Kingmaker Scenario

A Kingmaker Scenario occurs when two sides in a conflict are evenly matched and the third one is unable to overpower the other two. The third power then becomes the "King Maker": while it cannot win itself, it can ally itself with one of the other two, breaking the stalemate and bringing victory to its chosen side ("crowning them king"). The term "kingmaker scenario" comes from the game theory and was most likely inspired by Richard Neville a.k.a. Warwick the Kingmaker, who, while not suitable for the British throne himself (owing to a lack of Royal Blood), installed and deposed two sovereigns in short succession during the Wars of the Roses.

In multiplayer games, a Kingmaker is a player who, while unable to win himself, is able (or forced) to determine which of the other players will win. This will usually end up being the one who the "kingmaker" likes more at the time. This scenario is disliked by players except in games where personal politics play a role in the game's outcome by design (such as Diplomacy), and a game where such a scenario occurs frequently is referred to as having the "Kingmaker Problem".

A more detailed examination of possible Kingmaker Scenarios can be found in this article, at least until the link dies. A subtrope of Melee a Trois. Compare X Must Not Win.

No Real Life Examples Please. They tend to degenerate into a political Conversation in The Main Page.

Anime and Manga

 * In the fourth round of Liar Game, Akiyama's group become this by having control of their own 5 man alliance and the Extras alliance.

Board Games

 * Quite frequently appears in Settlers of Catan, where two players will be roughly equidistant from winning when a third rolls a seven. Seven allows you to steal a card from a player and put the bandit on any part of the grid to block production, which can be used to cripple a vital resource.
 * White Wolf's Prince of the City, based on their Vampire: The Requiem RPG was probably designed for this. The winner is the player who has the most Prestige and is not in Torpor at the end of the final round. This means that one player can have an insurmountable lead over the others in terms of Prestige, only to have every other player beat him into Torpor and hand victory over to number two. "Everybody loves him! He must DIE!"
 * Common in Risk. Also commonly averted in that most time the Kingmaker can later win through careful planning.
 * Avalon Hill's King Maker is based on this trope. Set during the War of the Roses, each player controls a group of nobles, and tries to capture the royalty in each of the two conflicting houses. Once one of the houses is wiped out, the winner is the one with the most senior member of the surviving house. Neville the Kingmaker, mentioned above, is one of the stronger nobles in the game (though Percy is the most powerful).
 * While not official, it is very common in a Monopoly game for a player to just hand over all his savings and property to another and leave the game.

Card Games

 * This can come up in multi-player Magic the Gathering games. If one plays frequent games with more than two players, Kingmaker decks can become handy things to have...
 * This is at the heart of Apples to Apples, which switches which player is the "kingmaker" every round. The main strategy of the game is appealing to the judge's ideas of a given quality.
 * Played properly, though, the judge doesn't know who played which cards until he picks the winner.
 * In three-person Kaiser, a player with the 5 and the 3 but an otherwise weak hand is in this position. In the standard four-person version, they're simply a support to their ally.
 * The Jyhad: The Eternal Struggle CCG is designed around this. In multiplayer games, the rules dictate that a player can attack another player on his turn, and only that player. In return, he need only defend himself from attacks by one other player, with Hunter > Player > Hunted proceeding clockwise. This leads to (and indeed, the game encourages) discussions, arguments, and dealmaking among the players, as a player can offer not to attack his target, leaving the target free to devote resources to attacking his own target, or the player can offer concessions to keep another player off his back while he launches a full-fledged assault. In addition, when a player is eliminated, the hunting order skips over him to the next player, making long-term planning a must.

Film

 * The Good the Bad And The Ugly. Both Angel Eyes and Tuco know the name of the grave site - Blondie is initially disadvantaged as he only knows the name on the grave itself, but will eventually become the Kingmaker and the other two know this. After Angel Eyes finds out Tuco's half of the secret, the film spends a little time watching the two fighting over who gets Blondie. (He sides with  but out of choice. No-one gets to tell the Man With No Name what to do.)
 * Inverted in the ending of Gore Vidal's The Best Man, in which two presidential candidates, played by Henry Fonda and Cliff Robertson, are tied in their race for the nomination.

Literature

 * Criston Cole, the Lord Commander of the Kingsguard was literally called "The Kingmaker" in A Song of Ice and Fire. He championed Viserys I's younger son Aegon II over his chosen heir, elder daughter Rhaenyra. This led to the Dance of the Dragons, a bloody civil war that tore the realm in half (including the Kingsguard itself) and led to the death of most of the Targaryen dragons. Aegon II won, but was ultimately succeeded by Rhaenyra's son.
 * Raul re Flo, who is more or less an expy of the original Warwick the Kingmaker, is also this in Arcia Chronicles. However, his nickname is "King of Kings".

