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A Keystone Army[[note]]A ''keystone'' is the wedge-shaped stone piece at the apex of a masonry vault or arch, which is the final piece placed during construction and locks all the stones into position, allowing the arch to bear weight. If it is removed, the whole structure falls, AndThatsTerrible. A lot of people do know what a keystone is, but its usage in architecture has dwindled with steel frames.[[/note]] 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.

General forms include having the MacGuffin-weakness be the HiveQueen of an enemy HiveMind, the sole [[PowerSource source]] of all the enemies' powers, or the [[FantasticFragility lone connection]] to AnotherDimension. Just like a real keystone, once it's destroyed, the rest falls apart.

Of course, [[NoPlansNoPrototypeNoBackup no one ever considered that someone would aim at their army's one weakness]]. After all, it would be rather anticlimactic to destroy the KillerRobot's central control but have a backup one come online in another location.

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.

Despite the name, only occasionally is the vital component a CosmicKeystone. A type of InstantWinCondition. See also DecapitatedArmy, for when the keystone is the leader. StraightForTheCommander exploits this trope. The armed forces equivalent of a TerminallyDependentSociety. See also FantasticFragility. Compare UntouchableUntilTagged, when individual NighInvulnerable fighters get mobbed after an unlucky hit.

Similar, but not to be confused with the ''Keystone Kops''. No relation to Pennsylvania's[[note]]"The Keystone State"[[/note]] National Guard.
----
!!Examples

[[foldercontrol]]

[[folder:Anime and Manga]]
* In LightNovel/BakaToTestToShoukanjuu, 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.
* This could be said of Zero in Anime/CodeGeass. During the invasion of Tokyo during the Black Rebellion, Zero's vaguely-justified retreat in the middle of the battle proved to be fatal for his troops' morale, and without him there to lead them they quickly fell to Brittanian forces.
** {{Lampshaded}} by Diethard earlier in the series, when Zero has gone missing and some of the Black Knights wish to retreat and leave him for dead, rather than risk themselves by sticking around looking for him[[note]]Not cowardice so much as the leaders not wanting to risk everything on the retrieval of one man[[/note]]. Diethard points out that Zero is the glue holding the organization together: lose him and they've lost everything.
* ''Manga/{{Naruto}}'': the army of [[spoiler:ninja resurrected by [[{{Necromancer}} Kabuto]]]] was controlled and created by his technique, and thus can be stopped by defeating him. However, just killing him won't [[NoOntologicalInertia get rid of it]]; he has to actually be forced to stop it.
** The first Naruto Shippuden movie features a nigh-invulnerable terracotta army that is animated by an EldritchAbomination type being called Mouryu. The only practical way to stop it is by sealing this creature, which is in itself a pretty tall order once its soul and body have reunited. Which, in a surprising moment of GenreSavvy, is why they sealed the soul & body in two different places. Mouryu, unfortunately, has shades of AsLongAsThereIsEvil going on.
* SpaceCruiserYamato 2. Desslok's flagship has a brigade of robots which Kodai (Wildstar) defeats by blowing up their central computer.
[[/folder]]

[[folder:Comics]]
* In a 1970's [[{{Comicbook/Avengers}} Avengers]] story, [[{{Thanos}} Thanos's]] space fleet is attacking Earth. It turns out that all of the multivaried aliens in the fleet are depending on a single gizmo on one ship to translate for them so they can understand each other. The Avengers destroy it, and then win fairly easily. Somewhat justified when it turns out that Thanos [[XanatosGambit arranged the whole thing]] as a distraction from his ''real'' evil plan, and didn't especially care whether the space fleet succeeded or not.

[[/folder]]

[[folder:Fan Works]]
* The square miles of regenerating undead army in ''Fanfic/WithStringsAttached'' are powered by the Heart of Evil, in a ruined city miles away from where the four and Jim Hunter are surrounded. Having been made a little stupid by rage, and never having grasped just how effective Ringo is at finding stuff, Jeft never imagined that the little band could escape their encirclement, get to the city, go straight to the Heart of Evil, semi-accidentally defeat its powerful guardians, and destroy it, which instantly destroyed all the undead. Which ''really'' pissed Jeft off, since not only did he not punish Jim Hunter for his HeelFaceTurn and the four for instigating it, but that heart was ''expensive''.
[[/folder]]

