''Our Worlds at War'' (2001) was a DC CrisisCrossover that brought nearly every hero and team, and a few villains, together to stop an entity known as Imperiex, whose stated goal was to destroy the entire Multiverse in order to start a new, "perfect" universe (apparently the existing Universe had some strange flaw that bugged the crap out of him, so ''of course'' destroying it all and starting over was the best solution). Having already destroyed several worlds, Imperiex heads straight for Earth, which as it turns out, is a literal CosmicKeystone which holds the entire multiverse together. So alien refugees from all over the cosmos head to Earth to make a final stand.

Franchise/{{Superman}}, being the primary character of the event, is forced to set aside his enmity against then-President ComicBook/LexLuthor, General Zod (who ruled an Eastern European nation) and even Doomsday and ''ComicBook/{{Darkseid}}'' in order to combat this cosmic threat. Superman's greatest challenge wasn't simply physical (Each Imperex "Probe" was 8 feet tall, stronger than Superman, invulnerable, and if it ''did'' have its armor pierced, it would detonate with the force of an atomic bomb), but emotional. Superman could only watch as allies and family became wounded (Half the Franchise/JusticeLeagueOfAmerica), were killed (Franchise/WonderWoman's mother, ComicBook/{{Steel}}, ComicBook/{{Aquaman}}, many others), went missing (Superman's parents both presumed dead during the arc), and even stood by helplessly as ComicBook/LoisLane went sobbing into ''Luthor's'' arms when her father died in a kamikaze blitz while defending the White House.

The crap ''really'' hits the fan when Imperiex is defeated (wait for it)... and ComicBook/{{Brainiac}} appears, absorbs Imperiex's immortal essence, becoming a PhysicalGod, and sets upon ''absorbing'' the entire universe (instead of destroying it). Turns out Brainiac had orchestrated the ''entire'' scenario (including using Lex Luthor's daughter to guide him into doing everything that had to be done to destroy Imperiex), and with all the universe's defenders dead, defeated or at the very least severely weakened, nothing would be able to stop him...
!!This Crossover contained examples of:
* AfterlifeExpress: The Black Racer, who goes carting around fallen heroes. This causes ComicBook/YoungJustice to crash-land on Apokolips when Lil Lobo disobeys ComicBook/{{Robin}} and chases after the Racer.
* AnyoneCanDie: Superman's parents, Wonder Woman's mother, Lois's dad, Aquaman (taking all of Atlantis with him), Guy Gardner, Steel and Impulse (sorta), though they all eventually get better. Oh, and Doomsday.
* BackFromTheDead: Everyone above ''eventually'', but Steel gets it within the arc when Darkseid bonds Steel's corpse to the Aegis Armor.
** ComicBook/SgtRock came back and served on Luthor's cabinet during the war. At the end it is implied that Amanda Waller got John Constantine to revive him for the duration of the war.
* BigBad: At first, Imperiex is thought to be the main villain, but it turns out Brainiac was the true mastermind.
* DefiantToTheEnd: All over the place: Lil' Lobo (versus the entire Apokoliptian army), Steel (he discovered Brainiac was cloaked and tried to alert Superman, but Luna appeared and point-blank blasted him. Before he died, he wrote out "Warwor-" in ''his own blood'', but this went unnoticed by Superman).
* DiscOneFinalBoss: Imperiex ends up this to Brainiac.
* DwindlingParty: Half the Justice League is K.O'd in the opening battle of the arc, and supporting characters drop like flies throughout all chapters. Only the Justice Society was immune to this, making their rescue of the Daximites from Imperiex (he was using them as an energy source) seem easy (They DID lose Hippolata, but that happened in an issue of Wonder Woman).
* EnemyMine: The entire crossover lives, eats and breathes this concept.
* EndingMemorialService: In an arc with this many deaths and death tropes, this trope was unavoidable. One notable "Funeral" was a large hologram of Aquaman placed in the Moses-like gap in the Atlantic Ocean left by Aquaman's defeat by/destruction of an Imperiex Probe.
* ExplosiveDecompression: When the Space Ark that everyone is on begins to buckle under the assault of two Imperiex Probes, an injured GreenLantern (Kyle Rayner) tries fruitlessly to keep the ship together. Diana, recovering from her own injuries, lends a hand (literally and emotionally), and Kyle whips it into high gear, preventing this trope with the use of SUPER-glue (and yes, it was "Super" replaced with Superman's insignia).
* {{Foreshadowing}}: About six months before the crossover, there was an ItsAWonderfulPlot issue of ''{{Superman}}: The Man of Steel'' in which Liri Lee of [[TimePolice the Linear Men]] showed Clark the BadFuture that would result if he gave up on Earth once Luthor was President. Luthor basically ruled the world, following a war which had seen the deaths of many heroes, including "the Patriarch" (presumably Aquaman), "the Queen Mother" (Hippolyta) and "the Young Speedster" (in the event, Impulse only ''almost'' died, but then, Superman [[ForWantOfANail hadn't left]]...)
* FromASingleCell: The army of Lil' Lobos!
