There was a war. It happened years ago, maybe even thousands of years. Characters reference it, especially if they took a part in it: the Shell-Shocked Veteran never managed to get over what he experienced back then, while the Phony Veteran, on the other hand, will never shut up about how many brave things he did in it.
Sometimes people will use the war as a reference point for placing events on a timeline - something happened a few years before the war, or somebody did something after the war.
Maybe people still have to deal with its consequences. The war happened, and it left its ugly mark on the world. But it's never shown to the audience - we never see a single flashback from the war, are never shown more than just a glimpse of what happened. The war will be referenced, but otherwise left mysterious, unexplained. Why it happened, how it ended, and what all the things that took place there (which people talk about like it should be obvious) actually were, are never explained. The war is only a mysterious event of the past, included mostly to add a bit of mystery and give people excuses for insane ideas. Needless to say, this trope can easily be processed into Fanfic FuelWorld War Three is often used as this, as are the Fourth, Fifth, Sixth and so on. For that matter, so are the First and Second. Stories set After the End have the tendency to do this with the war that caused the apocalypse. If the war never stopped, may be a Forever War.
If said war is going on presently but the characters themselves aren't involved, then it is Behind the Black.
Often used as a sub-trope of Cryptic Background Reference. Compare also Cataclysm Backstory.
Please, avoid shows referencing Real Life conflicts. We already have plenty amount of information on them.
Code Geass has the Britannian invasion and conquest of Japan, which we see only in flashbacks concerning young Lelouch, Suzaku and Nunally and the one from first episode opening sequence. There also was an alternate version of the Napolean wars, where Napoleon conquered Great Britain, making all the British aristocracy run to America, creating the Britannia Empire.
The many wars of the Ancient Belka in Lyrical Nanoha. We have been told some general info about it, such as how it destroyed Old Belka and led to the current age where physical-based weapons were banned, but otherwise, it's a big question mark. As Nanoha ViVid revolves around the Reincarnation of two prominent figures from that war, many readers hope that it will eventually give more insight to that period.
Cowboy Bebop has the War on Titan, which shaped the lives of Spike, Vincent, and Grencia years before any action related to the main story took place. We get a few glimpses of it in flashbacks however.
Last Exile also has the war between its two major powers, which is mediated by The Guild. Much of the action of this war takes place BEFORE the actual story, as a few episodes in we discover that one of the planets these factions live on is dying, essentially taking them out of the conflict for good. They're pretty much refugees after this point.
In Blue Gender, we don't actually see humanity get overtaken by the Blues.
The War with "Them" in Sora No Woto is a complete mystery that has become filled with myths.
Mysterious conflict with Mazinkaiser SKL, whose conseqences are related to events of the series.
Naruto has the first three Shinobi World Wars; though they play a major role in the backstories of a good chunk of the main cast, we still know very little about these conflicts outside of a few offhand mentions and short flashbacks, plus a single gaiden story starring Kakashi.
Pumpkin Scissors begins with the graduation ceremony of a class of army cadets being interrupted by an announcement that the war everyone thought they were going to be sent to fight in had just ended. The series itself is about a team working to help repair all the damage that was inflicted on their country during the war.
The war between the Spiral Warriors and the Anti-Spirals in Tengen Toppa Gurren Lagann. Parallel Works #8 details this a little focusing on Lordgenome's involvement.
In Super Dimension Fortress Macross, the backstory tells of a massive war between the Zentradi and a group known as the Supervision Army. The war ended long ago, and Britai's force is conducting mopping-up operations when they come across the Macross. The storyline is carried into Macross7, where we get some understanding of what the Supervision Army was, but the war itself is still an offscreen event.
Heavens War in Darker than Black serves as the backstory for most of the main characters, and is the driving force behind Hei's actions his sister Pai disappeared at the end when Heavens gate exploded, and Hei is trying desperatly to find her.
In the second arc of The Authority, Jenny Sparks mentions how Earth cut down all connection with the alternate Universe, The Sliding Albion, after the First World War erupted there. Characters from Albion mention briefly that between that event and Albion's invasion on Earth, that world has seen eight other world wars.
Also, the fourth story arc (and first written by Mark Millar) showed us a glimpse of another alternate Earth, which had thirteen continents. A world war that erupted there ended with armies of AdolfX exterminating all non-black people on all of them. The Engineer finds the idea of sending a group of superpowered white supremacists there quite interesting.
In With Strings Attached, Grunnel talks about how the Tayhil and their monsters conquered most of Baravada some 200 years ago, and how the skahs rose up to take back the place.
