having their entire upper echelon of leaders gutted at least twice, the Enclave has managed to regroup and recover from their losses almost every single time. Only time will tell if the Enclave has finally been defeated once and for all, after the destruction of their headquarters at Adams Air Force Base in Broken Steel.
It happens often in Crusader Kings. If you declare war on an enemy king, quickly defeat him and conquer his provinces, you can force him to hand over his titles to you. All of his vassals will then become yours, and any that had taken arms to defend their former liege will cease hostilities. On easier settings, this means that you can conquer the totality (or almost) of the map with relative ease.
Justified in EarthBound after defeating Giygas, in that he was the source of all the enemies' evil thoughts.
For that matter, defeating any Sanctuary guardian causes all the enemies in the area to run away from Ness and his friends.
Played with in Lords of Magic: Defeating an enemy lord removes their faction from the game, and the game ends when only one lord's faction remains. However, if two lords ally together and one is defeated, the surviving lord takes control of the defeated lord's faction, and if a lord dies without allies, their soldiers become neutral wandering bandits and swear to avenge themselves against the lord who defeated them.
In Dragon Age: Origins, when the Archdemon is killed, the Darkspawn all turn and flee. Justified in that he's mindlinked to them: the Darkspawn Chronicles DLC, in which you play a Darkspawn, describes this as a voice in your head telling you to kill.
Lampshaded early in the game when Alistair explains how to end the invasion: "We cut off the snake's head."
Justified in Super Robot Wars Original Generation: after Bian Zoldark is defeated, his troops scatter, but only because he previously gave them the order to do so if he was killed.
Although quite a few of them try to regroup under his second in command to continue their war. Essentially everyone in the force that wasn't in on the plan of submitting to the Federation upon his defeat or those that only agreed thinking they wouldn't lose and changed their minds when they did, kept fighting. And when these guys are defeated a few remenants go into hiding and again try to restart the war in the next game.
Happens in Bastion. On the second level, there's a large windbag guarding the core being supported by many smaller windbags. Defeating the head windbag will cause the others to shift their loyalty towards you... and then spontaneously die.
A Lord of the Rings game for the Game Boy Advance has mechanics which embody this trope. The battlefield is divided into three stripes - the middle and the flanks. All three can have leaders on them - either named characters or generic captains. Those generate command points, which are required to make units act on the battlefield. When a stripe is devoid of those - either because they died or left for another one - only the natural rate of command point generation applies, which happens to be "one point or none at all". So, basically - without a leader, units stand without orders, and the enemy can slaughter them however he pleases.
In Centurion: Defender of Rome the death of a general is followed by the general retreat of a significant part of his army. If the units aren not very brave to begin with then it overlaps with Keystone Army. Also provides an strategical example; if the player loses Italy, taxes are no longer collected in any province.
And if that's not enough, numerous enemy provinces have the enemy general out in front — meaning that he's likely the first unit to fall in battle.
In Killing Floor, defeating the final enemy, the Patriarch, immediately ends the game, even if some of his mooks are still alive.
Once you kill the leader of the Lutadors in Inversion all the rest basically give up. Despite the fact they had basically won the war a month ago but it's probably best not to think too much about it.
In Commander Keen Episode 3: Keen Must Die, Keen faces the Vorticons, a dog-like alien race, who try to do everything to kill Keen. When Keen kills his arch-enemy Mortimer McMire, the vorticons instantly return to their previous peaceful life, as it turns out Mortimer used to be enslaving the whole race with "mind-belts".
In the third level of Scott Pilgrim vs. The World (the club level), a few bouncers usually lead the charge against your characters. Once they go down, the current wave of mooks panic and book it instantly.
In Splinter Cell Pandora Tomorrow, the capture of Darah Dan Doa leader Suhadi Sadono effectively ends the conflict in East Timor.
At the end of Raiden Fighters, chances are you've killed the so-called "dictator" controlling the enemy forces. Credits then say they start running away.
