"Death to the Scourge! And death to the living!"The heroes have been cornered by the unstoppable hordes of evil, with nothing left to do but hunker down and make the last of their remaining ammo count, when suddenly they hear approaching hoofbeats. They scarcely dare to look - but yes, that is The Cavalry approaching, with the bad guys falling back in their path. A Red Shirt leaps to his feet, waving and shouting for help - and is promptly run through with a sabre. Basically any instance in which apparent rescuers turn on the heroes - due to either having a secret agenda or mistaking the protagonists for bad guys. As a subverted expectations trope, most examples will be spoilers. Subtrope of Hope Spot and Friend or Foe. Compare with: The Cavalry, Changed My Mind, Kid, Betrayal by Inaction, Big Damn Heroes and Always a Bigger Fish.
— Grand Apothecary Putress destroying a united Horde and Alliance force at the Wrathgate, World of Warcraft
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Anime and Manga
- Read or Die It's already been suggested that the British Library has a secret agenda, but the full extent of it isn't clear till they show up in force.
- In the first episode of K, our hapless protagonist, Yashiro Isana, is being chased by people who want to kill him-and he has no idea why. He is saved by a total stranger, Kuroh Yatogami who just wants to kill Yashiro himself.
- A rare example of this happening to the antagonists shows up in Mobile Suit Gundam SEED Destiny when Chairman Durandal tries to reinforce his game-ending super-weapon by bring up the Home-guard forces. However, the Calvery in this case happened to be lead by veterans of the previous war who had had enough of Extremists and had a rapport with the protagonists. A few minutes after entering the battle they switched sides, and took a good chunk their forces with them.
- Ninja Scroll. Despite what he said earlier about the wisdom of remaining neutral, the lord of the Mochizuki clan turns up with his forces to help the protagonists take on the Devils of Kimon. But when Kagerou reports to her lord, he runs her through with his katana, as he's actually the Big Bad in disguise, and has been all along.
- In Code Geass: Tales of an Alternate Shogunate, this happens to the villains; Suzaku and Euphemia's other forces have realized that Perry is trying to make Japan his own property, and as such, instead of helping him, fight alongside Lelouch and the Black Knights against him.
- Hellsing Ultimate Shortly after a battalion of Nazi vampires attacks London an army of some 3,000+ knights in helicopters and gunships were deployed by Section XIII Iscariot. The arrival of this force caught the Millennium forces initially off-guard and relieved the surviving population of London and they were greeted as saviours until an order came to eliminate civilians as well. Then they started indiscriminately mowing down everyone, human and vampire alike.
- In Saint Beast, Goh, Shin, Rey and Gai turn against Luca and Judas at the moment they are most needed due to brainwashing, which isn't lifted until it's too late. Then, after Judas and Luca are banished to hell the others plot to free them with the help of the Goddess. The Goddess, however, is plain old sick of fighting and sells them out to Zeus. They get Taken for Granite and have their souls sealed away in darkness. And Judas and Luca are still stuck in hell.
- Baka and Test: Summon the Beasts gives us the "FFF". On the one hand, they'll help out in summon wars just as much as the next student in the class. On the other hand, if they so much as hear rumors that a girl's been even relatively nice to you...
- In One Piece, during the Arlong arc, after being under Arlong's thumb for eight years and all Marines attempts to save them had been blockaded by the Arlong Pirates, Cocoyashi Village finally had marines reach the island...only for them to be there to confiscate Nami's money, preventing her from buying the village from Arlong.
- In Thunderbolts, right after Baron Zemo revealed they were the Masters of Evil, most of the team revolted. They were getting their heads handed to them when the Avengers and the Fantastic Four broke in; one even expressed the view that she never thought she would be grateful to see them.
Then Zemo revealed they were mind-controlled.
- Usagi Yojimbo: Happens to Lord Mifume, which leads to his death, and to Lord Noriyuki's father.
- The Gurkhas do this to Sasha in the Kim Possible fic Nights In The Big City.
- In the Lyrical Nanoha fanfic Rise of the Phoenixes, Jane Smythe does this to Miles Fairbrass and Harry Henriksen.
- In The Cries Of Haruhi Suzumiya, Nagato, in a Knight Templar-ish act to try and improve the situation, does this to Kyon.
