Enforced Cold War

When two hostile parties are at the edge of a direct conflict that would undermine the series or make it unacceptably violent for the premise, a higher power (ranging from parents or school officials up to god-like beings) will often intervene to maintain the status quo.

This can have the effect of forcing two groups of enemies into regular contact with each other, while preventing the fight from escalating, driving the plot into other forms of conflict (e.g., school contests, supporting opposing factions of minor groups, etc.) and not coincidentally mimicking the sort of uneasy peace that typified the Cold War between the US and the USSR during the late 20th century (where the threat of total nuclear annihilation was the higher power keeping the peace between the US and Russia).

Often allows the Bad Guys to taunt the heroes with impunity. If so, there will almost certainly be at least one scene where one of the heroes has to be held back by their teammates, usually while they shout something like "Let me at em!"

It's common for there to be a pair of Star-Crossed Lovers separated from each other by being on different sides of the fight.

Anime & Manga

 * In To Love Ru, Rito is forced to go to school with Yami, the infamous alien assasin known throughout the galaxy who wants to kill Rito, but doesn't because she's best friends with his sister. She will however try if he does anything ecchi towards her which has Rito narrowly escaping death on a daily basis
 * In Last Exile, the countries of Anatoray and Disith are engaged in a long and bloody war under the supervision of the mysterious Guild, which directs the official battles which take place in the air between the country's respective air forces. The enforcement comes into play when one side gains too much of an advantage and is stopped forcefully by the forces of the Guild, in order to maintain the status quo.
 * In the Ah My Goddess universe, actual battle between the gods and demons is prevented by the Doublet System. Kill someone from either side and someone else from the opposite side would literally drop dead on the spot. As no one (save one notable exception) knows who their doublet is, this prevents the saner ones from simply starting Ragnarok early. For the insane ones there's always the direct intervention of the Daimakaichou or the Almighty.
 * One Piece:
 * Dual Parallel Trouble Adventure utilized a very literal example of this trope. The two waring mecha factions in the show only fought at pre-scheduled places and times, and the pilots were allowed to tap out when it looked like were going to lose. As in the real life Cold War, the point of this was to keep the world from being wrecked by the weapons of mass destruction possessed by both sides.

Comics

 * The planets Apokalips and New Genesis in The DCU. The first issues of New Gods and Mister Miracle explained the peace treaty between them, which additionally resulted in the characters of Orion and Mister Miracle.
 * "The Baron's Peace" in Girl Genius is one of these. Said Baron is a Type 1 Unfettered who will do anything to maintain peace, and he controls an unequalled superpower that lets him do so. He is an old and very experienced mad scientist, in a setting where most of them tend to die quite young. He leads the continent's largest military force, overwhelming air superiority, and the most advanced military hardware/clockwork troops/mutant brigades. After a Zombie Apocalypse he was personally responsible for rebuilding European society through willpower and force, and for sectioning off the parts of it which are toxic or infested. He doesn't care how much the nobility squabble over land or ancestry, but if a battle ever breaks out, he shuts down both sides with complete overkill. That is The Baron's Peace.

Fan Works

 * In fandoms where Ship to Ship Combat is prevalent, many of the major message boards have rules against flaming other members or ships, preventing the ship wars from becoming too terribly violent. The major Avatar: The Last Airbender boards in particular are known to clamp down heavily hard on flame wars between Kataang and Zutara shippers.
 * The Great Joel vs Mike Flamewar that broke out in MST3K fandom when the latter took over hosting duties for the former in 1994 pitted MSTie against MSTie and very nearly wiped out all life as we know it. These days on MST3K message boards, it's generally a topic discussed in only the most coldly logical, unemotional terms lest ancient hatreds come to the fore. Flaming people over it is swiftly and brutally punished by moderators.
 * This editor once wrote a short story centering around a pirate/ninja cold war, which had to be that way because all the cowboys, robots, samurai, and other cool things were forcing them to play nice on the surface.

Literature

 * The Gryffindor/Slytherin conflict in Harry Potter.
 * Including one specific scene in Chamber of Secrets with the "Let me at 'em!" mentality, in which three of the Gryffindor boys have to physically restrain Ron from hitting Draco Malfoy. (To be fair to Ron, Draco has just said he was sorry Hermione didn't get killed by the Basilisk.)
 * Comes out in full force around Quidditch matches, where Gryffindors and Slytherins usually end up in the hospital wing with antlers and leaks in their body. During the fifth book Harry had to walk with a protective guard of Gryffindors because the Slytherins kept trying to sabotage him.
 * The treaty between Watches in the Night Watch 'verse, which is openly inspired (bordering on the Anvilicious actually) by the actual Cold War. Add two millennia-old Chessmasters with magic powers enough to make the most absurd Gambit Roulette seem timid.
 * In The Dresden Files, the White Council and the vampire Courts start the series in an uneasy peace, enforced by the terms of the Unseelie Accords. Eventually Dresden is forced to choose between letting his lover die and starting the magical equivalent of World War III; this is of course a classic setup for Take a Third Option.
 * Inverted in the Isaac Asimov story, "The Gentle Vultures". In it, pacifist, herbivorous aliens go from world to world to help the survivors of the nuclear war that inevitably breaks out in every intelligent speccies except them. This help is given in return for tribute in the form of resources, creating a form of economic imperialism. When the aliens get to Earth, though, they discover that the Cold War situation is keeping nuclear war from occurring, and may do so indefinitely.

