This is a Sub-Trope of Prevent the War that focuses on World War I (aka The Great War).
It involves a Period Piece adventure story that takes place in the late 1800s or early 1900s with a plot that centers around stopping a plot that would lead into what would become World War I. The hero or heroes catch wind of the Big Bad planning to start a war between the various empires of Europe (usually with a False Flag Operation or two) and they are the only ones who can stop them.
The Steampunk setting and political intrigue of the time provides a rich atmosphere for a globe-trotting adventure yarn as the heroes race the clock to stop the bad guys. They will almost always succeed (unless Alternate History is involved), but this usually makes the story Harsher in Hindsight. Sure, the hero just stopped this particular villain from starting a world war, but we all know one happened anyway. Then there was a second one.
The writers may not reference this in order to avoid making the story too depressing but if they do, expect a dark form of It Will Never Catch On. In more rare cases, sometimes they fail, sometimes they fail and die, and sometimes they even start the war in the first place. Can overlap with You Can't Thwart Stage One and You Can't Fight Fate.
Alternately, a touch of Bittersweetness can be added with the addition of bloodthirsty villains with more fantastical Steam- or Diesel Punk analogues to modern-era weapons that the heroes kept from being used that would have made things so much worse. World War I was bad enough, but we could've had a weird World War I with Humongous Mecha, carpet bombings, augmented Super Soldiers, or nuclear weapons had our heroes not won the day.
World War II is much less prone to this trope, largely due to Hitler's Time Travel Exemption Act. A Cold War-flavored version of this trope also exists, in which the villain's goal is to set off a war (usually a nuclear one) between the United States and the Soviet Union.
Compare and contrast with World War III or World War Whatever, which, for obvious reasons, are set in modern day or the future.
Examples:
- In Rurouni Kenshin, after Kenshin defeats Shishio, who has a Social Darwinist ideology that is deliberately meant to recall that of Imperial Japan, Kenshin thinks to himself that hopefully he's prevented such an ideology from ever again arising or leading to problems in Japan.
- Steamboy broadly invokes the trope as steampunk weapons dealers try to sell steam powered war machines to Europeans and other powers itching to try them out, with the real world consequences of applying progress to war looming over the whole affair. Their primary designer even says (in 1866) "Shoot me and you'll set science back fifty years. But you won't stop it!"
- The League of Extraordinary Gentlemen: The Black Dossier covers this. Thomas Carnacki, medium, ghost hunter and member of the fourth incarnation of the League, has terrible visions of a devastating war in Europe. Intel leads the rest of the group, consisting of Mina Murray, Quatermain, AJ Raffles and Orlando, to confront their French counterparts, Les Hommes Mysterioux (Captain Robur, Arsène Lupin, Monsieur Zenith, The Nyctalope and Fantômas) in a battle at the Paris opera house in hopes of preventing the war. When the battle is over, the League withdraws, hoping that they have defused the situation. Both teams were played against each other by their German counterparts, the Twilight Heroes, a group consisting of various Mad Scientists such as Dr Rotwang and Dr Caligari. They are now free to plan the coming war with impunity.
- In Lilith this happens by accident, as Lilith's various alterations to the timeline have as a side effect that when Archduke Franz-Ferdinand is killed in Sarajevo the war is defused. This however only delayed things, as by the Thirthies Germany has annexed Austria-Hungary and that started a chain of events that ended up in the Great War.
- The film version of The League of Extraordinary Gentlemen involves the mysterious "Fantom" (eventually revealed to be Professor Moriarty) provoking the European nations, so that he can supply super-weapons and super soldiers for a world war. The term "world war" is even mentioned for audience members who weren't paying attention.
- In The Prince and the Showgirl, an underage Ruritanian king plots with the Germans to oust the pro-Entente prince regent, an action that might spark world war. Fortunately, an American showgirl is there to mend the relationship between the prince regent and his kingly son, thus putting an end to the plot.
- Sherlock Holmes: A Game of Shadows has Moriarty provoking the European powers, so that he can sell the weapons and the bandages that will be needed for a world war. He mentions that, even if his plan to launch the European nations into war is stopped, they'll do it themselves soon enough. He's just trying to hurry it along. Fortunately he gets Out-Gambitted by the Holmes brothers, who donate his money to the Widows and Orphans of War fund. Holmes even addresses Moriarty's remark about the war's inevitability by insisting that he's just trying to stop Moriarty from profiting, and put down the Napoleon of Crime while he's at it.
- The Three Musketeers (2011) contains perhaps the earliest version of this trope as far as the period goes. The villains were trying to start a war among the empires of Europe. While the dialogue focuses on the continent, the idea is still the same, not to mention that a war of the European powers at the time would have likely included colonies in Asia, Africa, and the Americas.
- Ironically, the original Three Musketeers novel puts their story smack in the middle of the Thirty Years War, a World War Zero if the world ever saw one. Preventing the war itself is never an objective, just making it turn out alright for France. (And given this is the time French took over from Latin as the common language of the European high class, history suggests they succeeded.)
- The Seven-Per-Cent Solution. The ultimate aim of the Big Bad is to start a world war. Holmes and Watson stop him, but Holmes notes that they have only delayed the war, not prevented it entirely (and he's proven to be correct when World War I occurs).
- Towards the end of Black City, Erast Fandorin is hired to investigate the assassination of Archduke Franz Ferdinand before the conflict escalates. Unfortunately, he never makes it to Austria in time.
- Hugh Stanton, the hero of Ben Elton's Time And Time Again 2014 travels back to 1914 to prevent the Great War. Initially by trying to save the life of Archduke Ferdinand and, when that fails, by assassinating the Kaiser. He succeeds, but it turns out that the War wasn't the worst thing that could have happened.
- Played with in Amnesia: A Machine for Pigs. The titular machine was built by Mandus, and is revealed to be his attempt to stop future wars from taking place after he had a prophetic vision of the horrors of the World Wars, including his children dying in the trenches. The machine ends up inheriting a piece of Mandus' soul, achieving sentience and carrying out horrific acts of its own in an attempt to fulfil its purpose, and Mandus ultimately decides to sacrifice himself in order to sabotage it, even though it means allowing the Wars to happen in the near future.
- The original Shadow Hearts takes place in 1914, with several parties mentioning the growing tension in the world. The Big Bad claims that his scheme will prevent the horrors he has foreseen, warning the heroes that standing against him will allow humanity to unleash many horrors in the coming century. They ultimately resolve to face whatever is coming, with the first game concluding on a note about the start of the War.
- In Titanic: Adventure Out of Time, the protagonist must recover four items from the Titanic before it sinks. Two of the items recovered prevent the Black Hand from getting funds, and thus World War I never happens. Of course, depending on if you recover the other two items, this will impact how well Hitler, the Soviets, and in one ending, even Commie Nazis do twenty years later.
