History Main / KeystoneArmy

* In ''LightNovel/BakaAndTestSummonTheBeasts'', during the summoner test wars, taking out the class rep is an InstantWinCondition. Naturally the students usually try to protect their leader, but sometimes the opposing class will bait them somehow and lure them away so as to weaken the rep's defenses and take them out.

* ''LightNovel/RebuildWorld'': JustifiedTrope due to a PsychicLink which bordered on HiveMind in some regards. The network led by [[spoiler:Katsuya, and consisting of his BattleHarem]] all fall comatose for days instantly after the leader's death due to the shared emotions and knowledge from said link and shock to their nervous systems.

* ''The Fleshmarket Vampire'' features protagonists Danny and Mercy- Mercy being a vampire who has trained Danny to fight other vampires- targeting Gabriel, the head of the nest that is currently hunting Edinburgh. For [[OurVampiresAreDifferent this breed of vampire]], killing a vampire causes all of that vampire's turned victims to become human again, so by killing Gabriel Danny and Mercy destroy most of the Edinburgh nest; the only two survivors are Danny's sister Aurelia and a woman who reverts to her chronological age of 90+, so she is left a senile fugure in an old folk's home.

* ''[[https://www.royalroad.com/fiction/47038/book-of-the-dead Book of the Dead]]'': Tyron tries to learn more about his new Necromancer class, by reading up on historical necromancers, and he's fascinated by the discovery that when Arihnan the Black was killed, his whole tremendous undead army -- skeletons, spellcasters, skeletal flying wyverns, you name it -- immediately collapsed. Meaning that Arihnan had enough personal power to be directly controlling all of them.

*** Once Anakin blows up the Trade Federation's control ship, the droid army shuts down.

** This was subverted in cut content of ''Film/AttackOfTheClones''; the Jedi blow up the droid control ship, shutting the droids down en-mass, [[ItOnlyWorksOnce only for them to all power up again seconds later when their new onboard backup systems boot up.]]
** ''Film/RevengeOfTheSith'': After Anakin kills the Seperatist leaders, he is able to shut down the entire droid army with the push of a button. Of course since Palpatine was behind both sides of the war the entire time, he made sure the droids were built with this flaw so that he could end the war instantly once he had accomplished his goals.

* ''Videogame/{{Tyranny}}:'' One of the great advantages [[EliteMooks the Disfavored]] have in combination with their superior training, equipment and tactics is a strange ability to get up from the most grievous of injuries and keep on fighting, living like the injury never happened. [[spoiler:Graven Ashe, Archon of War and their beloved leader, is the one healing them from this near-automatically by taking on their injuries and pain; he can survive them, even if they've worn him down over time... but if he dies, all those injuries catch up with the Disfavored and most of them just flat-out ''drop dead'', and others are permanently crippled by the injuries' return. Those that remain undergo a massive mental breakdown (to which not even your party member Barik is immune) and end up in no condition to fight; how much of that is just trauma from losing their beloved general and only leader they've ever known, and how much of that is the Aegis' fall taking their sanity with it]].

** ''Anime/DoraemonNobitaAndTheCastleOfTheUnderseaDevil'' have the [[AIIsACrapshoot Mad AI]] Poseidon, leader of theunderwater robot army who wiped out the inhabitants of Atlantis and is targeting the surface world. Once Poseidon is destroyed via Buggy's EatMe HeroicSacrifice, his robots and drones quickly come to a standstill.

* No matter how powerful or plentiful the enemy may be, in ''VideoGame/TheBattleCats'', if you destroy the enemy base, all enemies in the field will die. Be careful though, as the enemies can also do this to you whether it’s through Zombies[[note]] They have an ability which allows them to go under your meatshields and travel a fixed distance unharmed [[/note]] or Long-Distance enemies.[[note]] They, in exchange for [[ArbitraryMinimumRange their weakness]], can hit farther than the range they start attacking at.[[/note]]

** The climax of ''Anime/DoraemonNobitasGreatAdventureIntoTheUnderworld'', where Doraemon and friends, including their new allies Miyoko and Professor Mangetsu, battles the Demon Lord Demaon and his legions. The battle ends when [[AttackOnTheHEart Demaon's heart is pierced]], leading to his entire army's immediate downfall.
** ''Anime/DoraemonNobitaAndTheTinLabyrinth'' features yet another hostile robot race from Planet Chamocha, controlled by their leader Professor Napogisutora whose brain controls literally every robot in his army. Doraemon managed to fire a disc containing a ComputerVirus into Napogisutora's mouth, and then the virus causes the entire unit to malfunction and destroy each other.