- In the Lilo & Stitch/Star Trek fanfic Starlight, Jumba initially thinks Experiment 628 has come to save them, until he reveals his allegiance to the Borg.
- In The Swarm of War, the Space Marines, twice. First, the ones woken up as a Superweapon Surprise turn out to be largely corrupted by Chaos. Later, during a battle between the King’s Army and the Chaos, forces led by the Loyalist Space Marines attack and wipe out the Chaos… and then the other side; due to the King refusing to recognize the Space Marines as his superiors, his kingdom is branded as heretics.
- In the 'movie-within-a-fanfic' of Movie Night At Freddy's, Foxy is betrayed by his own crew when a thunderstorm threatens their ship.
- In Braveheart, William Wallace calls on his cavalry to help out the hard pressed infantry at the Battle of Falkirk... but Manipulative Bastard Longshanks had bought them off, thus securing an English victory.
- In A Bridge Too Far This happens to the British paras at Arnhem as they hear approaching tanks and assume they are the British armored units that are coming to relieve them. It turns out it is a German armored division.
- In Dead in Tombstone, Judah Clark arrives at the mine with a posse of armed men wearing badges. However, instead of being their to save Guerrero and Calathea, he is there to collect the gold that the Blackwater Gang owes him.
- In Die Hard 2, the Special Forces unit called in to deal with the hostage situation turns out to be working with the terrorists.
- When Dredd calls for backup at Peach Trees, Ma-Ma calls in support of her own in the form of some bought off Judges.
- Almost occurs in Jarhead when the Marines call in A-10 ground attack aircraft to provide support during a battle, only for the A-10's to mistake them for the enemy and nearly kill them all with the bombing run.
- Judge Dredd. After a shuttle carrying prisoners to Aspen Penal colony crashes, a Capture Team is sent to locate Dredd, a convict on the flight. The Capture Team Leader reports to Chief Justice Griffin.
Leader: There's no sign of Dredd. He appears to have survived the crash.Griffin: You are in error, Capture Team. No one survived the shuttle wreck. Understand? Just find Dredd!Leader: The pilot, sir. He's alive.Griffin: No one survived the shuttle wreck! Do I make myself clear?Leader: Yes, sir. [Shoots the pilot to death]
- A similar case at the beginning of Tomorrow Never Dies, where the Big Bad has the survivors of the British frigate he sank machine-gunned while they paddled up to his ship hoping for rescue.
- The end of the original Night of the Living Dead (1968) where the black survivor thinks he is rescued only to be shot by the redneck posse that mistakes him for a zombie.
- In Red State, three local boys are taken prisoner by the Church to be sacrificed, and the ATF shows up outside to arrest them. When a local sheriff accidentally shoots one of the hostages, the ATF command orders everyone killed.
- In Sharpes Battle, the seventh movie in a historical action series starring Sean Bean, this happens to the youngest member of the riflemen, a character who had been around since the first movie. Why couldn't they have used a red shirt?!
- In Sin City, when the Federal Agents arrive at the farm, Lucille naturally assumes that they are there to help and yells at them to not arrest Marv, as he's with her. And then they pour boxes of bullets into poor Lucille.
- An inversion in Star Trek: Nemesis. The Enterprise is barely holding its own against Shinzon's flagship, when suddenly they detect two more Romulan warships approaching. At first it looks like the situation's going From Bad to Worse, but it turns out that they're from a faction of the armed forces which remains loyal to the legitimate government that Shinzon's coup d'etat wiped out. They get their heads handed to them quite quickly, but the thought was there.
- Another inversion in Taras Bulba (1962). The Cossack forces arrive in support of the Poles, who are losing in a battle against the Turkish forces. It turns out that the Poles were merely holding back so that they could treacherously attack the Cossacks after they won the battle for them.
- The Jean-Claude Van Damme film Second in Command ends this way. The Moldovan armored division that arrived to relieve the US embassy is working for the Big Bad; they end up getting asses kicked by Marine helicopters.
- In Fight Club, once the Narrator realizes the full extent of Project Mayhem's plans, he goes straight to the police to tell them everything... only to realize the officers he's talking to are part of Fight Club.
- Happened in Ready to Rumble when Jimmy King got betrayed by his own faction "The King's Men".
