Cavalry Betrayal

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, Big Damn Heroes and Always a Bigger Fish.

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.
 * 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 9th Crusade lead by Archbishop Maxwell towards the end of Hellsing is first greeted as saviors by the people of London against the forces of Millennium. Then they start 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.

Comics
Then Zemo revealed they were mind-controlled.
 * 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.
 * Usagi Yojimbo: Happens to Lord Mifume, which leads to his death, and to Lord Noriyuki's father.

Film
"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]"
 * The end of the original Night of the Living Dead movie where the black survivor thinks he is rescued only to be shot by the redneck posse that mistakes him for a zombie.
 * Judge Dredd. After a shuttle carrying prisoners to Aspen Penal colony crashes, a rescue/capture team is sent to locate Dredd, a convict on the flight. The Capture Team Leader reports to Judge Griffin.


 * 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 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.
 * There's a bit of a Double Standard in that this is considered a very vile act while early in the battle a large group of Irish from the English army outright switch sides and start fighting the English and it is treated as a great laugh. Of course, that battle has the English King being the Trope Namer for We Have Reserves, but still.
 * The Irish were just mercenary soldiers who didn't even particularly like Longshanks, whom Longshanks openly stated he considered expendable. In contrast, the Scottish cavalry were led by rich Scottish lords who Longshanks bought off, who then betrayed their own subordinates to death. So the Irish betrayed Longshanks because he was a cruel boss and wanted to help the Scottish win their freedom, while the Scottish lords betrayed their own followers for Longshanks' gold.
 * In Die Hard 2, the Special Forces unit called in to deal with the hostage situation turns out to be working with the terrorists.
 * 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.
 * An inversion in Star Trek Nemesis, in which The Cavalry betrays the bad guys and sides with the Enterprise.
 * 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 literally boxes of bullets into poor Lucille.
 * 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 ""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.

Literature
" "Jaime Lannister sends his regards.""
 * A Song of Ice and Fire pulls one of these, when the "reinforcements" turn out to be invaders.
 * It happens a lot, most infamously at the end of Game of Thrones with
 * And it happens twice in quick succession in A Clash of Kings, when
 * During the Red Wedding,


 * Lloyd Alexander's The Prydain Chronicles: the reinforcements turns out to be traitors allied with Arawn.
 * 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 JRR 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. Only Aragorn had intercepted and captured it, and the ships are full of soldiers from southern Gondor.
 * In Graham McNeill's Warhammer 40000 Horus Heresy novel Fulgrim, the loyalist Marines on Isstavan 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, of course, 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 Mieville's Bas-Lag Cycle, sequel to Perdido Street Station and The Scar.
 * In The Dresden Files book Changes
 * In Robert E. Howard's "The Scarlet Citadel", inverted in Conan; he arrived on his ally's lands with his forces to find the ally arrayed with his enemies.

Live Action TV

 * 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.
 * In Season 4 of Lost
 * 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,
 * In the pilot to Firefly, Mal's independents are desperate for air support at the Battle of Serenity. When they hear the air support coming, they start to cheer...until Zoe calmly says that it's not their air support.

Professional Wrestling

 * This happens a lot in wrestling, though how often varies on how swerve-happy a given era and promotion is. A notable recent example was Matt Hardy running down to ringside with a chair during his brother Jeff's match with Edge at the 2009 Royal Rumble. Didn't take a lot to guess that Edge wasn't the one about to get smacked with the weapon.
 * 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 Dudleys... 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.

Tabletop Games

 * Warhammer 40000 turns this trope Up to Eleven. During the Horus Heresy, after the traitorous Warmaster led three Space Marine legions to conquer Istvaan III, seven ostensibly loyal legions of were sent to quash the insurrection. Three legions - the Raven Guard, Salamanders and Iron Hands - dropped to the planets' surface and engaged Horus' forces, driving them back. Or so they thought. In an event forever remembered as the Istvaan Drop Site Massacre, the other four legions - the Night Lords, Iron Warriors, Word Bearers, and Alpha Legion - landed and turned their guns on their brothers. Already weakened by battle and now caught between two huge forces, the three loyalist contingents were all but obliterated. Just as planned.
 * 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.