Raiden Fighters 2 ends with the same possible result. But the ending credits do say it's important to keep watch of the enemy. Cue Raiden Fighters Jet.
Played straight and then subverted in Homeworld; gameplay-wise, destroying the enemy flagship in the final level is an Instant-Win Condition, justified by the fact that said flagship contains the almost-literal God Emperor of the enemy faction and morale would almost certainly collapse immediately. (Though in practice the flagship is so far from the action that you won't get near it until the enemy have already been pounded flat.) Ambiguously canon semi-sequel Cataclysm then goes on to explore what happened next: A Succession Crisis deteriorated into full-blown Enemy Civil War and a few vengeful hardliners are still mounting raids on the titular Homeworld fifteen years later.
This trope forms a key game mechanic in the Langrisser series. Killing an enemy leader will cause the subordinate units in the squad to die automatically. Thus, taking out the leader first removes the threat from the map but limits the possible experience the player can gain, while killing each individual unit maximizes experience gained but requires more time and resources (since sub units get buffs from their leader's field of influence).
In Metal Gear Solid 3: Snake Eater, after Volgin dies, his troops still come after Snake and EVA and pursue them all the way to the lake. Even after Snake takes out The Boss and has his final confrontation with Ocelot, MiGs chase the WiG with the intention of shooting it down (though they back off due to orders from Kruschev.
Fans! uses this with a certain amount of justification: when the General—a warlord from the future whose cadre of troops had a loyalty bordering on worship—finally dies, Kath (who earlier mentioned that she'd "read enough fantasy to know how gods die") immediately called for the troops to lay down their arms. Though some try to fight on, the combination of watching their godlike leader die and Kath's assumption of authority cows them.
Rikk's troops in the relaunch have this problem too, being too dependent on him. When he's knocked unconscious in a fight, his team falls apart.
In The Order of the Stick, after Roy throws Xykon into the Gate, his remaining goblin underlings attempt to surrender (giving the reason that because Xykon's dead, no one's paying them, though "Start of Darkness" provides some alternate possibilities), but Belkar kills them anyway.
Start of Darkness contains an interesting subversion. A charismatic goblin known as The Dark One has united all the monstrous humanoids, so the humans fake a peace talk and kill him, hoping his horde falls apart. Instead, his army goes berserk and embarks on a year-long killing spree of everyone and everything they come across. The resulting slaughter is enough to cause the Dark One to ascend to posthumous godhood via Gods Need Prayer Badly.
The Grand Finale of Avatar: The Last Airbender is close to being a subversion, but fails on certain points. Fire Lord Ozai gets permanently depowered by the Avatar while at his strongest thanks to Sozin's Comet and his entire airship fleet destroyed, Azula gets dethroned before she can even be officially crowned, and the entire Fire Nation garrison at Ba Sing Se is defeated. However, the Fire Nation still effectively controls a major portion of the world, and should have a massive amount of reinforcements. Instead, the loss of their leadership and a major settlement is enough to make them all fall in line when Zuko (who shouldn't even be a legitimate heir) claims the throne. However in the comic continuation, Avatar: The Last Airbender - The Promise a few fractions of of the Fire Nation are still loyal to Ozai and tried to assassinate Zuko or continue the fighting.
In The Legend of Korra, after the defeat of the leadership of the Equalists and Amon being exposed as a bloodbender, the Equalists are no where to be seen in Book 2. Word of God states that there is a holdout, but they are of no threat and will not appear in the series.
In Book 2, the Northern Water Tribe army withraws from the South after Unalaq is killed. Possibly justified by the fact that Eska and Desna, who by the end of the season had enough of their father's abuse, ordered them to, as the heirs to the tribal chief position.
In the final episode of Superman: The Animated Series, Superman manages to defeat Darkseid, and throws his battered (but still living) body from the top of his battlements to the slaves below in a suitably dramatic manner, telling them that he is finished and that they can do as they wish with him. The trope is then horrifically subverted, as the slaves immediately help him to his feet and carry him into the castle again to recover, to Superman's horror.