- In Zulu, the defenders of Rorkes' Drift are at first relieved when a cavalry unit pulls up which had evaded destruction by the Zulu Army. But its commander point-blank refuses to augment the defense, claims there is no hope there, and continues retreating to the west, leaving the lone garrison high and dry. worst: their broken retreat spooks a contingent of alrready demoralised native troops into deserting and running, leaving the defence worse off than before.
- On Eraser, Kruger takes part in a rescue mission to save a compromised witness but it turns out his team is dirty. The witness and her protectors are killed and he is framed for it.
- On Last of the Mohicans, Colonel Munro finds out General Webb's army is at Fort Edward just two days march from him. Webb flat out refuses to help and advises surrender to the French.
- In The Lone Ranger, the army guy does a Face–Heel Turn after finding out he massacred innocent Indians.
"These men work for the railroad. So the question is... (points his gun at the Ranger) who the hell are you?"
- The infamous Order 66 in Revenge of the Sith.
- Suicide Squad: A helicopter arrives to extract Waller and the Squad from Midway City. However, the chopper fails to responds to hails from the Squad and then opens fire on them; having been hijacked by the Joker and his gang.
- In Lord Of The Rings: The Two Towers, it was at first believed that this was the fate of Merry and Pippin when the Urk-hai who had captured them were attacked by the Rohirrim army; the army leader telling Aragorn and the others they left no survivors. Fortunately, Aragorn found signs on the battlefield that Merry and Pippin managed to escape in the chaos.
- A Song of Ice and Fire pulls one of these, when the "reinforcements" turn out to be invaders.
- Tywin Lannister "helped" win Robert's Rebellion when his army sacked the capital instead of defending it for House Targaryen.
- Most infamously at the end of A Game of Thrones when the Goldcloaks turn against Ned Stark.
- And it happens twice in quick succession in A Clash of Kings, when Ser Rodrik's forces are betrayed by Ramsay Bolton's forces while they are about to launch an attack to retake Winterfell— and then mere pages later when Ramsay the Bastard of Bolton betrays Theon Greyjoy, the man who took Winterfell in the first place, after he opened the gates to his "allies."
- During the Red Wedding in A Storm of Swords, Roose Bolton and his men storm into the room. Catelyn thinks they're there to help her and Robb: however, Bolton is really in league with the Freys, cuts down his fellow northmen, and stabs Robb in the heart.
- Lloyd Alexander's Chronicles of Prydain: In The High King, a powerful ally, who should be the reinforcements the good guys need against the army of the local Evil Overlord, turns out to be a traitor allied with said Overlord. Perhaps a mild subversion in that he pretty much rode into town and announced this a day before the big battle.
- The Deluge (Polish Potop) when Prince Radziwil joins the Swedish, the Polish commanders who refused to join the treachery are imprisoned. There is a battle when their units try to break them out. Finally, the literal cavalry (the protagonist's unit) arrives... except he had unwittingly made a pledge on his soul to serve the prince, just in time to get himself stuck on the wrong side.
- Inverted in J. R. R. Tolkien's The Lord of the Rings when the Black Fleet of Umbar, allies of Sauron, arrive at the Battle of the Pelennor Fields to the rejoicing of his armies and the despair of the forces of Rohan and Gondor. Only Aragorn had intercepted and captured it, and the ships are full of soldiers from southern Gondor.
- In Graham McNeill's Warhammer 40,000 Horus Heresy novel Fulgrim, the loyalist Marines on Isstvan V do not realize that only three of the Legions going to confront Horus are loyal; their other "allies" turn on them as recounted below.
- Jack Chalker's Lilith: A Snake in the Grass features the sudden arrival of troops supposed to aid the witches in their attack on the protagonist's enemy, only for both defenders and cavalry to turn on the witches, the protagonist realizing he'd been an Unwitting Pawn in a plan to get rid of the witches.
- In Star Wars Legacy of the Force, series villain Jacen Solo has his fleet surrounded by Confederation vessels in orbit of Kashyyk. He is just beginning to despair when the supposedly allied Hapan armada shows up. Thinking that victory is in his grasp he contacts the Hapan Queen Mother, his lover who is in command of the fleet, to coordinate with her only to have her give him an ultimatum to surrender.
- Iron Council in China Miéville's Bas-Lag Cycle, sequel to Perdido Street Station and The Scar.
