Keystone Army

A Keystone Army is an invasion force or army that's seemingly unstoppable, except for one particular weakness 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, the entire army is immediately disabled. This plot device is handy as it allows a 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 Hive Queen of an enemy Hive Mind, the sole source of all the enemies' powers, or the lone connection to Another Dimension. Just like a real keystone, once it's destroyed, the rest falls apart.

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

Bee People are likely to be a Keystone Army if they have a Hive Queen, and as such trashing it is a good way to win the Bug War. Another common Keystone Army simply replaces the insects with Killer Robots or Grey Goo and the Hive Mind with a Master Computer or evil AI, and yet another popular variation has an Evil Sorcerer whose (often Undead, Mind Controlled or Demonic) minions will cease to be a problem due to No Ontological Inertia upon his death/defeat/distraction. An army of monsters may be defeated by killing the Monster Progenitor. Easily Thwarted Alien Invasions often employs such armies.

Despite the name, only occasionally is the vital component a Cosmic Keystone. A Variety of Golden Snitch more commonly known as an Instant Win Condition. See also Decapitated Army, for when the keystone is the leader.. The armed forces equivalent of a Terminally Dependent Society. See also Fantastic Fragility.

Similar, but not to be confused with the Keystone Kops. No relation to Pennsylvania.

Anime and Manga

 * This could be said of Zero in Code Geass. 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 . Diethard points out that Zero is the glue holding the organization together: lose him and they've lost everything.
 * Space Cruiser Yamato 2. Desslok's flagship has a brigade of robots which Kodai (Wildstar) defeats by blowing up their central computer.
 * Naruto: the army of was controlled and created by his technique, and thus can be stopped by defeating him. However, just killing him won't 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 Eldritch Abomination 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 Genre Savvy, is why they sealed the soul & body in two different places. Mouryu, unfortunately, has shades of As Long as There Is Evil going on.

Film

 * In Star Wars: The Phantom Menace, 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 Attack of the Clones; the Jedi blow up the droid control ship, shutting the droids down en-mass, only for them to all power up again seconds later when their new onboard backup systems boot up.
 * This is still quasi true in Revenge of the Sith 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 Star Wars example is of course the original, with the Death Star's exhaust port. Reused in Return of the Jedi with the Death Star's main reactor. The Battle of Yavin in A New Hope is arguably a subversion, given that the Empire came back and kicked the rebels off the moon shortly thereafter.
 * In The Lord of the Rings movies, destroying the One Ring destroys Sauron, causing his armies to flee while Mordor suffers what can only be called a catastrophic geographical failure.
 * Although at least as far as Sauron is concerned, the One Ring was actually his keystone and the reason he still exists in Middle-Earth. The entire country falling into the sea is somewhere trickier to explain...
 * The Borg Queen and her Borg in Star Trek: First Contact (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".
 * The curse in Pirates of the Caribbean which rendered the crew of the Black Pearl 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.
 * Done in the second Hellboy film, where the demon crown is the keystone.
 * In 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, 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.
 * The rogue robots in I Robot.
 * The Avengers. Destroying the mothership causes all the aliens to drop dead.

Literature

 * The Trollocs in The Wheel of Time 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.
 * In Eragon, the Urgal army in Farthen Dur was routed when Eragon took out Durza, thus breaking his mind control over them.
 * In JRR Tolkien's The Lord of the Rings, the destruction of the Ring 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 Theoden.
 * The Two Towers never does quite explain why the Uruk-Hai turned tail and fled just because The Cavalry had arrived, the film doubly so, since Theoden'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.
 * The Buggers and their Queen from Ender's Game. 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 Bug Wars, with the justification that They've learned.
 * 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.
 * The Cauldron-Born (an army of undead) from the Prydain Chronicles rampage without end until.
 * In Codex Alera 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.
 * In Independence Day the alien mothership powered the shields of the smaller city destroyers. It's destruction left the previously nigh invincible ships vulnerable to conventional attacks.
 * Zig-zagged in The Chronicles of Thomas Covenant, specifically The Power that Preserves. Lord Mhoram thinks that by killing Satansfist, the commander of the army besieging Revelstone, he'll be able to route the entire army. It doesn't work, because the ur-vile loremasters take command immediately upon Satansfists death and restore order. But when Covenant defeats Lord Foul, the ur-viles sense it and decide to call it quits, and the whole army crumbles.
 * Justified in 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.
 * In the post-Apocalypse novel Malevil, depending on a Keystone Army becomes the plan of attack near the end of the novel.
 * In the Heralds of Valdemar 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.
 * Wind on Fire: In The Wind Singer, the Zars are known for being unbeatable due to having infinite numbers.

Live Action TV

 * In the Doctor Who episode "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 Body Horror they've become, the Cybermen kill themselves en masse.
 * The Ood from "Planet of the Ood" have a form of hive mind. Destroying it would presumably kill all the Oods. The subversion being that the Big Bad tries to destroy it while the Doctor has to save it.
 * The armies of Vortigern in the 1998 Merlin series.

Music

 * In the rock opera by the Protomen 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.