The Powers Of Harmony has the War of the Sun and Moon, the conflict between Celestia and Nightmare Moon, which in this story is presented as having lasted several years. There are a few flashbacks to it, but for the most part it's offscreen.
When watching Airplane!, they never make clear which war "The War" was for Ted Stryker, though judging by when the movie was made, one might assume Vietnam. As farcical as the entire movie is, it could very well have been some other war entirely though. Hell it could be a war they (or Ted) completely made up as well.
"The War" in Zero Hour!, the film on which Airplane! is based, and which came out a quarter of a century earlier, was obviously World War II. This timeframe informs a lot of the gags and references in the latter film.
The Lord of the Rings begins with Galadriel recounting the story of the last war with Sauron, several thousand years prior. We only see the end of the final battle.
The Clone Wars were this for the Star Wars trilogy until the prequels came, then two cartoons and a lot of other things that explored it, so it ultimately avoids this trope.
The Time Machine (1960): In-Universe. The protagonist from 1899 traveled a couple of decades into the future. One of his friends' sons mention "the front" of "the war". It's obvious its the First World War, but being a time-traveler, he was unaware.
The Discworld series has two examples - the wizard wars which serve as an example of why wizards shouldn't actually cast spells, and the wars of the Evil Empire, which serve as the origin story of the Orcs. The latter may or may not be the same as the "big old wars" mentioned in "Troll Bridge", in which Cohen the Barbarian fought for a bright new future and the return of the king, and Mica the troll fought because a big troll with a whip told him to.
The First Wizarding War against Voldemort in the Harry Potter books.
The war against Gellert Grindelwald is even more obscure. All we know for sure is that it apparently took place around the same time as World War II. Fanon consensus is that Grindelwald and Those Wacky Nazis had some sort of alliance.
Besides the war, the trilogy also serves to conveniently set up most of the major players in the original Dune'verse, such as the Corrino Imperium, the House of Atreides, the House of Harkonnen, the Spacing Guild, the Bene Gesserit, the Suk Inner School, and the mentats.
Bujold's Vorkosigan Saga is full of these. Mad Yuri's War, the Cetagandan Invasion, the Komarr Conquest and subsequent Revolt all have a direct impact on the storyline decades after they took place.
In Sergey Lukyanenko's Watches books, the Treaty is signed between the Light and the Dark Others after a magical war that nearly destroyed everything. Hardly any details are revealed about the war. The beginning of The Film of the BookNight Watch shows a battle between two groups of barbaric-looking people without using any magic (the director hates magic), with each group consisting of two dozen men at most. This is likely meant to be symbolic, though.
Larry Niven's Future History (leading to Known Space) series deals heavily with relations between humans and the Kzin, but the early Man-Kzin Wars never showed up in the books just because Niven didn't like writing war stories. He did let other writers go back and fill that in later, though.
The aptly-named Vague War in Sergey Lukyanenko's Line of Delirium, which takes place decades prior to the novel. Many references are made to the war, but few details are revealed. Apparently, it was a big free-for-all with all known races but no alliances. The war led to the formation of the Human Empire. The author even throws in a funny story about humans spreading misinformation about their dietary needs (i.e. that we need spinach to survive). The aliens spend resources developing a spinach-killing virus and lose countless ships spreading it throughout the human worlds. When humans don't die, they surrender out of shock. On a less funny note, good luck finding spinach after the war.
The Aiel War in The Wheel of Time is mentioned in passing by numerous veteran Warders, Tam al'Thor, and others. Many other wars - the Breaking of the World, the War of a Hundred Years, Artur Hawkwings war of consolidation, all serve to create rich background for the series.
The ending at least of the Aiel War was covered in the Prequel novel, New Spring.
In The Sword of Truth series, many of the MacGuffins, events and plots of the entire series are a result of the direct influence of the events in the Great Wizard War that happened over 3000 years ago.
A Song of Ice and Fire has many of these. The most important is Robert's Rebellion, which is really the cause for all of the events in the series - giving the effect that the reader has plunged into the middle of the story rather than the beginning. More recently, there was Balon Greyjoy's Rebellion. Further back in time, we had the Targaryen Conquest, the Blackfyre Rebellion, and the War of the Ninepenny Kings. Going even further back then that, back to mythic times, there are the stories from various cultures about a great battle between good and evil, implied to be an ealier war with The Others.
Also, whatever it was that Tywin Lannister did the to inspire 'The Rains of Castamere'. "But there are no Reynes and Tarbecks"... "Exactly."