* In ''TabletopGame/{{Chess}}'', the king is all that matters. If the king cannot move without being captured, it doesn't matter what the other pieces are doing.
%% * The same goes for the Japanese game of Shogi.

* In every ''VideoGame/DynastyWarriors'' game except for Empires (which has different battlefield mechanics), defeating a general instantly causes his troops' morale to drop permanently to zero and any officers under his command to flee. So if you can get to the generals and put them down (without getting killed, of course), you can quickly swing the battle with a minimum of fighting. There are a number of tasks (particularly in 3) that are nearly impossible to accomplish any other way.

* In the near future mecha chapter of ''VideoGame/LiveALive'', Akira will fight the Crusader, a skull-themed biker gang, several times during the adventure. In battle a Crusader will appear flanked by several underlings, ranging from RC cars to over-sized toy robots. Defeating the crusader will destroy all the enemies as well.

** ''Anime/DoraemonNobitaAndTheCastleOfTheUnderseaDevil'' have the [[AIIsACrapshoot Mad AI]] Poseidon, leader of theunderwater robot army who wiped out the inhabitants of Atlantis and is targeting the surface world. Once Poseidon is destroyed via Buggy's EatMe HeroicSacrifice, his robots and drones quickly comes to a standstill.

* In ''Franchise/StarTrek'', as soon as the bridge of a starship is captured, the entire rest of the ship is implied to surrender. This is {{Handwave}}d by WordOfGod saying that capturing the bridge allows the boarders to use the ship's own anti-intruder measures against the rest of the crew. It's still a bit ridiculous seeing the thousand man crew of the ''Enterprise'' being defeated by three Ferengi.

** ''Film/{{The Avengers|2012}}''. Destroying the mothership causes all the aliens to drop dead. In the DVDCommentary, Creator/JossWhedon admits that he hated having to resort to this trope, [[EnforcedTrope but it was]] [[NecessaryWeasel necessary]] to give the heroes a clear victory instead of battling for seventeen more hours of cleanup.

A Keystone Army is an invasion force or army that's [[NighInvulnerability seemingly unstoppable]], except for [[AttackItsWeakPoint one particular weakness]] [[AchillesHeel in the form of a well protected but very fragile component]]. It can be an individual soldier, an object or whatever, but if you destroy or tamper with it, [[InstantWinCondition the entire army is immediately disabled]]. This plot device is handy as it allows a [[FiveManBand handful of heroes]] to win the war without having to depict them fighting off the entire enemy force. Just a daring raid on the enemy's stronghold and BAM! Evil is defeated. It also allows otherwise unstoppable foes to be suddenly stopped dead in their tracks.

BeePeople are likely to be a Keystone Army if they have a HiveQueen, and as such trashing it is a good way to win the BugWar. Another common Keystone Army simply replaces the insects with {{Killer Robot}}s or GreyGoo and the Hive Mind with a MasterComputer or evil [[AIIsACrapshoot AI]], and yet another popular variation has an EvilSorcerer whose (often [[TheUndead Undead]], [[MindControl Mind Controlled]] or [[LegionsOfHell Demonic]]) minions will cease to be a problem due to NoOntologicalInertia [[ShootTheMageFirst upon his death/defeat/distraction]]. An army of monsters may be defeated by killing the MonsterProgenitor. [[EasilyThwartedAlienInvasion Easily Thwarted Alien Invasions]] often employs such armies. WeCannotGoOnWithoutYou is a case of the player character of a video game being the keystone in their party's army.