Tabletop RPG

 * Warhammer 40000 loves this trope.
 * Probably the inspiration for the Zerg example: The Tyranids in Warhammer 40000 have certain breeds called "Synapse Creatures" that connect lower-tier organisms into the Hive Mind. 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 other units have to pass a morale check or flee. Ethereals exert Mind Control over the other Tau castes; the morale check represents the breaking of that control. However, the enemy must press his advantage as early and long as possible, because the Tau might return to the field, and they will be pissed.
 * Done literally with the Undead in previous versions of Warhammer, if the general (normally a Necromancer or Vampire) was killed the army would quite literally disintegrate. This was toned down in later editions.
 * The spin-off game Warmaster has kill the enemies general as one of the generic win conditions for all armies.
 * This is a classic trope for fantasy RPGs like Dungeons and Dragons. Often times the Always Chaotic Evil 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 Big Bad manages to terrify them enough into cooperating under his leadership. The evil races' fear of and/or devotion to the Big Bad is all that keeps them cooperating. If the Big Bad 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 Big Bad before his armies can attack the outmatched forces of good.
 * One of the most notable examples in Dungeons and Dragons 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.
 * In Chess, the king is all that matters.

Video Games

 * The Zerg in Starcraft are defeated when their Overmind is killed. The Expansion Pack, 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 Starcraft II, tasks you to destroy a space station with overwhelming forces of zerg crawling on it by taking down its coolant towers.
 * All the units of the Scrin harvest escort fleet in Command and Conquer 3: Tiberium Wars are powered by 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 are aware of its importance: it's ludicrously well-defended with everything from stasis shields, Storm Columns and a space fleet to phase fields that render it invulnerable temporarily. Additionally, their original invasion plan called for multiple redundant nodes, but they got sidetracked.
 * A popular mechanic in Turn-Based Strategy games such as Fire Emblem, the Final Fantasy Tactics series and the Luminous Arc series: kill the boss of the stage causes all other enemies to flee, stand still or outright melt away, an Instant Win Condition.
 * Metal Gear Solid 2 Sons of Liberty subverts this: is designed so that it wouldn't be a keystone if it was ever actually used, because it's almost invincible as long as it has backup. When it doesn't have backup,.
 * In Supreme Commander: 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.
 * Justified in a realistic way in the slaughterhouse level of Tom Clancy's Splinter Cell. 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 Burn Notice 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 Order of the Stick, 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 Belkar.
 * 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.
 * 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 structure/organism to die.
 * Occurs twice in the Total War games. First, eliminating the general leading an army causes that army's moral 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 member 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.
 * Total War 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.
 * In Sonic Heroes, there are gold enemies that take all nearby 'bots with them when destroyed.
 * In the single-player mode of the early Battlefield series games, your AI teammates are so incompetent that they will constantly lose ground if you're not being Rambo on the front lines next to them.
 * Subverted at the end of Bad Company 2.
 * In single player Diablo, all surviving monsters die when Diablo is killed.
 * In Dragon Age Origins, killing the Archdemon immediately ends the Blight. Justified since the Archdemon's will is what unites the Always Chaotic Evil Darkspawn hordes into an organized military force. The expansion 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 incredibly powerful, it can even  So unless , the only way to end a Blight would be to kill every darkspawn in existence. Which is impossible considering they outnumber just about everybody.
 * 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 use another golem against some darkspawn in the next room.
 * In every Dynasty Warriors 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.
 * F.E.A.R.'s Replica Soldiers depend on a psychic commander for their orders and become completely inert when the commander dies.
 * A standard clause for battles in Exit Fate. 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.
 * Mass Effect - The Rachni have Hive Queens, who reside on toxic planets. The employment of Krogan (a sentient species that evolved on a Death World, so can survive on Rachni Homeworlds and attack the Hive Queens) was the turning point of the war.
 * The Skedar from Perfect Dark. They had been at war with the 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.
 * Valkyria Chronicles III: Invoked by The Nameless with the operation to assassinate Maximillian. It doesn't work, because said attempt is thwarted by Selvaria.
 * The Elder Scrolls V: Skyrim 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.
 * The Aparoids in Star Fox Assault. 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.

Western Animation

 * In the episode "Dark Heart" from Justice League Unlimited, an army of 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 Martian Manhunter frees their power source from the corrupted leader controlling it.
 * The Joining from The Batman 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.
 * Super Robot Monkey Team Hyperforce Go! featured in one episode the Vreen, a Horde of Alien Locusts from the future. Their weakness was the present-day bug they evolved from; when it was found and destroyed, the Vreen disappeared.
 * The Kim Possible movie 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.

Real Life

 * A noted weakness in armies led by a dictator. When the leader is removed, the army is useless. It's worth noting that many dictators, including Adolf Hitler, Benito Mussolini, Saddam Hussein, Idi Amin, and Moammar Ghaddafi, tend to be far better at terrorizing their populations than actually commanding their armies.
 * Toward the end of WWII the Allies stopped trying to assassinate Hitler because they were afraid he might be replaced by a competent military commander, prolonging the war.
 * 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.
 * While the battle of the Alamo was an eventual loss, the damage inflicted on the attacking army was so intense because the leaders wore decorative hats, which made them a primary sniper target. One of the reason why the leaders of armies today don't have much to separate them from their troops.
 * During WW 2, german snipers managed to take out a fair share of senior officers by aiming for... soldiers with moustaches. Apparently, a british officer could bear to part with his stripes and insignias, but NOT the pride of their upper lip.
 * Inverted in the Valkyrie plot to kill Hitler. In real life, Hitler had such a hold on the country, the fear was that simply assassinating him would have caused the Germans to unite around the surviving Nazis, declare him a martyr, and fight to the last man. The army would, indeed, carry out any action ordered by Hitler in the event of his death, however.