Admiral Theisman's purge of the State Sec forces which refused to fall in line with the new government after the overthrow of the Committee of Public Safety. The only part of it shown is from The Fanatic, which itself took place away from the meat of the action. Not a typical example, as that particular conflict took place between two of the later books of the series.
Earth's "Final War" many centuries before the current timeline, where the planet was nearly rendered completely uninhabitable until several colonies sent aid to repair the damage.
Stephen King's The Dark Tower series occasionally reference the last war of the Gunslingers against the Good Man, and it's the backdrop against which Wizard and Glass is set. There is also an even older event implied to be a nuclear war, which is why the series is After the End in the first place.
In the Star Trek: New Frontier series, Calhoun and Picard (and their crews) discover that a species that's apparently been friendly is actually capable of an insidious level of mind control which they've hid successfully until then. In the next book in the series, there's been a Time Skip of several years and said alien species has been defeated after a fairly vicious war.
Triplanetary mentions the first and fourth Jovian Wars, which resulted in the formation of the Triplanetary League from Venus, Tellus, and Mars.
The 'Holy War' against the Ghouls in E. R. Eddison's The Worm Ouroboros, in which all the civilized ("polite") nations of the world of Mercury fought alongside each other, and that ended just shortly before the book's storyline begins.
In 1984, there is supposedly a vast war raging between the three superstates, but it has no actual bearing on the novel's plot. Of course, it could just be made up to make the party's rule seem legitimate.
Some kind of great war is implied to be happening in Lord of the Flies. The reason the children are on the island is because the plane that was evacuating them from an imminently about to be nuked Britain was shot down.
Live Action TV
The Last Great Time War from Doctor Who. It was never shown, we know only that it caused a lot of destruction and wiped out entire races, including the Time Lords, whose last survivor was the Doctor. We find out that the Doctor was the one who ended the war, killing every Dalek (apart from a few who got away, of course) at the cost of also killing every Time Lord, including his own children and grandchildren. The Doctor has to deal with the consequences of the Time War from time to time and sometimes he or somebody else makes a reference to some events of it, but it's still mostly a mystery.
Current showrunner Steven Moffat has gone on record saying that he will never show the Time War since there isn't enough money in the world to do it justice.
Doctor: You weren’t there in the final days of the war. You never saw what was born. But if the Time Lock’s broken then everything’s coming through, not just the Daleks, but the Skaro Degradations, the Horde of Travesties, the Nightmare Child, the Could-Have-Been-King with his Army of Meanwhiles and Never Weres – the war turning to Hell.
Older episodes referred to similar such events, including the survivors of the destruction of Phaester Osiris defeating Sutekh and sealing him in a pyramid on Mars, or the ancient war between the Time Lords and the Great Vampires shortly after the beginning of time.
Ancient battles between the Fledgling Empires (including Gallifrey) and the Racnoss were mentioned in "The Runaway Bride".
The Sontarans have been at war with the Rutan host for at least 60,000 years of the Whoniverse's timeline (mentioned in both Doctor Who and The Sarah Jane Adventures), and yet we've never encountered a Rutan and Sontaran together on TV once.
Played With in "The Doctor's Daughter". A war, fuelled by cloning machines, has gone on for 700 generations. It turns out that those 700 generations were cloned, shoved into battle and killed in the space of one week.
The great Cyber-Wars fought between human forces and the Cybermen were mentioned frequently, but the Cybermen were hardly even seen at the height of their power - mostly after the fact as The Remnant.
The Third World War and Eugenic Wars, all taking place on Earth and concerning only humanity. It was actually one war in the Original Series, but was later divided.
There are EU novels dealing with both, with The Rise and Fall of Khan Noonien Singh being the most prominent. Also, in the interest of reconciling the Eugenics Wars with the real life passage of the era in which they are said to have taken place (1990s), the Eugenics Wars were retconned into a far less grand scale war that happened mostly in the shadows of real life events.
The war between the Federation and Romulan Empire, which forms the backstory for the episode "Balance of Terror".
The ongoing war between the Federation and the Klingons at the time of the Original Series is mostly not shown.
Note that the Federation and the Klingons have gone to war many, many times: A few of these conflicts have been depicted on-screen, including in the Original Series: The brief Federation-Klingon War of 2267 (which was ended by the Organian Peace Treaty, imposed upon the belligerents against their will) took place entirely within the episode "Errand of Mercy". Another Federation-Klingon War took place during the first half of the fifth season of Deep Space Nine, which was effectively ended (and the terminated alliance suddenly restored) when the Cardassians joined the Dominion.
The Next Generation has the war between the Federation and the Cardassians, which was responsible for creating the Anti-Cardassian Maquis. Strangely, it wasn't mentioned in the first seasons, only later.