[[folder:Film]]
* ''Film/TheAvengers''. 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.
* Played with in ''Film/BattleLosAngeles''. The alien invaders are ferocious opponents on the ground and are driving back the US military from Los Angeles and other coastal battlefields even before their drone support shows up overhead. The drones themselves are controlled from a large command center that, when taken out, causes the drones to crash and the United States military to regain air superiority over Los Angeles, but the war is far from over and the aliens remain a tenacious force on the ground.
* On two levels in ''Film/EdgeOfTomorrow''. Every Mimic force comes with an Alpha. If that dies, the Omega [[spoiler:resets everything with foreknowledge of how things went down in the last loop]], adapting until the Mimics win. Killing the Omega, however, stops all the Mimics.
* Done in the second ''Film/{{Hellboy}}'' film, where the [[MacGuffin demon crown]] is the keystone.
* In ''Film/IndependenceDay'', the alien ships are rendered shieldless through a virus uploaded to the mothership, which apparently maintains a constant data link with both the motherships and the fighters. Despite claims that this would only last a few minutes, they never get their shields back.
* The rogue robots in ''Film/IRobot''. [[spoiler:Since VIKI was remotely distorting their programming to make them hostile to humans, they immediately became docile upon her defeat.]]
* ''Film/InTheNameOfTheKing: A DungeonSiege Tale'' has the Krug, formerly mindless beasts who have been magically uplifted by [[EvilSorcerer Gallian]] to serve as his army. Still mostly beasts, they fight using WeHaveReserves tactics. As soon as Farmer slits Gallian's throat, the Krug stop and, after standing around in confusion, drop their weapons and wander off.
* Arguably in ''Film/TheLastStarfighter'', the Xodan Command Ship's communications turret. It is what allows the entire armada of space fighters to coordinate. Without it, they're not as effective and taken out quickly with a MacrossMissileMassacre thanks to the Death Blossom.
* ''WesternAnimation/TheLegoMovie'' mimics ''Film/TheAvengers'' and ''ThePhantomMenace'' by having the Micromanagers drop dead with the destruction of the SupervillainLair.
* In ''Film/TheLordOfTheRings'' movies, destroying the One Ring destroys Sauron (this at least is justified, as it's a SoulJar), which causes his armies to flee while Mordor suffers what can only be called a [[LoadBearingBoss catastrophic geographical failure]].
* In ''[[Film/TheMummyTrilogy The Mummy Returns]]'', the unending army of Anubis turns to dust when the title's Scorpion King is killed and ordered to take his army with him. Justified, because whoever kills the SK, [[YouKillItYouBoughtIt becomes the legitimate master of the army]]. The good army is just preventing the anubites from getting out into the world, they knew they couldn't win on their own.
* ''Film/{{Oblivion 2013}}'' : [[spoiler: After the Tet gets blown up, the drones immediately deactivate.]]
* ''Franchise/PiratesOfTheCaribbean:''
** The curse which rendered the crew of the Black Pearl [[{{Immortality}} immortal and unstoppable]]. Jack and Will break the curse while the crew is in the middle of a climactic battle with Norrington's men, and the second that the pirates realize that their key advantage is lost, they surrender.
** The ''enormous'' Imperial Armada Beckett brought with him to annihilate the pirates in ''At World's End'': they all turn tail and run when the ''Endeavour'' is sunk, despite it being heavily implied that they vastly outnumber the entire pirate fleet. May simply be a case of LazyBackup. On the other hand, they were up against the Black Pearl, which was pretty infamous at the time for her actions, as well as the Flying Dutchman, all at once. Especially the Dutchman, who would probably have made it a curbstomp battle given how everyone keeps saying "Control the Dutchman, control the seas."
* The Borg Queen and her Borg in ''Film/StarTrekFirstContact'' (at least, those Borg who survived the engine coolant spill)
** And done in "The Best of Both Worlds" when the ''Enterprise'' rescues Picard/Locutus, who they use to destroy the Borg Cube...by telling them all to "go to sleep".
* In ''Franchise/StarWars: Film/ThePhantomMenace'', once Anakin blows up the Trade Federation's control ship, the droid army shuts down.
** Also, the Gungan army turns and flees as soon as their shield generator is destroyed. Justified, because the Trade Federation army had brought tanks to the battle, and with the shield down they could move in. And do.
** 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.]]
*** This is still quasi-true in ''Film/RevengeOfTheSith'', given that Anakin is able to shut down the entire droid army after killing the Seperatist leaders. This is largely justified as there was a serious risk of droid rebellion and the controls over those droids were necessary, not to mention that they didn't benefit from leaving their army around if they're already dead.
** The earliest ''Franchise/StarWars'' example is of course [[Film/ANewHope the original]], with the Death Star's [[AchillesHeel exhaust port]]. Reused in ''Film/ReturnOfTheJedi'' with the Death Star II's main reactor, not to mention Palpatine dying with it. These both are arguably subversions, however, given that the Empire kicked the rebels off the moon shortly after the Battle of Yavin and that the civil war still officially went on for another fifteen years after Endor.
* In the ''Film/TotalRecall2012'' remake, the BigBad is making an army of robots to take over the only other habitable area in the world. The protagonist finds out about a shutdown code for the army buried in his memories. When LaResistance leader tries to access the memory, this triggers a hidden code that allows the BigBad to find him. Also played straight when the protagonist destroys [[spoiler:the Fall, a gravity elevator from Europe to Australia, destroying the robots aboard]].
[[/folder]]

