Lilac: It was the moment you brought it to our homeworld!
Factions 1 and 2 of World A have been fighting for eons. Then out of luck, Faction 1 finds World B, which was completely unrelated to the war and had no involvement whatsoever. Faction 1 might send spies into World B, in hopes that it proves useful to them. Faction 2 later learns of World B's existence and sends its forces there, turning World B into another battleground for World A's war.
In a situation like this, the inhabitants of World B would be taken completely by surprise and must find a way to deal with this unexpected twist in events. Some may take sides among the various factions. Others may try to become intermediaries, hoping to find some kind of diplomatic solution. And there are those who want to become a third faction to drive out both.
The warring factions don't exactly have to be Trapped in Another World either, they just have to fight each other on World B, which had absolutely had nothing to do with them until now. The war could be either secretive or full-scale, old or pretty recent. The trope does not count if World B had public knowledge of Factions 1 & 2 and got involved beforehand, which would just be a neutral country being attacked by both of its neighbors. However, it can apply to an isolated island that gets invaded by two or countries elsewhere.
Compare Alien Invasion, which is a war between an invading world and the invaded one. Contrast Truce Zone, an area where warring agree not to fight, especially if the areas are neutral, have nothing to do with the war, or are too important to risk any damage. This may involve Invading Refugees. See also Proxy War when the offworlders try to use groups on the neutral planet against their enemies.
- Aura Battler Dunbine is a very unique isekai series not just by preceding most of the modern hits of the genre, but also having the alternate world of Byston Well undergo a Mass Teleportation into the modern world a.k.a. Upper Earth.
- Considering the first time Aura Battlers appeared, the Upper Earth military attacks first, resulting in the annihilation of most air forces, the liberal use of nuclear weapons, and up to total destruction of entire cities including national capitals. Drake later takes advantage of his affiliation with those from the United States (i.e. Shot Weapon and Todd Guinness) to acquire a staging ground for conquering the rest of Upper Earth. At the same time, one of Drake's more powerful allies indiscriminately attacks cities in Europe, causing the continent's nations to cast their support for the heroes.
- What really separates this apart from other series is how Aura is at least ten times more potent in Upper Earth than in Byston Well. Shou witnesses first hand that small missiles only made for destroying Aura Battlers have the power of mini-nukes, killing over a million people in Tokyo, and Aura Barriers are able to shake off nuclear weapons like nothing. The whole final arc is about the rebels stopping Drake before the world turns into a desolated wasteland.
- In The Rising of the Shield Hero, the Four Heroes are all summoned via a magical ritual to the realm that the Kingdom of Melromarc is located, each of the heroes summoned from our world (or perhaps multiple, alternate versions of our world) with the goal of helping fend off the various monsters and magical threats that plague the land via the Waves of Calamity. The ritual equips them with their own magical, sacred weapons (a spear, a sword, a bow and a shield) that imbues them with uniques stats and abilities. The Waves of Calamity are the result of two or more parallel worlds fusing together. Some of the worlds already have their own wars going on, which have now poured into the kingdom, resulting in even more chaos.
- Blackest Night: The main plot is about the war between the Black Lantern Corps and all the rest of the Lantern Corps of the universe, who have to team-up against their will to stop a bigger evil and not become the next Black Lantern. In the last issues, the intergalactic war goes to Earth since Nekron found the Entity of Life hidden on Earth, guiding all his Corps to Earth so he can eliminate it and all forms of life on the universe.
- In Saga, the long-running war between the planet Landfall and its satellite Wreath has been outsourced to other planets, as the leaders of both planets quickly realized that they could not wage war on each other's worlds without causing drastic environmental effects on their own.
- Wild CATS: This was the series' original concept, with agents of the good-coded Kherubim and evil-coded Daemonites fighting a Secret War on Earth. Subsequently things got more complicated.
- Angelic Resonance: Although not a war, the story starts with the Time-Space Administration Bureau's pursuit of the Huckebein going horribly wrong and throws them in the Empire of Misurugi. The Bureau quickly pick up the First Princess and her maid abroad while the Eclipse-infected killers go on a rampage. Then Leonard's Amalgam faction gets involved.