* GoryDiscretionShot: Used a lot, though averted in Lil' Lobo's death. He had a crap-ton of spears and holes in him and was red from the top of his hair to the bottoms of his boots. And he was STILL STANDING. Then we get the trope played straight, and there's nothing left of Lobo except a puddle of clumpy red goo.
* IllNeverTellYouWhatImTellingYou: ''ComicBook/{{Impulse}}'' #77 opens with this from the narrator:
-->"Okay, let's say, just for the sake of argument, that you ''haven't'' already read the first part of this star-spanning saga in the pages of ''ComicBook/YoungJustice'' #36 (still on sale!)...\\
And let's say that you ''don't'' know that the entire Young Justice gang has crash-landed on the volatile and unforgiving surface of Apokolips, home of the dark god Darkseid and about a million other unpleasant people...\\
And let's just assume that you don't know that, following the crash, Superboy and Robin had a fight over priorities and the team split up: Robin's team to stay with the downed ship and effect repairs...\\
... Superboy's team striking out across the harsh terrain in search of the fallen hero Steel, whose seemingly lifeless body was seen being spirited away by the grim herald of death, the Black Racer!\\
Well, if you don't know all that, ''we're'' not gonna tell ya.\\
Man, we'd hate to be you...
* InnerMonologueConversation: Among Superman, Brainiac and Imperiex at the end.
* KilledOffForReal: Unlike many of the above mentioned deaths, Maxima and Strange Visitor stayed dead.
* MadeOfExplodium: The Imperiex Probes.
* MoreHeroThanThou: Not specifically stated, but Superman IS the ONLY being capable to surviving a mano-a-mano fight against an Imperiex probe, so it eventually does fall upon him to take down every single one when everyone else fails to do the job.
* NotMeThisTime: Superman vs. Zod just before the first Imperiex prob hits Earth
* NotTooDeadToSaveTheDay: Superman soaks himself in yellow sun radiation and is about to obliterate Brainiac by destroying War World's "heart"... when he sees the essence of Imperiex floating in Brainiac's veins! So if he destroys Brainiac, Imperiex is free and will destroy the universe anyway! This causes Superman to shake off his homicidal rage and come up with a plan to dispose of ''both'' immortal threats without breaking his code against killing. Also a slightly humorous example when Young Justice is saved by an ''army'' of Lobo clones (each one came from a drop of Lobo's blood due to his over-powered regenerative abilities)
* OneSteveLimit: Subverted in ''ComicBook/{{Supergirl}}'' #59, when Buzz is looking for Linda and calling out her name, only to be answered by Linda Petrullo, a young wooman trapped in an overturned car. Buzz is content to leave her there, saying, "I don't care if you're ''Creator/LindaHamilton'', you're not the Linda ''I'm'' looking for."
* PlotRelevantAgeUp: Sort of: Brainiac holds Lex Luthor's daughter, just an infant, but projects her as a 20-year-old woman to gain Luthor's trust.
* PlutoIsExpendable: Brainiac converts Pluto into a new [[PlanetSpaceship Warworld]] and it's later destroyed when Superman defeats him.
* PortalToThePast: Essentially what Luthor and Darksied's "Temporal Boom Tube" is.
* RaiseHimRightThisTime: [[spoiler: Lena is reverted back to infancy and returned by Superman to Lex. Clark ''begs'' Lex to Raise Lena Right This Time.]]
* ScrapbookStory: Each issue written by Jeph Loeb uses excerpts from a well-known speech from either before, during or after a major conflict: the Gettysburg Address, JFK's inauguration speech, FDR's address to Congress following Pearl Harbor, and General Douglas [=MacArthur=]'s retirement speech. It's a very nice effect.
* StableTimeLoop: Imperiex exists to correct a flaw diffuse through the fabric of all space-time in the universe. [[spoiler: As Superman is tossing Imperiex through a Boom Tube back to the Big Bang which will scatter his essence beyond all hope of recollection he realizes that this makes him the very flaw he is trying to correct]].
* SuperMode: Superman dives straight into the heart of the sun and saturates his entire body with solar radiation, making him strong enough to move a planet...
* TheWorfEffect: Doomsday. A single blast from Imperiex turns the once invincible monster into [[StrippedToTheBone a mere skeleton]].
* UnspokenPlanGuarantee: Superman uses J'onn's telepathy to concoct a plan and beam it right into Luthor, Steel, and Darkseid's brains, only asking them aloud over and over "Will it work?!" And since we have no idea what that plan is, then of course it does! (The plan which defeated the Big Bads, btw? Superman sent them back in time to a moment after the Big Bang, which technically gives them both what they want: Brainiac is atomized and spread across the universe, while Imperiex is allowed to craft the universe as he sees fit, but realizes that the "imperfection" that had been bugging him since his birth at the dawn of time was actually his presence there, making a StableTimeLoop.)
* YouHaveGotToBeKiddingMe: Basically Brainiac's response to Superman ''pushing'' his ''entire'' planet, even as Brainiac has FULL THRUSTERS going in the opposite direction!