- In The Dresden Files book Changes has a double whammy. In the climactic scene, Martin arrives at a standoff between Susan and the Red King... only to take down Susan, revealing himself as The Mole. Except he does this so Susan will be able to kill him and become a vampire, allowing Harry to wipe out the Red Court.
- Less notable but still important in the same scene was Lea disguising herself as one of Lords of the Outer Night so she could distract the vampires during the double whammy.
- In Robert E. Howard's "The Scarlet Citadel", inverted for Conan the Barbarian; he arrived on his ally's lands with his forces to find the ally arrayed with his enemies.
- Inverted during the Battle of Thull Mardu in The Belgariad, when King Drosta Lek Thunn of the Nadraks ends up clearing a path for the Western forces, betraying his fellow Angaraks, knowing that siding with the Mallorean forces would only mean subjugation, whereas siding with the West might get his fellow Angaraks' boots off of his neck.
- Happens to the bad guys in the second The Heroes of Olympus book. The giant Polybotes is leading an army of monsters against New Rome, and he's pleased to see the Amazons (whom he had earlier tried to bring over to his side) coming to join the battle. Turns out they're here to help the Romans.
- In Animorphs, this is a common role played by the Andalite military: The final book has the Andalites ready to destroy the human race rather than let it become a resource used by the conquering Yeerk Empire. Earlier books had other Andalites with plans was based on the same well-intentioned extremism: Arbat, who was probably just a rogue faction rather than representing the Andalites in general; and Alloran, who developed a biological weapon to eliminate the alien race of Hork-Bajir.
- The Inciting Incident of The Dinosaur Lords is Jaume being ordered to perform this on Karyl, whose forces are being slaughtered by their common enemy. He charges towards Karyl's forces, making it seems as if he's going to rescue him, only to send the blast od hadrosauruses' sonic attack towards him, crippling Karyl's defence and ultimately leading to man's (first) demise.
- In A Tale of Two Cities, soldiers arrive during the siege of the Bastille only to turn their guns against its defenders.
- Shadow Police: In The Severed Streets, Quill is being pursued by the Ripper. Neil Gaiman arrives to pick him up in a car and spirit him away. However, he is actually there to deliver Quill into an ambush.
- The Crimson Shadow: A rare inversion. During a naval battle between the Avonese and Eriadorans, ships come sailing from Baraduine (an island ruled by Avon), making the former celebrate. Instead though they attack the Avonese ships, since it turns out that Baraduine would like to become independent too.
Live Action TV
- Game of Thrones: During the last days of Robert's Rebellion, Tywin Lannister's forces entered King's Landing as allies, then proceeded to sack the city in Robert's name. Varys and Jaime guessed what was about to happen, but their warnings went unheeded.
- In Season 4 of Lost A boat arrives, that the castaways believe is going to rescue them, but in fact brings a team of mercenaries with orders to kill Ben and anyone who gets in their way. Some of them do get rescued anyway, but it doesn't help.
- A similar thing happens at the end of season 1, with several characters getting on a raft and finding a boat which they think will rescue them, but instead it's the Others, who kidnap Walt and torch the raft.
- The priceless Star Trek: Voyager episode "Message in a Bottle". The Prometheus is getting trashed by Romulans when some Starfleet ships arrive, only to attack everybody, since they know the ship had been taken over by Romulans earlier.
- It doesn't help that the EMHs accidentally fire a torpedo at a Defiant-class ship.
- Happens in Person of Interest season 1 in that episode in which Carter finally takes down Elias.
- The first time, she tries to evacuate a crime lord who flips on Elias. Carter is shocked when she walks back outside to see her backup has fled. This also happens to one of the Dons when he realizes his muscle has fled but later shoots at them as they leave.
- When Carter and the Dons hole up in a safehouse, she notices cops surround the building. She recognizes them but realizes they aren't there to help.
- This happens a lot in wrestling, though how often varies on how swerve-happy a given era, region and promotion is.
- Happened at the creation of the (in)famous Alliance, while Chris Jericho & Kane were being beaten down at the hands of the nefarious WCW crew. Then ran in for the save Tazz, Rhyno, Justin Credible (wait a minute)... Raven, The Dudley Boys... holy shit, Tommy Dreamer! Rob Van Dam! Jericho & Kane were promptly pronounced screwed when Paul Heyman left the announce table to proclaim that this invasion had been TAKEN! TO THE! EXTREEEEME!