Live Action TV

 * Deliberately used in Survivor - the eliminated contestants vote between the final two for the winner.
 * Subverted in that, since Cook Islands, seasons have regularly ended with a final 3, this was to avoid a Kingmaker situation. The winner of the last immunity challenge would either be a dominant force that would take out their opponent, or they would be the Kingmaker, and hand the million dollars to another player. Of course, now you just have strong players trying to (and succeeding in) bringing two unpopular contestants to the jury vote.
 * Kingmakers have appeared in almost every season since the fourth season in Marquesas.
 * In the first season, the final three consisted of Rudy, who everyone liked, and two fairly-hated contestants. Richard Hatch threw the last immunity challenge, forcing the other hated contestant to be kingmaker, which in this case is a bad thing; she had to vote Rudy off (she would have lost the final vote against him), which made her even more hated, giving Hatch a close win. If Hatch had been kingmaker, he almost certainly would have lost the final vote no matter who he let through.
 * The Apprentice usually does a take on this in the final episode of the season - eliminated contestants will come back and work for the finalists on a big final project. In some cases, they do their jobs in an exemplary manner. In others... not so much.
 * Hells Kitchen does this as well, with pretty much the same results except for the addition of a Badass sous chef threatening to hunt people down if they ruin the finalist's chances.

Tabletop Games

 * One of the scenes in the sample campaign from Nobilis is explicitly this. There's a miracle duel between two rival powers in the home base of the PCs. The GM's advice points out that, since the players are a handful of demigods of the same power level as the competitors, the winner is going to be the one who gets help from the PCs.

Video Games

 * Nearly all turn-based strategy games reach a point where the contest is only between the most powerful players or AI, but the other factions are strong enough to tip the balance. A few, such as the Civilization games, even have a mode of victory where the strongest have to gather the support of weaker nations over their competitive rivals. It should be noted, however, that in a strict Kingmaker Scenario, it must be literally impossible for the kingmaker to win, not just highly unlikely.
 * The Drinking game in Yohoho Puzzle Pirates can result in a Kingmaker Scenario when played with three or more players. It is possible (and not terribly uncommon) for two players to be one move away from a win simultaneously, while a third player is active with possible moves that would allow them to "steal" 20 points from either of the two leaders, allowing the other leader to win on his/her next move.
 * Can happen in Galactic Civilisations II quite a bit, as the AI plays smart enough to make several-way-stalemates a common occurrence.
 * Part of the plot of Shin Megami Tensei Nocturne: the Demi-Fiend himself cannot create a Reason (due to his demonic body), but he can choose to support those created by others. Whichever Reason he supports will succeed without fail (or as close as you can get in an Atlus game). The problem with this system:
 * Occurs early on in Digital Devil Saga: the Maribel and Solids Tribes are at a standstill, so the Embryon's tactician suggests forming an alliance with the former to destroy the later.
 * Kingmaker scenarios are fairly common in more open-ended roleplaying games, as they allow some player agency without having to account for every possible action the player might carry out.
 * In the Orzammar portion of Dragon Age, you come into such a scenario, between the named-heir Pyral Harrowmont, and King's son Bhelen Aeducan. Makes for an interesting scenario when your origin is Bhelen's older sibling . Of course, no matter what your origin is, you can't become the ruler of Orzammar yourself.
 * Also applies to deciding Ferelden's ruler if your character isn't a human noble. Otherwise, there's also an option to marry one of the contenders for the throne.
 * In Might and Magic VII, you play as a quartet of new royals. Halfway through the game, you have to choose between allying with the side of light or the side of darkness. The side you ally with wins, because, all for the sake of getting one more Control Cube. IF it were possible to not choose a side at all, you would then have to kill the leaders of both sides, and you'd never be able to repair the Heavenly Forge anyway or the portal to the Gateweb ...
 * The Human-Elf War can look like this, although your influence can both tip the balance to one side and change the nature of the peace (after a few relatively minor skirmishes and actions, or after a bloody war).
 * In Might and Magic III, one of the quests is to give eleven (out of thirty-one) Orbs of Ultimate Power to the King of Good, Neutrality or Evil, which gives them theoretical governance over Terra and destroys the other two kings.
 * The Darkling class in Dokapon Kingdom is made for kingmaking; they can't advance in the quest for gold, and only the player in last place can fall to darkness, but the class is designed for dragging other players down. Also, through an exploit, the Darkling can be used to give overpowered equipment to another player with no penalty: Darkling gear vanishes when you stop being a Darkling, but only if it's in the Darkling player's hands. (In theory, a Darkling player could trade his equipment off to another player and then ask for it back when he comes out of it, but expecting the other player to follow through is a wee bit unlikely.)
 * In Fallout New Vegas, this is more or less the situation going on at the start of the game, with neither the NCR, Legion, or Mr. House having the capability to defeat one opposing faction without falling to the other. It's up to the Courier to break the deadlock in any way they can..
 * In Mario Kart 64's Battle Mode, anyone who runs out of Balloons when there are still 2 or more players left becomes a Mini Bomb Kart. They've officially lost that match but can drive right up to any remaining players and explode in their faces, then respawn in a few seconds to do it again.
 * In The Elder Scrolls V Skyrim the Player Character is this, depending on which side you choose in the Civil War campaign. Whichever faction you side with wins, placing their respective Jarl on the throne.

Western Animation

 * In Exo Squad, the Pirate Clans become the kingmaker in the Neosapien War, since despite their military strength being too small to beat both Exofleet and Neosapiens, their allegiance proves crucial for either side to win the war.