Videogames

 * 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, including you.
 * Some aren't happy about it. Others are. Whatever it is, it gets justified quick when they think you're responsible for the whole mess and a psychopath.
 * 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, there is a genuine example of The Cavalry during the battle of Angrathar when the Horde helps the Alliance. But then the Lich King himself comes, and Putress shows up...
 * Played straight by the Horde in The Broken Front quest. The Alliance are attacking a Scourge encampment, and the Horde join in...only to completely ignore the Scourge and try to kill Alliance troops.
 * The famous Wham Level of Modern Warfare 2 ends with one of the most brutal cavalry betrayals in fiction: 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
 * 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.
 * A variation occurs in Dragon Age, where the cavalry simple ignore their allies and leave them to die.
 * Towards the end of Awakening
 * 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.
 * An extremely rare villain version happens in Assassin's Creed Brotherhood, during
 * 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..

Web Comics

 * Order of the Stick: Done by to Zora.

Web Original
"The Nostalgia Critic: Benzaie! Clench your fist and strike a blow for our side! Benzaie: *Punches out an already dazed Linkara*
 * In The Gamers 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: ...he's on our side."

Western Animation

 * Used in a Cliff Hanger in the 2002 He Man and The Masters of The Universe cartoon: 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 King Claudus is literally 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.

Real Life

 * This happened at least a couple of times early in The American Civil War, since neither side had a standardized uniform yet, and the approaching blue-uniformed troops turned out to be Confederates.
 * And in another case, the grey-uniformed soldiers the union forces were firing on were union cadets in their grey cadet uniforms.
 * Some accounts of the Battle of Waterloo include the French leadership attempting to rally their forces by claiming the approaching Prussians were French reinforcements. In the event, the French army retreated before the Prussians arrived. No one is quite sure what the French plan was for when the Prussians arrived and their troops discovered they had been misinformed.
 * Make that pretty much all accounts - for instance, Marshal Ney writing to Fouché 8 days after the battle mentioned that it was Napoleon's ADC La Bedoyère who brought him the false news that the troops approaching the French right flank (i. e. the Prussian I Corps led by General Zieten) were Grouchy's Frenchmen. Napoleon gambled that this lie would motivate his troops enough to make the final attack on Wellington's centre a success and achieve a breakthrough while the troops on his right held off Zieten's. But the attack was repulsed, and it was the French centre that now collapsed. However, Zieten's troops did drive into the French right. In the process there also at first were a few attacks on the Nassau troops Wellington's left flank (Papelotte and Fichermont) because the Prussians mistook them for French as they wore French-style uniforms and as it was getting dark it was not easy to tell the Nassauer's dark green from the French dark blue. (Also, at that point Bülow's IV Corps had already been in action for hours, fighting for the village of Plancenoit behind the French centre and right).
 * The Battle of Barnet in the War of the Roses. A cavalry leader took his force charging off on a wild-goose chase, and while they were gone, the two sides shifted positions about ninety degrees. And there was heavy fog.... When the cavalry came back, they mistakenly attacked their own allies' flank. A fictional homage to this battle appears in Lord Kalvan of Otherwhen by H. Beam Piper.
 * The Soviet Union didn't exactly act as the heroic cavalry when the Polish Underground started its ill-fated Warsaw Uprising against the occupying Nazis in 1944, to say the least.
 * Stalin LOVED this trope, as he encouraged the Pro-Western factions in Eastern Europe to rise up when the Red Army advanced, let them bleed themselves out against the Germans until they were destroyed, then kicked the Germans out and set up his own puppets.
 * 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 organization (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 tough 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 duly monstered by the English. When they began to retreat, the French knights charged right over them, because their commanders were too impatient to wait for the Genovese to withdraw.