- The Conversion Bureau: Some stories have the war between Earth and the Solar Empire Equestria spilling over into canon Equestria.
- Some Gate fics or inspired fics have several gates opening to the Special Region from Earth. The result is this trope as the nations or factions war with each other, but take the fighting to the Special Region with different policies over how to deal with the inhabitants.
- In Weight of the World, the people of Remnant have been at war with Salem and her Grimm for millennia. Atlas, a Kingdom of Remnant, discovers Earth's existence and secretly sends spies to the planet to see if its people and resources can be useful to their side. A decade later, an Atlas traitor tells Salem about Earth's existence. Out of the desire to kill any and all potential hosts for Ozpin, Salem sends hordes of her Grimm to Earth, including Spawner-types, which spawn an endless amount of Grimm. This pulls Earth into Remnant's war. Earth is not happy to be dragged into another world's war, and the Terran death toll rises so much that some parties consider using nuclear weapons on Remnant in order to kill the Grimm at their source and save their world.
- Conversely, Dust And Echoes has the UNSC discovering Remnant in the midst of the war with the Covenant. They know the Covenant will find the world sooner or later(indeed the Action Prologue is just that happening, before the story proper shows How We Got Here), and that Remnant's people and technologies offer something that can turn the war around in their favor. But High Command has punted the Idiot Ball for once, and is being very careful to ally/annex Remnant as positively as possible.
- In Star Wars: Revenge of the Sith, this is how the Utapauns regard the Clone Wars between the Galactic Republic and the Separatist Confederacy — at least, until General Grievous and the Separatist Council set up shop on Utapau. Tion Medon ostensibly tries to sell this attitude to Obi-Wan Kenobi when he shows up looking for Grievous, before whispering the Jedi Master the truth.
Tion: Greetings, young Jedi. What brings you to our remote sanctuary?
Obi-Wan: Unfortunately, the war.
Tion: There's no war here, unless you brought it with you.
- This Island Earth: The Metalunans kidnap a pair of human scientists to help in their war with the Zagons.
- Transformers is often about the Autobots and Decepticons fighting on Earth, with humans being caught in the middle. The Michael Bay-directed Transformers series has humans repeatedly change sides during the war. The first couple movies have them work alongside the Autobots after constant attacks by the Decepticons, even forming the N.E.S.T. government organization. The next couple movies show some humans turning on the Autobots, from those wanting them to leave and take the fighting someplace else to outright helping the Decepticons. The last few movies feature them hunting down both groups with only a few remaining loyal to the real heroes of the story. However, the humans never truly sided with Decepticons, except for Dylan Gould. In the first three movies they stand with Autobots. But after an Autobot betrays his fellow Cybertonians and Earth as whole, leading to millions dead, the humanity could no longer trust Autobots. In Age of Extinction humanity fights for itself, having Lockdown as a temporary ally. In Last Knight, humans once again stand for themselves, allying with Megatron only because he could deliver the weapon they were searching. However, then Cybertron arrives near Earth and Quintessa plans to destroy it, humans ally with Autobots once again to save their world. The first movie's tagline is even "Their war. Our world."
- Wonder Woman (2017): The warring Germans follow fleeing British spy Steve Trevor across the veil into Themyscira, a land neither side previously knew existed. Upon seeing the Amazons, the Germans assume they are aiding the British and immediately attack, leading to a bloody beach battle.
- In Animorphs, the Yeerks were invading Earth anyways to obtain hosts, but it ends up the strategic battlefield for the Yeerk/Andalite war.
- Good Omens: Earth is prophesied to be the site of the final battle between Heaven and Hell, which would have the unfortunate side effect of ending the world. Defied by the Anti Anti Christ Adam, who tells the assembled angelic and diabolic legions to pack it in and go somewhere else.
- Confederation of Valor: In the backstory, the Confederation became embroiled in a war with a rival alliance referred to as "the Others", and is made up of species that had long since evolved away from war, made First Contact with humanity and gave them advanced technology in exchange for fighting the war for them.