Live Action TV

 * Star Trek:
 * The Organian Peace Treaty, forced on the Federation and the Klingons by Sufficiently Advanced Aliens who found war in their backyard distasteful. This lead to a series of more direct Cold War metaphors, such as both sides intervening in a border planet's war.
 * Deep Space Nine also had the sixth season opening arc in which the Dominion was in control of the station. To avoid collateral damage, Bajor did not ally with the Federation, so the main characters affiliated with the Bajoran government stayed on-board and worked side by side with the occupation forces. (Then again, they did eventually form an active resistance cell.)
 * Though in general, Deep Space Nine subverted this, as the Dominion and the Federation, who were hostile toward each other since they met in season 2, eventually did go to war at the end of season 5.
 * In the 1970s Battlestar Galactica Classic, the fleet encounters a planet named Terra, occupied by two factions on the brink of nuclear war with one another. In this instance, the Galactica plays the role of the sufficiently advanced being, shooting down both factions' missiles as they attempt to launch and intimidating them into pursuing peaceful negotiation.

Tabletop Games

 * Dungeons and Dragons:
 * Most drow lives like that, in the framework of Lolth theocracy: otherwise infighting turns into Ax Crazy all-out war and then genocide -- and she needs them alive.
 * In Forgotten Realms setting: enforced in the House wars of Menzoberranzan: espionage, conspiracy and secret raids against rival houses are permitted, even encouraged, but woe be to those who get caught. Failures to enforce it caused whole cities to be utterly destroyed (Golothaer, Bhaerynden aka Telantiwar aka Great Rift), ruined and taken by external force (Ched Nasad) or weakened and massacred by neighbours (Maerimydra). Menzoberranzan barely escaped the same fate twice during Silence of Lolth (seven months) alone. And Baenre rule Menzoberranzan because it was their matron who said "stop the madness" when all-out fight began, Menzoberra was killed and the cavern itself seriously reshaped. They are that aggressive.
 * Warhammer 40000: Oddly enough, given that in the forty-first millennium there is only war, this occurs for varying stretches of time between the Imperium of Man and various alien empires simply due to the fact that while the Imperium or said alien empire is tied up elsewhere a hostile peace exists between them. This occurs mostly with races such as the Tau Empire, which has on-off peaces with the Imperium on its border, rather than Ax Crazy races like the Orks or all-devouring galactic munchers like the Tyranids.
 * In Magic the Gathering's Ravnica Block, the different guilds keep the peace using the Guildpact. It sets statutes and ordinances for each guild so that each of the ten guilds has its own place. Of course, this doesn't stop the various guild leaders from trying to find as many loopholes as possible.

Theatre

 * Older Than Steam: Romeo and Juliet is, of course, the Ur-Example.

Video Games

 * An interesting variant shows up in Knights of the Old Republic: the Republic and the Sith are very much openly at war with each other, but both sides are dependent on trade with the neutral planet of Manaan, which supplies a unique and very valuable mineral used in medical supplies that neither army can do without. Thus, both the Republic and the Sith maintain a military presence and an embassy on Manaan, but have to avoid direct conflict lest the government cut off trade with the aggressor, and many of the quests on the planet are outright illegal acts of espionage against the Sith.
 * Heavily implied to be the sole purpose of Ravens' NEST in Armored Core (and its later incarnations: Nerves Concord, Global Cortex and Raven's Ark). When Companies are strong enough to wield walker mechs, floating battleships and everything in-between, only skilled, non-affiliated Ravens can keep or even enforce a level playing field between feuding Companies. This is subverted in later incarnations of Armored Core 3 timeline, where the then Raven organization, Ravens' Ark, was, in essence, "bought" by Companies, triggering the conflict in Last Raven.

Web Comics

 * Girl Genius: here one case is shown and another is mentioned.

Web Original

 * Red vs. Blue revolves around two ostensibly warring factions who rarely attack each other and never do it effectively, with the only medic blatantly working for both armies. This should have been a bigger tip-off that Red and Blue were in fact commanded by the same government, and the war was a construct. Of course, the teams are also idiots.

Western Animation

 * In X-Men: Evolution, the Xavier Institute students and the Brotherhood boys attend the same high school, at least keeping them civil on campus. Magneto also kept the latter party from accidentally exposing the existence of mutants.
 * In the late second season of WITCH, the heroines are unable to defeat Big Bad Nerissa because she possesses the Heart of Meridian. Only two people can take it from her, and one of them is imprisoned inside of it. The other? They disguise him as a student at their school, and for a brief time, Hilarity Ensues as Fish Out of Water  stumbles through the trials and tribulations of high-school life. However,
 * In the third season of The Venture Brothers, The Guild of Calamitous Intent forces Affably Evil supervillain The Monarch to give up Dr. Venture as his arch-nemesis and find somebody else to antagonize. He doesn't take it well.
 * Code Lyoko: The Supercomputer is the team's main resource, and Aelita's survival is dependant on it. It also happens to house their enemy XANA

Real Life

 * Anthropologists claim to have discovered tribes in which it was a common custom for the young GlorySeekers of rival tribes to fight each other until the elders decided it was enough and ordered it stopped. This sounds more like a tournament then a war (though with real weapons like the earlier knightly tournaments). However the fine distinctions between crime sprees, war, sport, and comparatively normal mayhem might be difficult with tribal relations given the low organization.
 * Note however that claims of this sort of thing are generally very contentious and heavily laced with Unfortunate Implications. Anthropology has a lot of variant mileage.
 * Arguably, the original Cold War was enforced as well, not by God or Sufficiently Advanced Aliens but by fear of the consequences -best-case scenario was a bodycount to rival the First World War, worst-case was the end of human civilisation - if it went hot.