- WCW Bash at the Beach '96, which, in turn precipitated the biggest Face–Heel Turn in pro wrestling history. After Sting, Randy Savage, and Lex Luger had taken a beating from The Outsiders (Scott Hall and Kevin Nash), Hulk Hogan comes down to the ring, ostensibly to help out Sting and Savage (with Luger having been knocked out)...and then he leg dropped Randy Savage, revealing himself to be the Outsiders' mystery partner and heralding the beginning of the New World Order.
- Matt Hardy running down to ringside with a chair during his brother Jeff's match with Edge at the 2009 Royal Rumble. This one still managed to surprise because Matt had legitimate gripes with Edge and even if you did not know that, Edge had sent Matt packing from Monday Night Raw after stealing his girlfriend (well, Kane stole her first but we digress).
- After BxB Hulk became Open The Freedom Gate Champion, Davey Richards, who lost in the first round of the tournament to decide who would be holding the title, ran out to attack him. The surprise here was that it was Richards' opponent, Yamato, to seemingly came to stop him only to join Davey's attack.
- At ROH SoCal Showdown, Colt Cabana ran out to stop Scotty 2 Hotty from performing his finishing move, the worm, on guest referee Johnny Fairplay, only to give Fairplay the worm himself.
- Joe Bravo and Escobar came out on a February 2015 World Wrestling League show to help Mr. 450 defeat Americas champion Phenomeno BJ. Then they turned on him before Richard Negrín demanded a referee count three on Mr. 450, bringing the Americas Title into the Gentlemen's Club.
- Warhammer 40,000:
- During the Horus Heresy, the Imperium sent seven legions to Isstvan V to crush Horus's insurrection. Three legions formed the first wave and met horrific resistance. When the other four landed, they expected relief and reinforcement; unbeknownst to them, their "reinforcements" were in league with Horus and the three loyalist legions were suddenly caught in a pincer attack between eight traitor legions. Out of hundreds of thousands of loyalist legionaries, only a few thousand escaped the Drop Site Massacre.
- This also often happens when Inquisitorial forces show up to assist. They only tend to be called when the situation has gotten to the point where the only answers are 1) Kill It with Fire and 2) No Witnesses.
- The Grey Knights used to have an Apocalypse formation that invoked this trope. They could only deploy if the enemy had at least one greater daemon or warp rift on the table and if, at any point, there were no longer any Chaos models on the table control of the Grey Knights would transfer to the opposing player and the Grey Knights would immediately begin slaughtering their allies to make sure there were no witnesses (why this couldn't wait until after the battle was over is never satisfactorily explained).
- Due to the cold hard logic they use when devising their battleplans, the Iron Hands Chapter of Adeptus Astartesare more than willing to fire on their allies if they have calculated that this is the most efficient way of defeating the enemy.
- In Half-Life, the US Marines sent in to quell the alien invasion of Black Mesa are also there to contain all information of the outbreak by killing every Black Mesa employee they meet, and that includes the Player Character, Gordon Freeman.
- Some aren't happy about it. Others are. Whatever it is, it gets justified quick when they come to think that Gordon is responsible for the whole mess and a psychopath (the latter due to the fact that Gordon by that point has proven to be a One-Man Army and has been cutting their ranks down quite a bit).
- They get some karmic justice in the add-on Opposing Force, where the Black Ops soldiers deliver them their own Cavalry Betrayal.
- Also performed in They Hunger.
- Happens to Captain Gordon in Disgaea: Hour of Darkness.
- The twist toward the end of Mega Man X: Command Mission.
- The Komato in Iji combine this with Always a Bigger Fish. They're there to kill the Tasen, but you're...well, there.
- In World of Warcraft, the battle of Angrathar contains a genuine example of The Cavalry when Horde wolf-riders charge into battle to help the beleaguered Alliance. But when the Lich King himself emerges, Putress bombards the entire battleground with Blight, Horde, Alliance, and Scourge alike.