* During the UsefulNotes/SpanishConquestOfTheAztecEmpire, following the events of La Noche Triste, the surviving Spaniards and their Tlaxcaltec allies fled north while being pursued by a far larger Aztec army that cornered them in the plains of Otumba with no possibility of escape. Seeing his enemy weak and outnumbered, the Aztec general Matlatzincatzin followed the Mesoamerican custom, spread his forces and ordered them to take as many men alive as possible to sacrifice them back in Tenochtitlan. However, UsefulNotes/HernanCortez knew from previous battles that, as a general rule of Mesoamerican warfare, if he killed the general himself, the Aztecs, now deprived of their leader, would stop fighting and rout. There was only one chance. Cortés surprised the Aztecs by using his last 23 horses in a cavalry charge that he led himself, something that the natives had never seen before (even if they had already seen the horses, they might be still under the impression that horses were only used as pack animals), killed the general and captured his standard. As predicted, the Aztecs broke lines and returned to Tenochtitlan. Cortés then took refuge in Tlaxcala, rebuilt his forces and conquered Tenochtitlan the following year, when the city had just been ravaged by a convenient plague of smallpox (caught from a Spanish slave that got captured during La Noche).

* During the UsefulNotes/SpanishConquestOfTheAztecEmpire, following the events of La Noche Triste, the surviving Spaniards and their Tlaxcaltec allies fled north while being pursued by a far larger Aztec army that cornered them in the plains of Otumba with no possibility of escape. Seeing his enemy weak and outnumbered, the Aztec general Matlatzincatzin followed the Mesoamerican custom, spread his forces and ordered them to take as many men alive as possible to sacrifice them back in Tenochtitlan. However, UsefulNotes/HernanCortez knew from previous battles that, as a general rule Mesoamerican warfare, if he killed the general himself, the Aztecs, now deprived of their leader, would stop fighting and rout. There was only one chance. Cortés surprised the Aztecs by using his last 23 horses in a cavalry charge that he led himself, something that the natives had never seen before (even if they had already seen the horses, they might be still under the impression that horses were only used as pack animals), killed the general and captured his standard. As predicted, the Aztecs broke lines and returned to Tenochtitlan. Cortés then took refuge in Tlaxcala, rebuilt his forces and conquered Tenochtitlan the following year, when the city had just been ravaged by a convenient plague of smallpox (caught from a Spanish slave that got captured during La Noche).

*** While this is also a common tactic against Chaos forces (who have to deal with ambitious subordinates, OverzealousUnderlings, personal rivalries, and religious schisms), it doesn't always work (especially if the commander was jut a figurehead for TheManBehindTheMan). Another Literature/CiaphasCain has a warlord with a permanently CompellingVoice be killed by Cain, causing his army to attack (and his pilot to starve to death waiting for his return).

** In Alpha Legion Chaos Space Marines specifically mitigate this trope by having no centralized leadership. In the tabletop, this translate to the opponent being forced to kill every character in the army before they can earn a victory point for assassinating the leaders. In the lore, thanks to a BackupTwin and an IAmSpartacus culture, nobody's entirely sure if their original leader is dead or not or weather the many warbands follow any overall structure anymore.

* Following the events of La Noche Triste, the surviving Spaniards and their Tlaxcaltec allies fled north while being pursued by a far larger Aztec army that cornered them in the plains of Otumba with no possibility of escape. Seeing his enemy weak and outnumbered, the Aztec general Matlatzincatzin followed the Mesoamerican custom, spread his forces and ordered them to take as many men alive as possible to sacrifice them back in Tenochtitlan. However, Cortés learned (from the Tlaxcaltecs or from La Malinche, depending of the version) that if he killed the general himself, the Aztecs, now deprived of their leader, would stop fighting and leave. There was only one chance. Cortés surprised the Aztecs by using his last 23 horses in a cavalry charge that he led himself, something that the natives had never seen before (they were under the impression, in fact, that horses were only used as pack animals), killed the general and captured his standard. As predicted, the Aztecs broke lines and returned to Tenochtitlan. Cortés then took refuge in Tlaxcala, rebuilt his forces and conquered Tenochtitlan the following year, when the city had just been ravaged by a convenient plague of smallpox (caught from the Spaniards themselves).

** Though not as such in the rules, this is a common way of defeating Ork Waaghs in related material -- a Waagh is the result of a powerful Ork Warboss gathering together a number of warbands and going to war; killing the Warboss means that, without his charisma and domineering personality lashing them together, the assembled Orks will return to their normal habits of shooting at each other as well as the non-Orks. Best demonstrated in the ''Literature/CiaphasCain'' novel ''Death or Glory''; when Cain kills the Warboss, one of the Orks watching the fight tries to order the rest to attack, only for another to say something along the lines of "'oo says ''you're'' da new boss?!?" and attack one another.