[[folder:Literature]]
* Justified in ''Literature/{{Animorphs}}''. When the heroes take control of the Yeerk Pool Ship and Visser One, the Yeerk general, it provides the Andalite army with comprehensive intelligence on Yeerk military, that allows them to turn tides in the war and defeat the Yeerk Empire.
* {{Zig-zagged}} in ''Literature/TheChroniclesOfThomasCovenant'', specifically ''The Power that Preserves''. [[SupportingLeader Lord Mhoram]] thinks that by killing [[TheDragon Satansfist]], the commander of the army besieging Revelstone, he'll be able to route the entire army. It doesn't work, because the [[EvilSorcerer ur-vile loremasters]] take command immediately upon Satansfists death and restore order. But when Covenant defeats [[BigBad Lord Foul]], the ur-viles sense it and decide to call it quits, and the whole army crumbles.
* In ''Literature/CodexAlera'' without their Queen the Vord are just animals. Dangerous ones, but manageable. Although normally they possess the ability to give birth to new subsidiary queens, so killing one may rout the Vord in the area, but won't destroy their threat.
** The Vord Queen in Alera explicitly says [[spoiler:she blocked this ability in all other Vord since other Queens immediately tried to kill her]]. Of course, this doesn't guarantee what happens after [[spoiler:her death]]
* The Buggers and their Queen from ''EndersGame''. Somewhat subverted in that the Buggers actually know that this is their weakness and actively hide the Queen among the rest of the ships. It takes a genius tactician like Ender to figure out which one is the Queen ship, and even he can't do it in the middle of combat. On top of that, the Queen usually isn't even with her troops, being capable of instantaneous communication from halfway across the galaxy. As a result, Ender ends up having to wipe out whole fleets at a time.
** It's played straight in the narration of the past [[BugWar Bug Wars]], with the [[JustifiedTrope justification]] that [[spoiler:the [[HiveMind Buggers]] only saw killing a HiveQueen as killing, which was part of the reason for Humanity's fear and hatred of the species; when they happened upon a human colony, they dismantled our technology to see how it worked - ''after'' they "dismantled" the ''colonists'' to see how '''they''' worked. They didn't understand how much that would piss us off any more than they could comprehend that we would kill a sentient queen, rather than the nonsentient workers.]] They've learned.
* In the ''Literature/HeraldsOfValdemar'' series, the kingdom of Hardorn has waged two wars against Valdemar with forces composed of a few mages and loyal generals and a mass of mind-controlled conscripted troops. In both cases, killing the mage freed the soldiers, who then turned on their generals and ripped them to pieces.
* In ''[[Literature/InheritanceCycle Eragon]]'', the [[OurOrcsAreDifferent Urgal]] army in Farthen Dur was routed when Eragon took out [[TheDragon Durza]], thus breaking his mind control over them.
* In Creator/JRRTolkien's ''Literature/TheLordOfTheRings'', the destruction of the Ring [[DecapitatedArmy kills Sauron]], which confuses and thus incapacitates the parts of his armies which were more directly controlled by his will (e.g. the orcs), which makes them easy game; the not-magically-controlled human armies had various natural reactions, some surrendering and some keeping on fighting.
** On a smaller scale, Sauron's human army from Harad are routed when their chieftain is slain by King Théoden.
** ''The Two Towers'' never does quite explain why the [[ImplacableMan Uruk-Hai]] turned tail and fled just because TheCavalry had arrived, the film doubly so, since Théoden's men were already surrounded in a sortie while the cavalry were flying into spikes.
*** Easy reason: Lord of all horses, being ridden by what's basically a pissed off archangel, while a horn loud enough to make a mountain shake is blaring, and being bounced off the walls of the canyon. Urk hai or not, that's going to route you. Add in that any boosts from Saruman were nulled the night before by the Ents.
*** Not the mention that in the book the force disparity was much less. Rather than battering down the door the the throne room the Uruk-Hai were fighting and failing to take two fortified and entrenched positions, one of which was famous for never, ever falling when they were attacked in the rear by an army more than a quarter of their total.
* In the post-Apocalypse novel ''Literature/{{Malevil}}'', depending on a Keystone Army becomes the plan of attack near the end of the novel. [[spoiler: The BiggerBad is marching his army toward the hero's castle, he rules his men 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 Cauldron-Born (an army of undead) from the ''PrydainChronicles'' rampage without end until [[spoiler:Taran recovers the [[InfinityPlusOneSword Enchanted Sword]], Dyrnwyn, and stabs one with it - instantly destroying '''all''' of them]].
* In the ''Starfire'' series, the alien Bugs (big spider monsters with insane numbers on their side) are all but invulnerable... until they are stopped by a tiny flaw in their evolution: kill enough of them at once and the others feel the pain of their deaths. Kind of like a whole race of Obi Wan Kenobis, all feeling a billion voices crying out in pain only to be silenced. Only these guys all have a fatal stroke when they feel them.
* In the ''Literature/WarriorCats'' book ''Firestar's Quest'', there's a large horde of HiveMind rats. Once Firestar kills the leader, they're too confused to fight because they have absolutely no idea what to do now, and those who aren't killed scatter.
* The [[OurOrcsAreDifferent Trollocs]] in ''Literature/TheWheelOfTime'' series are the vicious and terrifying foot soldiers of the Shadow. Unfortunately, though they are violent and bloodthirsty, they are also, by nature, selfish and lazy. The only way they can be utilized effectively as a military force is by having Myrdraal control them with a mental link and the use of fear. This makes them a great danger, but if the Myrdraal dies, so do all the Trollocs linked to it. ({{Downplayed}} in that each Myrddraal officer is only linked to a platoon or at most a company - you need to kill all the Myrddraal to take out the whole army, and they won't all be in one place.)
* ''Literature/WindOnFire'' series: In ''The Wind Singer'', the [[TheHorde Zars]] are known for being unbeatable due to having infinite numbers. [[spoiler: The titular Wind Singer's music, however, [[DePower strips off their magic]] and [[NoImmortalInertia makes them age]].]]
[[/folder]]