The implication is that the Federation was pulling its punches more than a little. Their defensive action shook the Cardassian Union nearly to the point of collapse.
However, the new BSG's First Cylon War will be getting screentime in the upcoming Blood and Chrome series. A licensed video game also purports to cover that period, even being played from the perspective of Commander Adama as a rookie pilot, but it mixes and matches so many elements of the original series and remake that it probably belongs in its own separate continuity.
The conflict between Dharma Initiative and the Others was only hinted at in the first four seasons of LOST, though the Season 5 gives us a pretty clear picture of it. The conflict between the Others and the US Army is even more obscure.
The war with the Magogs from Andromeda, the end of which caused the Nietzschean revolution, may count too, as it started at the beginning of the first episode. The episode then jumps 300 years in the future, to the main plot, not only skipping the downfall of the revolution, but also the civil war among the ones who caused it, the High Guard.
Played with in That Mitchell and Webb Look where a group of people are holed up in a nuclear bunker playing a game show and broadcasting in the vain hope somebody is actually watching. It all focuses around "The Event"; the only interesting thing any of the characters can think about but are sworn not to talk about for various reasons. The most probable event would be a nuclear war.
Or alternatively whatever caused the majority of the human population to become zombies ('Them').
Babylon 5 has the Dilgar War, the first major interstellar war that the humans got involved in, as well as the previous Shadow War, which took place around a thousand years previously. We do see a glimpse of that era, mostly just some less advanced looking Minbari ships, but nothing of the war itself.
Also the Telepath War. We are shown only before and after.
And the first Narn war for independence from the Centauri, about 100 years prior to the beginning of the series.
Along with the numerous wars the Centauri Republic was waging with its neighbors in the third season, none of which are seen, or indeed the numerous smaller wars between the members of the League of Nonaligned Worlds in the same season.
There are also allusions to various minor conflicts that the Earth Alliance took part in. Not to mention the Earth Minbari War, which we heard much about, but saw little of, until the prequel movie In The Beginning.
Power Rangers falls back on this one a lot, to the point where Epileptic Trees have grown due to some wars sharing the same rough dates. Some of these conflicts are original while others are carried over from the original Super Sentai counterparts:
Mighty Morphin' Power Rangers had a great battle between Good and Evil 10,000 years ago. Zordon was with the good guys, Rita with the bad guys under the command of Zedd. We have no other clues about it beyond that, despite the many ancient magical characters who were there and make none-too-informative references to it.
Power Rangers Lost Galaxy is so vague that we don't even know if there was one big event or just a bunch of isolated incidents. Either way, a lot of backstory happened 3,000 years ago.
Power Rangers Lightspeed Rescue had Bansheera and company put in the can 5,000 years ago. Her entire motivation for attacking in the present is to destroy the city and reestablish her palace upon the demons' sacred ground, granting 'ultimate power.' The fact that she had it then meant she had to have been a lot worse than anything witnessed during the Lightspeed series.
In The Walking Dead, the military is mentioned to have been overtaken by the Walkers and their abandoned equipment and corpses are seen lying about in the streets of Atlanta and outside of CDC, but it's never shown how the military were defeated.
The Firefly pilot (and one other episode) contain a flashback to the "Independence War", but other than that it's just talked about, although its aftermath is the prime motivation for many of the characters.
Kamen Rider Agito is a non-direct sequel to Kamen Rider Kuuga, most explicitly referenced by the police-made G3 Powered Armor being based on data from Kuuga himself. According to the backstory, Kuuga's old enemies the Grongi fought a war against another monster group, the Lords, who eventually won and became the villains of Agito. Kamen Rider Decade directly references this when the heroes go to an Alternate Universe version of Agito and land right near the end of the Grongi-Lord War.
In Merlin there are increasingly frequent mentions of a war waged at some nebulous time before/around King Uther's time in which ancient/recent kings were pitted against the sorcerous High Priestesses.
Scrubs has the Janitor mention overthrowing Kyle, the former leader the Brain Trust ("A dictatorship masquerading as a democracy") during The Revolution of '02, specifically The Battle of the Basement Supply Closet. This probably never happened.
The Core War in Bionicle, although we do see a very brief account of its more important moments in one of the Flashback comics.
Dungeons & Dragons adventure I12 Egg of the Phoenix. The War of Ending between the forces of Evil and Old Empyrea. Doc and the silver dragon Falx fought in the war, during the adventure the PCs find a message that dates back to it, and one mission involves scouting one of the Castles of Ruling that played a major part in the war.