- The famous Wham Level of Modern Warfare 2 ends with one of the most brutal cavalry betrayals in fiction: The team holds a cabin against an endless stream of attackers while a huge amount of data is copied on a portable hard drive and most of them are dead when all of the data is secured. The last men make a frantic dash down a hillside without any real cover while under fire from lots of automatic rifles, machine guns, and rocket launchers and just 14 meters from the extraction point the player gets downed. When you regain consciousness a few seconds later, your last teammate Ghost drags you by your jacket and finally helicopters of Shadow Company appear from behind you and mow everything down with their miniguns. And when you finally stumble the last steps to the evac chopper, General Shepherd takes the hard drive and shoots both you and Ghost in the chest. The last thing you see is your bodies being thrown in a ditch, a soldiers emptying a canister of gas over you, and Shepherd throwing his burning cigar at your face. You really didn't see THAT coming.
- Likewise Fire Emblem Genealogy Of Holy War, although the lead-up makes it a bit easier to see it coming. Chapter 5 looks like it's going to be the final battle against all of the antagonists, but rather unexpectedly Velthomer switches sides and starts attacking Freege's army as soon as you get close to Velthomer, double-crossing its allies. All this in spite of the fact that Arvis has now been revealed to be in league with the Loptyr Sect. The chapter ends with Sigurd being led to Belhalla, ostensibly to be cleared of all charges, only to learn that he is still labeled as a traitor and will be killed, along with much of the party (though their fates are mostly left ambiguous).
- Its midquel Thracia 776 features a sort-of Meta example: late into a grueling defence chapter, an army of green (NPC) Thracian Dracoknights arrive and start heading for the city. The player will probably be exclaiming "I'm saved!" at this point, as the Non Player Characters fly in and clear out the enemies. Until they actually reach the city, upon which they start attacking you. Turns out, they're on neither side: they just want the city back under their control, meaning you're just as much of an obstacle to them as the enemy army. Notable in that it's possibly the only time in the entire series where green NPC units can actually attack you.
- There's a Japanese DOS-era tactical RPG called Legend of the Seven Heroes 2 that ends like this. After a Climax Boss battle against the enemy nation's Big Bad General and his army, an army from the friendly nation shows up, and the main character assumes they're there to rescue them. However, the friendly nation's General arranges a truce with the Big Bad General and decides to kill the main characters, because he figures that the main characters' survival would bring peace between the two nations and thus be politically disadvantageous to both of them. The final battle of the game thus has your 7 characters being attacked from the North of the battlefield by the Big Bad General and his army and from the South of the battlefield by the not-so-friendly nation's General and his army.
- Towards the end of Dragon Age: Origins: Awakening you have the option of doing this to the City of Amaranthine. The city has been overwhelmed by darkspawn while Vigil's Keep has been ambushed by an even larger darkspawn army. You have the option to burn the city down with Arrows on Fire, killing all the people and darkspawn that your soldiers have trapped inside, freeing you to save your fortress. Fortunately, you can Take a Third Option and save everyone if you have built up the fort enough that it can stand without you.
- The original game, Dragon Age: Origins, has a "It Was His Sled" moment: when Alistair and the Warden light the beacon at the top of the nearby tower, signalling Loghain's forces to charge, Loghain orders a retreat instead. Cailian and the rest of the Grey Wardens, including Duncan, are overwhelmed. Whether the act constituted a betrayal or not is still debated in- and out-of-universe.
- This happened in the Backstory of one of the towns in Dragon Quest VII. When the monsters first appeared and started terrorizing the town and kidnapping people, one man convinced his neighbors to stand against them. He went ahead to start the fight... and everyone else chickened out. To twist the knife further, this betrayal caused his sister Matilda to hate everyone, allowing the monsters to turn her into a Tragic Monster Barrier Maiden.
- An extremely rare villain version happens in Assassin's Creed: Brotherhood, during Sequence 8, where after destroying all of Cesare Borgia's support, he sees one last chance in his Papal Guard army arriving at Roma's gates, only for them to arrest him on orders of the new Pope.
- After the finale of Left 4 Dead, the characters are rescued by the military in an APC. The subsequent comics show that the army locked them up out of fear that they were carriers and intended to kill them before a horde rush allows them to escape.
- In Ys Seven, after getting the last crest from the Sea Dragon on Ruin Island you go back to your ship only to be attacked by a giant Titano, Just in the nick of time the Dragon Knights show up and Save you. That is only half true however, They are actually here to rescue Aisha and Arrest you for "kidnapping" her and assassinating the King who was murdered mere moments after Adol leaves said King's bedroom.