[[folder:Live Action TV]]
* In the ''Series/DoctorWho'' episode "[[http://tvtropes.org/pmwiki/pmwiki.php/Recap/DoctorWhoNSS2E13Doomsday Doomsday]]", when the space-time rift that pulled through an army of Cybermen and a ship full of Daleks is closed, it starts to suck them back in.
** In "The Age of Steel", the Cybermen are stopped when the program preventing them from feeling emotions is disabled - upon realizing the nature of the BodyHorror they've become, the Cybermen ''kill themselves en masse''.
** The Ood from "[[http://tvtropes.org/pmwiki/pmwiki.php/Recap/DoctorWhoNSS4E3PlanetOfTheOod Planet of the Ood]]" have a form of hive mind. Destroying it would presumably kill all the Oods. The subversion being that the BigBad tries to destroy it while the Doctor has to save it.
* The armies of Vortigern in the 1998 ''[[Series/{{Merlin1998}} Merlin]]'' series. [[spoiler: Once Merlin disposes of their king, they cease fighting, and Vortigern's rival Uther is shortly thereafter crowned king.]]
[[/folder]]

[[folder:Music]]
* In the rock opera by Music/TheProtomen called "The Father of Death" Dr. Wily, as a Dangerously Genre Savvy Villain, tricks Dr. Light into committing terrorist acts and sacrificing Sniper Joe to destroy his "robot control tower." Turns out Wily has a second control tower located in a fortress and he was only waiting for Dr. Light's attack to give him an excuse to deploy his robot army and declare martial law.
[[/folder]]