An attempt was made to rework the World Of Greyhawk setting with The Greyhawk Wars (the war Iuz was preparing for in the time period of the original boxed set). The war itself was represented as a stand-alone board game only. It produced some interesting novels set in the aftermath of the War where characters often have to deal with the consequences, but the canon version of the war itself is relatively obscure.
Exalted has a few examples: the Primordial War, the Aftershock War, the Usurpation and the Balorian Crusade. All of these are provided some level of detail (specifically who was fighting and why), but the exact events of the wars are generally shrouded in mystery (typically because they all involved reality being damaged to some degree).
And those are just wars involving the Exalted. The occasional hint is dropped regarding wars waged by gods in the era before humanity, and even occasionally to conflicts involving the Primordials prior to the existence of Creation.
Autochthonia has its own version in the Elemental War (so named because it so devastated the mechanical ecology that it drove thousands of elementals violently mad), which was noteworthy for being an extremely violent, ethnically driven total war in a world where most fighting is skirmishes to steal resources and supply lines.
RuneScape has the God Wars which had raged on for 4000 years, making up the entirety of the Third Age. Despite ending 2169 years ago, the repercussions are still felt today: many races are driven to extinction, down to Last of His Kind or is a Dying Race; the gods are forced to depart from Gielinor. Many quests focus on this time: the Cave goblin-dwarf railway is postponed due to the discovery of related artifacts, the player rediscovers the myriad, a Dying Race of Energy Beings, human-vampyre tension runs high but if another war breaks out, Guthix would be reawakened to destroy the world and remake it. Except now he's dead.
Castlevania has "The Demon Castle War" in 1999 where modern-day soldiers attacked (and died; given the zombies) Dracula's Castle. This war also heralded Dracula's true and final death. There is no game that covers this.
Freelancer has the Coalition / Alliance war, the beginning of which was shown in Starlancer but which lasted for another century afterward and the winner is not the one the first game would indicate, as well as the 80 Years War between Rheinland and the GMG.
Star WarsKnights of the Old Republic has the Mandalorian Wars. Much of the information about it is from allusions and As You Know statements in the games, and lots of characters you run into in both games are war heroes or veterans from one side or the other.
For the longest time, StarCraft had the Guild Wars, which were referenced only in vague snippets as a civil war whose consequences still loomed over the Terran worlds. Fast forward many years, it's been more-or-less explained away with tie-in literature. Still nothing in the games, but this series has always assumed you did the reading first.
The Keyblade War happened in the background story of Kingdom Hearts long before the events of the game, and the main goal of the villain is to start a new one.
The Backstory for The Elder Scrolls III Morrowind provides another example from the Elder Scrolls universe. The "War of the First Council" and "Battle of Red Mountain" thousands of years ago set the stage for the plot of the game. The devout, Daedra-worshipping Chimer and atheistic, scientific Dwemer came into conflict in the land now known as Morrowind. After years of fighting, they were forced to team up to drive out the invading Nords. Their alliance remained under the leadership of Chimeri Lord Indoril Nerevar and Dwemer Dumac Dwarfking, known as the "first council." It was a time of great peace and prosperity for both races. However, the Dwemer Dug Too Deep beneath Red Mountain and unearthed the Heart of Lorkhan, the creator god. Chief Tonal Architect, Lord Kagrenec, crafted tools to tap into the power of the heart, hoping to allow the Dwemer to transcend mortality. The Chimer, seeing this as a blasphemy against the real gods in the Daedra, attempted to stop the Dwemer, reigniting their war. Forces led by Nerevar and Lord Voryn Dagoth infiltrated the Dwemer Red Mountain stronghold. Exactly what happened next is up for intense debate, but the Dwemer disappeared from existence, Nerevar was slain, Dagoth Ur and the Tribunal used the tools on the heart to achieve godhood, and Azura cursed the Chimer with dark skin and red eyes, transforming them into the modern Dunmer.
Kinda adverted with Fallout 3 with the "Operation Anchorage" simulation, where you (virtually) fight a small part of the war to reclaim Alaska from the Chinese before the bombs dropped, though notes around the facility record that the simulation has been repeatedly rewritten at the orders of a general, and bears less and less relation to the reality.
Transformers: Has this to varying degrees, the Cartoon itself is a straight example, as the war has restarted on earth and the million year war on Cybertron is only alluded to.
Adventure Time has the Mushroom Wars, which caused the end that the series takes place after. Recent season have delved further into into just what happened then, revealing that what seems to be a Fantastic Nuke was involved, leading to the return of magic and a wave of mutations that resulted in the many oddities of Ooo.