- The player character pulls this off in the endgame of Fallout: New Vegas if they're fighting for Mr. House or an Independent Vegas. While you and the Securitrons are fighting alongside the NCR for the majority of the battle, you're also secretly trying to activate the Sealed Army in a Can. Once the Legion is routed, you then have to get General Oliver to retreat, either through force or through diplomacy.
- Inverted in Homeworld: during the final battle a large squadron of Taiidan frigates with two destroyers exits from hyperspace, but they then identify themselves as the Taiidan Rebellion coming to support you.
- Also played straight in Cataclysm: at one point the Kiith Manaan carrier Caal-Shto shows up with reinforcements, but when the Raiders you're fighting attack them the Caal-Shto is promptly revealed to have been infected by the Beast, and is now an enemy ship.
- The American soldiers and pilots in Desert Strike, who are stranded behind the enemy lines and waiting for the player to rescue them, aren't exactly immune to friendly bullets.
- Happens in Command & Conquer 3: Tiberium Wars when Killian's reinforcements turn against Nod's Temple Prime. Kane's Wrath reveal that it was actually Alexa who ordered LEGION to pretend to be at the head of Killian's forces.
Kane: "Of course I could not have planned for an ambush BY MY OWN FORCES!"
- In Dead Rising, if the player follows the path to the best ending, the heroes will call for help. What they get, instead, is a spec ops squad come to ensure the truth behind the zombie outbreak remains a secret, and everyone is fair game, living or undead.
- Sonny begins and ends with these. At the start, the ZPCI come in helicopters, apparently to wipe the zombies currently attacking you, only to turn on you instead as they don't want witnesses. Finally, the last battle of the game is against Galiant the Paladin, whose help you just impersonated and who you just helped in taking down Baron Brixus. Veradux reasons he won't accept the truth and turn on them, so he decides to turn on him first and take you along.
- Pokémon Sun and Moon: Wild Pokemon will sometimes call for help if they are losing a battle. Usually this help arrives in the form of another member of their own species, more rarely a Pokemon of another species will show up. And sometimes, their predator will show up, and attack the Pokemon that called for help.
- The Order of the Stick: Done by Tarquin to Zora.
- In The Gamer's Alliance, a regiment of Proninists disguised as Scun Loyalists manage to fool the Maar Sulais army into letting them get closer during the Second Battle of Victoire. They reveal their true colours by attacking the surprised Maar Sulais defenders who then are forced to flee as they're being outnumbered.
- Parodied in the Anniversary Brawl between the cast of That Guy with the Glasses and The Angry Video Game Nerd when Benzaie shows up.
The Nostalgia Critic: Benzaie! Clench your fist and strike a blow for our side!Benzaie: *Punches out an already dazed Linkara*The Nostalgia Critic: ...he's on our side.
- In Worm, Echidna has stalemated Eidolon and her clones are whittling down the combined forces of the Protectorate and the Undersiders. Then the Travelers, the Undersider's allies, show up to help, but Trickster, the leader of of the Travelers, betrays everybody to support Echidna, his Protectorate and ex-girlfriend.
- Inverted in Counter Monkey. During the Thieves' World campaign, the players attacked a cult and the DM expected that they would need help by their employer, Tempus Thales, who came with six guardsman to serve as backup. However, The Cavalry Arrives Late and the party wins the day without assistance. When they heard approaching troops, they assumed it was more cultists and ambushed Tempus as he entered the room, inflicting Facial Horror and driving the campaign Off the Rails in the process.
- Used in a Cliff Hanger in He-Man and the Masters of the Universe (2002): Adam, deprived of his sword and stuck facing down a group of villains led by Skeletor, thinks he sees the Masters on the horizon. They turn out to be Skeletor's minions using the Masters' vehicles.
- In Thundercats 2011 during The Siege of Thundera, trusted General Grune shows up toting a Flare Gun that seems to bring the enemy Lizards' Walking Tank fleet to a halt, only for him to perform an Insignia Rip-Off Ritual and an Evil Costume Switch while debuting his nasty new Lightning Gun, while trusted General Panthro...isn't and King Claudus is stabbed In the Back.
- This was part of Megabyte's grand strategy during the Web World War: he waited for Mainframe's CPU fleet to take heavy casualties before deploying his own forces, and the moment the invasion ended, he immediately directed his troops to destroy the city's remaining defenses. It was this betrayal that ultimately allowed Megabyte to conquer Mainframe.