[[folder:Tabletop RPG]]
* A ''TabletopGame/{{BattleTech}}'' example would be the initial Clan invasion. When a lucky hit takes out their supreme war leader, the entire hitherto unstoppable Clan advance practically grinds to a halt for months as their Khans return to their distant homeworld cluster to elect a new one. (Choosing a new ilKhan requires a vote by ''all'' the Khans, not just the few actually running the invasion at the time.)
* In ''TabletopGame/{{Chess}}'', the king is all that matters.
** The same goes for the Japanese game of Shogi, which is not unlike Chess.
* This is a classic trope for fantasy [=RPGs=] like ''DungeonsAndDragons''. Often times the AlwaysChaoticEvil races of orcs, goblins and trolls are just as apt to fight each other as they are to attack the humans and other goodly races, until a BigBad manages to terrify them enough into cooperating under his leadership. The evil races' fear of and/or devotion to the BigBad is all that keeps them cooperating. If the BigBad is slain, the evil races will just as willingly turn on each other and the army will disintegrate. Needless to say, [=PCs=] are typically the ones who are tasked with destroying the BigBad before his armies can attack the outmatched forces of good.
** One of the most notable examples in DungeonsAndDragons takes place in the War of the Lance in the ''{{Dragonlance}}'' setting. Takhisis, the Queen of Darkness and ruler of the evil gods, keeps her five Dragonarmies united through ruthless discipline and their fear of her. When she is banished back to the Abyss by the [=PCs=], the Dragonarmies turn on each other and begin fighting for power. The canon post-War setting includes five territories that are each held by a mutually hostile Dragonarmy, and are just as apt to fight each other as to attack the forces of good.
** This trope actually comes up mechanically in Dungeons and Dragons rather than just from a story point of view. There are several types of monsters who are able to create "spawn" (usually these are undead like vampires and wraiths) and for some of those monsters, those spawn are created completely subservient to their creator. It is possible to build an army of monsters like this, however killing the one who created the spawn in the first place will break the bond of servitude and cause them to act freely. This could be catastrophic for anyone that was using such a monster to create a personal army or just a way to dissipate an army with a single blow.
*** Though with the potential to turn into a NiceJobBreakingItHero moment as for some of these monsters their creator's control also serves as an effective PowerLimiter and/or RestrainingBolt -- kill the "master", and suddenly all its former created "slaves" may be finally free to achieve their true horrific potential...
* ''TabletopGame/IronKingdoms'': for both Warmachine and Hordes, the Warcaster/Warlock is the motive force keeping the walking tanks/giant monsters going, so killing one shuts down/frees their personal troops, and killing all of them (if there's more than one) is the most common automatic win condition.
* ''TabletopGame/{{Warhammer 40000}}'' loves this trope.
** The Tyranids in ''TabletopGame/{{Warhammer 40000}}'' have certain breeds called "Synapse Creatures" that connect lower-tier organisms into the HiveMind. Killing a local Synapse Creature causes the portion of the Hive under its sway to become disoriented until another one can move in to take its place.
** Similarly if the Tau Ethereal is killed, all Tau units have to pass a morale check or flee. Ethereals are the Tau ruling caste and spiritual leaders, and the death of a cadre's overseeing Ethereal will cause the entire cadre to [[HeroicBSOD fall back into retreat regardless of how well they were doing before]]. {{Subverted}} in that while Tau initially fight with limited effectiveness, they eventually recover, and their fear and doubt will be replaced with a [[TranquilFury cold anger]] over [[MoralEventHorizon what you just did]]. Cue UnstoppableRage... [[BeamSpam with plasma guns]].
*** However as of 6th this has been removed, now it just gives the other guy an additional kill point.
* In early versions of ''TabletopGame/{{Warhammer}} Fantasy'', if the general of an Undead army (normally a Necromancer, Vampire or [[{{Mummy}} Tomb King]]) was killed, the army would quite literally disintegrate. This was toned down in later editions as leader units were {{nerf}}ed and killing them became a less daunting prospect, but it's still bad news.
** Dogs of War (essentially a lot of mercenary companies scraped together into an army) don't much care about their general... but if the Paymaster is killed and his treasure chest captured, [[OnlyInItForTheMoney they're likely to run for it.]]
** While only mentioned in the fluff, it is generally considered common Warhammer knowledge that killing a major Warboss will inevitably cause any WAAAGH to eventually collapse in on itself, as weaker Warbosses and Orcs start to [[EvilPowerVacuum turn on each other to claim the title]]. However, this has no representation in the rules -- no petty infighting is going to make them stop in the middle of a fight they're already engaged in.
** The spin-off game ''Warmaster'' has "kill the enemy's general" as one of the generic win conditions for all armies.
[[/folder]]