- In Invasion: Earth by Harry Harrison, a group of alien ships requests aid from Earth against an incoming group of invaders. There is a big twist at the end.
- The Liberation of Earth, a classic short story by William Tenn has two alien species "liberating" Earth from each other as part of a galactic war. They leave the planet wrecked nearly to pieces, and the whole thing is barely a footnote for them.
- Magic, Metahumans, Martians and Mushroom Clouds: An Alternate Cold War: In May of 1968, France suffers a brief military incursion from an Alternate Universe where the Bonaparte dynasty never fell and took over the world, with them now seeking to becoming Multiversal Conquerors.
- In X-Wing: Starfighters of Adumar, the neutral planet Adumar is approached simultaneously by both the New Republic and the Imperial Remnant, as its large war production industry (for the Adumaris' wars with each other) could readily be upgraded to manufacture weapons for the continuing galactic war.
- Falling Skies has the Earth suffer from an Alien Invasion, which then evolves into a battleground for other alien races working for or against humanity.
- Star Trek: The Original Series:
- Inverted in "A Taste of Armageddon". The Enterprise crew are conducting negotiations with the Eminians when the computer that simulates their war with a neighboring planet records a hit on the ship, and the Eminians demand that Kirk and his crew report for execution to "simulate" deaths from the attack. Kirk instead destroys the computer, and the Federation mediates an end to the war.
- Two episodes deal with the Space Cold War between the Federation and the Klingon Empire bleeding over onto neutral planets.
- In "A Private Little War", the Klingons give muskets to one set of tribes on a pre-industrial planet so they can conquer their enemies in exchange for allegiance, starting a Proxy War. Kirk is forced to arm the rival tribes similarly to restore the balance of power.
- In "Errand of Mercy", however, the natives turn out to be Sufficiently Advanced Aliens and force both sides to back down, mediating a truce.
- V features a conflict between the Visitors and a rebel group known as Fifth Column who want save humanity and other races from being devoured. It is mentioned that said Visitors are currently at war with other races as well.
- In the Space: 1999 episode "The Last Enemy", the Moon drifts into a star system that contains two planets, on opposite sides of their sun, which are at war with each other. Both sides land battleships on the lunar surface in order to launch missiles at the other planet and are subject to massive retaliation by their enemy, endangering Moonbase Alpha.
- Downplayed in Doctor Who: Earth sometimes becomes strategically important in the Forever War between the Sontarans and the Rutans, leading to anything from a scout to an invasion plan by one side or the other in various episodes. However, the two sides have never actually squared off on Earth, which is a long way from the main battlegrounds and remote enough that by the time one side takes an interest in it the other is long gone.
- Dungeons & Dragons: The Plane of Hades is the primary battleground in the unending Blood War between the devils of the Nine Hells and the demons of the Abyss. Hades' native Yugoloths have mostly relocated to a different Plane, Gehenna, and have built a thriving lifestyle out of being arms dealers and mercenaries in the never-ending conflict.
- Most forms of the Transformers boil down to this: The war between the Autobots and Decepticons spills over to Earth. Sometimes accidentally, sometimes on purpose. Usually depending on which side found us first.
- Freedom Planet: Torque points out that the war his team has been putting up with against Brevon's army has resulted in many innocent worlds being destroyed by Brevon's hand, and the reason he and Brevon came to Avalice is for the kingdom stone. Brevon wants the kingdom stone for his army, while Torque wants to ensure that Brevon doesn't get it by any means. Upon hearing Torque's story, Lilac, Carol, and Milla all agree to help him stop Brevon, because they consider Torque a friend and since their planet is just as much on the line.
- Iji opens with an Alien Invasion of Earth by the Tasen, but it soon turns out that they're only here because they're on the run from the Komato, who have long been waging a war of extermination against the Tasen.
- The Sid Meiers Alpha Centauri expansion "Alien Crossfire". Two ships from warring factions of an alien race, the Manifold Caretakers and Manifold Usurpers, crash land on the same planet being colonized by humanity's remnants.