- The protagonists of Titan Maximum weren't actually happy to see Troy Hammerschmitt and his Titan Megamum, but that's only mostly because he's The Ace while they are a Ragtag Bunch of Misfits with bad publicity. Everyone else is happy to see him. And then Troy hands his super-advanced robot to the Big Bad, because he was angry that his father considered him an Inadequate Inheritor.
- Popular theory holds that the Soviet Red Army pulled such an act during the Warsaw Uprising, when Polish resistance fighters sought to rid the capital of the Nazi General Government and military. Research from Polish historians shows this as a complicated and narrow aversion: the Red Army certainly did not actively fight the Polish resistance in the Uprising (as they had, for example, during the onset of the war when they invaded Poland alongside Germany)—they didn't fight anyone at all, and in all likelihood, were badly spent from edging out bloody victories against the largest German army groups of the war during the liberation of Belarus. The Red Army instead decided to stand aside during the battle (despite some propaganda broadcasts from Moscow suggesting Soviet support was imminent). Soviet tank units were denied fuel for advancing even if they wanted to, and only after repeated pressure from the Western allies did the Red Army offer basic assistance to the Polish Home Army . Soviet leadership, Stalin included, chose to focus on other fronts and take Warsaw afterwards—possibly reflecting the very high level of distrust between the parties involved.
- Both Russia and Germany made extensive use of captured tanks. In North Africa, a tactic Rommel returned to was to spearhead his armour with captured British tanks. Even though captured vehicles had been clearly repainted in German insignia (to conform with international law and to safeguard the crews if captured) tired British troops in forward positions recognised the familiar silhouette and sound of what they took to be their own tanks - and relaxed their guard for just long enough. This trick was also used in Russia, although as the tide turned it came back to bite the Germans. The USSR captured so many German tanks that it created special armoured regiments with them - whose job was to spearhead attacks and deceive the Germans into thinking it was their own tanks approaching them. In other fronts, the USSR deliberately painted its tanks in the same disruptive three-colour camoflage used by Germany. Quite often even this deceived German watchers for long enough, or had them holding fire while trying to ascertain if these were captured Russian tanks being used by German units.
- A naval variant involved the British submarine HMS Morse, that would patrol the approach routes to German submarine bases in France, quite openly on the surface. Tired U-boat crews returning from the Atlantic would see only one of their own - as the Morse was originally a U-boat, captured intact off Iceland. The deception worked right until the morse ran up the White Ensign and fired its torpedos at an unprepared target.
- In an inversion of the above, Hitler also played this role in the areas of Europe the Soviets occupied prior to 1941. Indeed, many guerrilla organizations (such as the Forest Brothers and the UPA) saw him as a potential savior, only to find themselves surrounded by Feldgrau and Panzers gunning for them (though Hitler did allow some anti-Soviet fighters to join his puppet organizations).
- The Battle of Bosworths Field. The Stanleys, nominally allied with King Richard III, waited until he was separated from his army and charged the King's Bodyguard in the rear.
- This was how the Bosnian uprising under Husein-beg Gradaščevićes leadership against the Ottoman Empire was squashed in 1850. After routing the imperial army for more than a decade, the final battle that would decide the fate of Bosnia until this day was being fought in what is today the Stup neighborhood in Sarajevo. After defeating the Ottomans, the exhausted Bosnian rebels were overrun by what were supposed to be their reinforcements from Herzegovina- who thought the country would be better off in the crumbling Ottoman Empire.
- The French did this to their Genovese allies at the Battle of Crecy. The Genovese crossbowmen, forced by their French paymasters to fight without their pavises (huge shields that protected them from the murderous English longbows as they reloaded), were massacred by the English. When they began to retreat, the French knights charged right over them, either because their commanders were angered by the "cowardice" of the Genovese or were simply too impatient to wait for the exhausted Genovese to withdraw. The French knights paid for their arrogance and bad tactics, however, as they were also crushed by the English forces, faring little better than the crossbowmen had.
- Italy in World War I. Before the war they had an alliance with Germany and Austria-Hungary against France, Britain and Russia, but, between the long-standing Italian claim to some Austro-Hungarian territories, the German emperor having done everything in his power to deserve the Italian king's hate and actually having better relations with France and Britain than with their own allies, first refused to enter the war, and when they did it was on the other side.