[[folder:Video Games]]
* Killing the opponent's king unit in [[ExactlyWhatItSaysOnTheTin Regicide mode]] in ''VideoGame/AgeOfEmpiresII'' gives you instant victory, regardless of how many other units and resources the other player still has. Of course, losing your king will do the same to you. In several campaign scenarios the objective is killing one particular enemy commander or destroying one enemy building too.
* In the single-player mode of the early ''VideoGame/{{Battlefield}}'' series games, your AI teammates are so [[RedShirt incompetent]] that they will constantly lose ground if you're not being Franchise/{{Rambo}} on the front lines next to them.
** Subverted at the end of ''Bad Company 2''. [[spoiler: Destroying the scalar weapon and killing Kirilenko makes Bad Company think that the Russians will no longer invade the US. Cue a bunch of American tanks rolling up beside them to tell them that the Russians have just started to invade through Alaska.]]
* All the units of the [[StarfishAliens Scrin]] harvest escort fleet in ''VideoGame/CommandAndConquer 3: Tiberium Wars'' are powered by [[GreenRocks Tiberium]] radiation emanating from a "Relay Node" established at the original Tiberium meteor's crash site, in Italy. While the Node has an operational range that goes as far out as the Moon, its destruction stops every Scrin unit ''on Earth'' dead in its tracks, and some even break apart. The Scrin [[GenreSavvy are aware of its importance]]: it's ''ludicrously'' well-defended with everything from [[TimeStandsStill stasis shields]], [[ShockAndAwe Storm Columns]] and a [[CoolStarship space fleet]] to [[ImmuneToBullets phase fields]] that [[CrazyPrepared render it invulnerable]] temporarily. Additionally, their original invasion plan called for multiple redundant nodes, but they got [[ObstructiveBureaucrat sidetracked]].
* In single player ''VideoGame/{{Diablo}}'', all surviving monsters die when Diablo is killed.
* In ''VideoGame/DivinityDragonCommander'', capturing the enemy's capital building and holding it for a turn results in you taking over '''all''' of their territory and surviving units.
* In ''VideoGame/DragonAgeOrigins'', killing the [[BigBad Archdemon]] immediately ends the Blight. Justified since the Archdemon's will is what unites the AlwaysChaoticEvil Darkspawn hordes into an organized military force. The expansion ''[[VideoGame/DragonAgeOriginsAwakening Awakening]]'' subverts this -- one of your tasks as the newly appointed Warden Commander in Amaranthine is to investigate why and how Darkspawn are still making fairly organized attacks on the surface (though not on the same scale as a full blown Blight).
** This particular Keystone is also much harder to destroy than most examples. Not only is the Archdemon itself [[ThatOneBoss incredibly powerful]], it can even [[spoiler: [[BodySurf transfer its soul to the nearest darkspawn and thus be reborn]].]] So unless [[spoiler: a Grey Warden sacrifices himself/herself by taking in the Archdemon's soul (an act that destroys both of their souls) or partakes in Morrigan's Ritual which transfers the Archdemon's soul into Morrigan's developing child]], the only way to end a Blight would be to ''kill every darkspawn in existence''. Which is impossible considering they outnumber just about everybody [[spoiler: and the Broodmothers can generate more at a ridiculous pace]].
*** This is why the first Blight lasted something like ''two hundred years''.
** A smaller example occurs in ''Awakening'', where a darkspawn uses a control rod to make a bunch of golems attack you. Killing the darkspawn deactivates all the golems and allows you to loot his control rod, allowing you to [[HoistByHisOwnPetard use another golem against some darkspawn in the next room.]]
* 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.
* ''VideoGame/TheElderScrollsVSkyrim'' has evil wizards who will often summon up two or more zombies or elemental familiars to fight you. If you kill the wizard, all of their summoned creatures die/vanish. This can comically extend to entire packs of vampires or a dungeon full of necromancers, as they will raise their fallen comrades, who then proceeds to raise more of their comrades. However, if you allow them to do this while keeping track of the last "alive" one, killing him results in the entire room dissipating into dust.
* A standard clause for battles in ''VideoGame/ExitFate''. Even though you and your opponent has multiple units to fight with, as long as you knock out their primary leader, the rest will flee/surrender and you win. (You get a better rating if you take them all out, though.) However, the same goes for you; if the enemy defeats the unit that represents the main character, it's immediate game over.
* ''[[VideoGame/FirstEncounterAssaultRecon F.E.A.R.]]'''s Replica Soldiers depend on a psychic commander for their orders and become completely inert when the commander dies. Although this is subverted in the expansions (where Paxton Fettel reactivates them from beyond the grave just a couple hours later) and later games (where another psychic commander and then Alma herself take control of them).
* TheFederation in ''VideoGame/FTLFasterThanLight'' is guaranteed victory once you destroy the [[FinalBoss Rebel Flagship]], even if the sector, which includes the Federation's Home Base, is completely covered in Rebel-occupied nodes.
* It's mentioned in the backstory of ''VideoGame/KnightsOfTheOldRepublic II'' that after [[spoiler:the massive amount of death and destruction at Malachor V]], Revan decided that turning his enemies into allies was a better idea - and, particularly in the construction of HK-47, this trope was his primary idea on how to accomplish that: kill or convert one influential person, watch everything they held together crumble on its own.