Darkseid: I am many things, Kal-El... But here, I am god.
In Justice League, Darkseid is killed for good, and in the two-part finale, it's revealed that his forces remained fanatically loyal, but now splintered and fought each other over his legacy. That is until Lex Luthor accidentally resurrects him and Darkseid pulls the fragments back together to invade Earth.
Lampshaded at the end of the second Robot Chicken Star Wars special. A commander orders for all ships to converge on the rebels and open fire, only to be told by his 2nd in command that the Rebels have won because they blew up the 2nd Death Star AND killed the Emperor.
Ancient armies often did behave this way. Many of Alexander the Great's victories were based on having his Phalangial infantry hold off the numerically superior enemy while Alexander and his Companion horsemen cut their way through to the enemy leader and either killed him or forced him to flee the field, causing the collapse of enemy resistance. This was the tactic that won the day at the Battle of Issus and led to the collapse of the Persian empire.
There were a few cases where this backfired, notably at the battle of Chalons, where Attila's army was having a successful go at fighting the Roman's allies, the Goths, until the old king Theodoric the Goth fell off his horse and died, upon which a cry of "Avenge the King!" went out amongst the Goths, who proceeded to drive off the Huns.
The U.S government has been guilty of this, killing off high-ranking members of drug cartels and terrorist organizations again, and again, and again, and again, and again, and again, and again. The success rate of these incidents has varied; sometimes it does the job, while in others it gives the group an extra incentive to hate America.
This is a problem with such groups that have a large following of devotees. If you cut off the head, this will only strengthen the devotees because that person will be seen as a martyr.
The US and other modern militaries also subvert it, with a very particular chain of command. Taking out a general just means that command devolves upon the next guy in line who, if the general did his job correctly, knows all the same information. Taking out several links in the chain of command at once, such as by a lucky artillery hit on a command post, is another matter however.
Primate research ran across a case like this: an exceptional individual led his troop on a successful campaign of expanding the troop's territory against all the other troops nearby. Removal of said individual from his troop and territorial boundaries returned to their prior state. They returned him to his troop and the "conquest" started all over again.
While there wasn't a death of the head of the military for Japan (at least, in a military operation), the war continued on for many a Japanese soldier because they refused to believe Japan would surrender. It took one person nearly 35 years to stand down because he was waiting for orders from his CO.
In the lesser version of war: any time the best player of a sports team is injured/leaves/retires, it'll be tough for the team to win without him. Which is why it's usual to have a defender following the standout (and possibly injure him out of the game...).
The Prussian Army came very close to this trope in the battle of Auerstedt (14 October, 1806) when its commander, the Duke of Brunswick, was incapacitated by a (as it turned out, mortal) wound. King Frederick William III, who was present on the field, then failed to appoint a new commander-in-chief or to try to take command himself. The resultant loss of co-ordination between the various parts of the army (two-fifths of which were not even brought into action until the retreat already started) enabled Davout's outnumbered corps to score a remarkable victory.
When in late 1813 some Allied roving corps attacked French-held Bremen, they could not make any impression on the city's fortifications. Then a rifleman belonging to the Lützow Free Corps managed to kill the French commandant with what even contemporaries described as a lucky shot. Shortly afterwards the garrison, which consisted largely of Swiss soldiers, capitulated.
On February 22, 2002, Angolan rebel leader Jonas Savimbi was killed in battle by government forces. Three weeks later, his army, the National Union for the Total Independence of Angola (UNITA) had signed a ceasefire with the ruling Popular Movement for the Liberation of Angola (MPLA), and by August had disarmed and disbanded its military forces, bringing the Angolan Civil War to an end after 27 years.
A political example: the 1968 campaign of Robert F. Kennedy was cut short by his assassination. He remains the only presidential candidate to have been killed while on campaign, and after his death, the U.S. Secret Service's mandate was extended to protect presidential candidates.
Causing this was the whole idea of blitzkrieg: fast-moving tank columns disrupt supplies and command and control, leaving the enemy army leaderless.