- Starcraft starts as one of these at the beginning. The Terrans were the only known sentient beings in the Koprulu sector with the Confederacy as the primary superpower. Then the Zerg and Protoss show up, who fought each other for a longer period of time and brought the war right to their doorstep.
- Starwing Paradox: Two great powers on the living planet of Meguriboshi have fought whoever knows how long for powerful energy resources. Their main weapons, the Air-reals, can only have their full power activated by have pilots with superior aptitude. As a result, they select Earth to recruit the players in hopes of finding said pilots.
- Super Robot Wars X has the X-Cross fight sandwiched on a two-sided front, with one being the Empire of Misurugi waging war on the inhabitants of Al-Warth. How does such a nation with abysmal weapons besides its Bug-inspired drones do that? They summon people from existing wars (hence the Mariemeia Army insurrection, Zogilia-Free Pact Alliance War, all the conflicts within Gundam: Reconguista in G, with some Universal Century antagonists for good measure) and recruit the more antagonistic factions to do the fighting for them.
- World of Warcraft: This has happened no less than four times in the overall timeline of the setting.
- The first time was the Orcish Horde on Draenor — twisted by the Burning Legion — which fought a war of annihilation against the Draenei (in an attempt by the leaders of the Burning Legion to wipe them out for their "betrayal" 50,000 years ago, which translates as "not joining them in their war to eradicate all life in the universe") and eventually expanded the war into an invasion of Azeroth.
- The second time was the armies of the Alliance invading Draenor in retaliation, carrying the war back to its world of origin. Twice — immediately after the first invasion, and subsequently during the events of The Burning Crusade against the Legion itself (and a few others).
- The fourth time was (yet another) invasion from an alternate timeline of Draenor to Azeroth, and the counter-invasion in Warlords of Draenor.
- The premise of Xenoblade Chronicles X starts with Earth getting blown up in the crossfire of two alien forces. While one of the two factions actually was trying to kill off humanity, the planet was blown up by accident before they had a chance to actually destroy it. Humanity just barely managed to escape, and most of the fleeing ships didn't even make it out of orbit.
- Kill Six Billion Demons: The Universal War raged across 777,777 worlds as a group of insane God-Emperors with dreadful Enlightenment Superpowers discovered The Multiverse, laid claim to entire planets and then ravaged each other's territories. Those that survived the Conquest didn't fare much better, being ruthlessly exploited for their resources.
White Chain: Having cut God's work, the Demiurges began to open all the multiverse. The Second Conquest was long and brutal. ...At first, they pretended their war was civilized, but then their singular hunger for dominion consumed them. And they went mad. Worlds burned, Allison. They burned.
- The Justice League second season finale has Thanagarians destroy an invading Gordanian warship and offer their assistance in building a shield generator against future attacks. Then it turns out their plans are a bit different and more... desperate.
- Star Wars: The Clone Wars: Occurs with somewhat increasing frequency because of Palpatine's machinations forcing many of the neutral planets to side with either the Republic or the Separatists. This is most prominent in the episodes "Jedi Crash" and "Defenders of Peace" in Season 1, when Jedi Knight Anakin Skywalker, his padawan Ashoka Tano, and Jedi Master Aayla Secura crash land on Maridun. When the native peaceful Lurmen agree to aid the grievously injured Skywalker, the Separatists arrive not long after to claim the planet as their colony. Despite the combined efforts of the Jedi, their Clone Troopers, and the native Lurmen successfully destroying the Separatist forces, the Lurmen ultimately agree to join the Republic.
- Teenage Mutant Ninja Turtles (2003): The Season 3 premiere, which consists of two back-to-back Multi-Part Episodes, is all about this trope as Earth finds itself caught in the crossfire of an Evil vs. Evil conflict between the Triceraton Empire and the Federation. Initially, the Triceratons invade Earth as part of their hunt for the Fugitoid, a Living MacGuffin who possesses the blueprints for a teleportation device that both sides seek to weaponize. Later, the Federation also gets involved and secretly make a deal with the US government to capture the Fugitoid. The three-part episodes where the two rival empires finally duke it out is appropriately titled "Worlds Collide".