* In the ''VideoGame/{{Langrisser}}'' series, when a commander is killed, all of its surviving soldiers are also removed from the map. It's often to the player's benefit to take out as many of the enemy's soldiers as possible before killing the commanders to get the most experience points.
* ''Franchise/MassEffect'' - The Rachni have {{Hive Queen}}s, who reside on toxic planets. The employment of Krogan (a sentient species that evolved on a DeathWorld, so can survive on Rachni Homeworlds and attack the Hive Queens) was the turning point of the war.
* ''VideoGame/MetalGearSolid2SonsOfLiberty'' subverts this: [[spoiler:Arsenal Gear]] is designed so that it wouldn't be a keystone if it was ever actually used, because while it's almost invincible as long as it has backup, when it ''doesn't'' [[spoiler:it's so weak the BigBad is willing to leave it to the QuirkyMinibossSquad as a way of ''killing them off'']].
** The SOP can be seen as the keystone in ''MetalGearSolid4''. Sure, [[SuperSenses they give you more awareness and perfect response,]] [[BottledHeroicResolve and suppress PTSD,]] but if you lose it, [[HeroicBSOD expect withdrawal]]. Half of the game is consist of Liquid Ocelot try to find a way to control it and Solid Snake's attempts to hinder him.
* The [[ScaryDogmaticAliens Skedar]] from ''VideoGame/PerfectDark''. They had been at war with the [[InnocentAliens Maians]] for a couple of centuries preceding the game's story, and are only stopped for good when the destruction of their home planet and murder of their king crushes their morale.
** One of the bonus missions shows they're GenreSavvy about this to the point that they have ''three'' back-up kings ready to take control of their forces in case the one from the main game bites it.
* Quite a few {{RTS}} will have this as a condition on various campaign levels. The player's goal is to destroy a single unit or structure, and doing so nets a victory, no matter how many enemy units are left on the field.
* In ''VideoGame/SonicHeroes'', there are gold enemies that take all nearby 'bots with them when destroyed.
* Justified in a realistic way in the slaughterhouse level of ''Creator/TomClancy's VideoGame/SplinterCell''. The bad guys are mercenaries working for cash, and Sam's boss informs him that once he kills their leader/employer, the remaining bad guys will say "screw this" and all go home (since the guy who writes their checks is now dead).
** Although it should be noted that if you're in the line of sight of an enemy soldier when you kill this boss, they do continue to shoot at you, which can quickly turn your mission success screen into a mission failed screen, especially since your controls lock up at this point.
** The show ''Series/BurnNotice'' makes the same remark. Michael points out that if you take out the person handing out paychecks, mooks and mercenaries won't stick around to avenge him.
** Something similar happened in ''Webcomic/TheOrderOfTheStick'', when Roy throws Xykon into the portal, he is destroyed. The Goblins in the throne room surrender saying that no one is paying them anymore (though it's more likely that no insanely powerful lich is threatening them anymore). Unfortunately, the goblins tried to surrender to [[HeroicComedicSociopath Belkar]].
* The Zerg in ''VideoGame/{{Starcraft}}'' are defeated when their [[HiveMind Overmind]] is killed. The ExpansionPack, ''Brood War'', subverts this; without the Overmind to direct their actions, the Swarm launches into a mindless frenzy and slaughters half the Protoss population. ''Then'' lots of backstabbing intrigue about control of the Swarm happens.
** A similar effect occurs when a Cerebrate is killed, except the Cerebrates only control specific broods, or sections of the swarm.
** Note that the Zerg examples are campaign-only. In regular maps, the closest thing there is to a keystone is the Protoss Pylon, which powers all the surrounding Protoss buildings. Take down the Pylons, and the buildings are disabled.
** The campaigns themselves are rife with examples where the key to victory against overwhelming odds lies in destroying weak points; for example, "Shatter the Sky", one of the two alternate penultimate missions in ''VideoGame/StarcraftII'', tasks you to destroy a space station with overwhelming forces of zerg crawling on it by taking down its coolant towers.
* The Aparoids in ''VideoGame/StarFoxAssault''. As you would expect from a race of giant alien insects, they keep multiplying as long as their queen is still alive. Luckily, they suffer from apoptosis, so killing the queen also kills them all instantly.
* In ''VideoGame/SupremeCommander: Forged Alliance'', destroying the Quantum Rift of the Seraphim stops their invasion dead in its tracks.
** In both the ''Supreme Commander'' game and its expansion, armies will deactivate if their Armored Command Unit is destroyed. In multiplayer mode, losing one's ACU is a condition to lose the game. Justified in that the ACU contains the only person on the field, and that all the other units are robots under his or her control. [=sACU=]'s are shown to sometimes be piloted by humans, but only in story missions.
* The Commander in ''VideoGame/TotalAnnihilation'' is a good example of this trope. If it dies, you lose and your whole army blows up. The same goes for the oppenent.
* Occurs twice in the ''VideoGame/TotalWar'' games. First, eliminating the general leading an army causes that army's morale and fighting capacity to decrease, making them much easier to defeat (though it's not an instant win). On the strategic map, removing all the adult male members of a faction's royal family (via Assassination, or by bribing / marrying them into your own faction) causes that faction to lose, no matter how many territory or armies it controls.
** ''TotalWar'' also includes an {{inversion}}. If a general has a lot of losses, their leadership bonus goes negative. Assassinating a general replaces him with a newly-promoted subordinate who starts at zero. So, killing an enemy's lousy general improves his army's chances for victory.
* ''VideoGame/ValkyriaChroniclesIII'': {{Invoked}} by The Nameless with the operation to assassinate [[BigBad Maximillian]]. [[ForegoneConclusion It doesn't work, because said attempt is thwarted by Selvaria.]]
* Subverted in the ''VideoGame/{{Warcraft}}'' Universe with the Scourge. The only thing keeping the Scourge from becoming an unstoppable army which would consume everything is the Lich King, who [[HiveMind controls]] the entire Scourge and stops them from doing so.
* The ''{{X-Com}}'' series ''loves'' this trope. In the first game, the entire alien army is run by a giant brain on Mars. In the second game, the leader is hidden underwater. In the third game, you have to seal off the gates to their dimension.
** The third game, ''Apocalypse'', is more of an inversion: In order to seal the gates off, you have to have already destroyed their entire city, and nearly every living thing in it. The gates are the last [[OrganicTechnology structure/organism]] to die.
** Given a nod in XCOMEnemyUnknown. After you take an alien base, your NumberTwo believes that you've broken the invasion. The Head Researcher, however, raises an eyebrow at that, mutters under her breath something about "Too easy", then walks off to begin researching the spoils. [[spoiler: [[GenreSavvy She's right.]] Turns out the base was only an ''outpost'' of the main force. Played straight, and Justified, at the end of the game however. Taking out the [[FinalBoss Uber Ethereal]] kicks off a reaction that [[LoadBearingBoss causes the alien]] [[TheMotherShip Temple Ship]] [[LoadBearingBoss to implode]], taking the majority of their ships, supplies, and leadership with it.]]
* A popular mechanic in TurnBasedStrategy games such as ''Franchise/FireEmblem'', the ''VideoGame/FinalFantasyTactics'' series and the ''VideoGame/LuminousArc'' series: kill the boss of the stage causes all other enemies to flee, stand still or outright melt away, an InstantWinCondition.
* The highly unique role-playing/real-time strategy hybrid ''VideoGame/{{Sacrifice}}'' uses this as a foundational rule -- for both the player and their opponents. Each level is essentially a glorified arena for two (or more) wizards to compete in, and each wizard is a combination of a mobile base, resource gatherer and spell provider. DeathIsASlapOnTheWrist applies because each wizard has a connection to a mystical altar, which rapidly revitalizes them; death merely prevents them from summoning new troops or casting spells, but their army keeps on fighting. Only by defiling the wizard's altar and then slaying them can they be removed from the battlefield -- and when that happens, all of their troops drop dead, because they no longer have their master to sustain them. Each battle, then, is a constant struggle by all of the players to take out the rival wizards.
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[[folder:Western Animation]]
* The Joining from ''WesternAnimation/TheBatman'' are an army of robots. They are defeated twice by this: the first time by a self destruct code that had been built into the various parts they were made off. The second time they were defeated by a signal to their mothership ordering them to go offline.
* In the episode "Dark Heart" from ''JusticeLeagueUnlimited'', an army of [[NanoMachines self replicating robots]] is defeated when the heroes destroy the titular dark heart commanding them.
** From the same series, an alien army is defeated when the MartianManhunter frees their power source from the corrupted leader controlling it.
* The ''WesternAnimation/KimPossible'' [[TheMovie movie]] [[WesternAnimation/KimPossibleMovieSoTheDrama So The Drama]] has this. Dr. Drakken distributes toy robots around the world which turn out to be giant killer robots that he can activate with a command signal he broadcasts from his headquarters. When he launches the worldwide attack, Kim and Ron foil it by knocking out the main broadcasting tower, causing all the robots to revert back to their harmless toy forms.
* ''StarWars''' use of this trope was lampooned in one of the ''RobotChicken'' specials, where one of the Imperial commanders scoffs at the idea that "The rebels won!", since they ''do'' still have a big-ass fleet. Apparently killing the Emperor ''and'' destroying the Death Star is enough for the rest to call it a day.
** Explained in the books by having Palpatine use the Force to unify his armies to fight more efficiently. With his sudden death, the morale blow of the Death Star being destroyed, and the loss of the fleet commander on the super-star destroyer the Imperial Fleet was thrown in disarray. And due to Darth Vader's habit of Force-Choking incompetent underlings, the Death Star had the Empire's best and brightest aboard when it blew up. Plus, you've just seen the other side blow up a ship the size of a moon. The average Imperial officer is going to think "the rebels have a superweapon, ScrewThisImOutOfHere" not "lucky shot"
* ''SuperRobotMonkeyTeamHyperForceGo'' featured in one episode the Vreen, a HordeOfAlienLocusts from the future. Their weakness was the present-day bug [[StableTimeLoop they evolved from]]; when it was found and destroyed, the Vreen disappeared.
[[/folder]]

[[folder:Real Life]]
* A common problem on Ancient Greek battlefields: when the general died, the whole army tended to rout. This was exploited by the Thebans during the Battle of Tegyra. Outnumbered four to one by their Spartan opponents, they went straight for the officers, whose death threw the Spartan army into terminal paralysis.
* 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 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.
[[/folder]]
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