Pretext for War

""Of course you realize, this means war!""

- Groucho Marx, Duck Soup

The leaders of Viridia and Tyria want to go to war. Not for a silly reason, but due to anything from good old fashioned jingoism, greed, political/economic/religious differences, or a simple historical grudge. However, they can't just out and out declare war, that would be uncivilized! And more importantly, it would make them look bad to the international community, which isn't good politics. So instead they will wait for or manufacture a Pretext For War out of whatever should come their way.

Did a Tyrian pig farmer lose a pig when it cross into Viridia by accident, whereupon a Viridian caught and cooked made it his pet? This Means War! Maybe a Viridian girl disappeared near the Tyrian border? Tyrian slavers must want to capture Viridian women since theirs are so ugly! A favorite is for Star-Crossed Lovers from both sides (preferably royalty) to elope, causing both sides to assume the other kidnapped their heir.

One interesting and ironic variant is when hardline elements from both sides will collaborate to stage a high profile assassination or other incident to kickstart a war, proving just how well they work together to achieve their goals despite hating each other's guts. This one is especially common when one or both nations have a Reasonable Authority Figure as a head of state, since it can force their hand to war, or if they're the assassination target, get them out of the picture and make them an unwitting martyr. False-Flag Operation is one of classical moves too, and almost XX century theme song.

Point is, an incident any two sane heads of state would quietly defuse is treated as a Rant-Inducing Slight in order to start the war.

A technical term for this is the latin term Casus Belli, or case for war.

Since wars of aggression have technically been banned, you'll find that these are a lot more common today than they were previously, since both sides are at pains to show that the other side started it. As a result, the history of many a 20th-century war reads like a really, really dark Idiot Fawlty Towers Plot.

See also War for Fun and Profit.

Comic Books

 * Tintin and the Broken Ear. A Corrupt Corporate Executive encourages Tintin (then working as the aide to a Banana Republic's dictator) to declare war on a neighbouring country. Tintin point-blank refuses, so the executive frames him and he's forced to flee the country in a staff car. The car is fired upon by the border guards and the incident is then used as an excuse for war.

Film

 * The Marx Brothers' Duck Soup. Essentially just bankruptcy and personal slights.
 * Some of which were hypothetical slights that didn't actually happen, at that.
 * The plot behind The Princess Bride.
 * Star Trek VI: The Undiscovered Country featured to try to veer both nations onto the path to war by . Given how the Federation was definitely in the stronger economic and military position at the time, many fans have since hypothesized that the respective  were goaded into "dying on their feet rather than living on their knees" by Section 31 machinations.
 * Star Wars has many thanks to the machinations of Chancellor Palpatine. In The Phantom Menace, he directs the Trade Federation to invade Naboo over a trivial dispute, and in Attack of the Clones he deploys the clone army secretly created by his apprentice Count Dooku to Geonosis, sparking the Clone Wars.
 * When Guy de Lusignan becomes King of Jerusalem in Kingdom of Heaven, he releases Reynald in order for him to do something to start a war with Saladin.

Literature

 * The Discworld book Jingo is named for jingoism. The excuse for war is a worthless island.
 * A strategically vitally located island with plenty of mysterious ruins on it, admittedly, but still, fairly worthless compared to what war would mean.
 * As one character pointed out, the island being strategically located only mattered if there was a war, and the only reason they were fighting was because of the damn island.
 * Which was why a more politically acceptable excuse to go to war and seize the island (the attempted assassination of a diplomat) was engineered.
 * In Small Gods, Vorbis
 * The manufacture of such a pretext is a major plot point in both the film and book versions of The Princess Bride.
 * Crest of the Stars has a complex one. First is the destruction of the Gosroth with United Mankind insisting on setting up a joint investigation committee, with the suggestion that tensions are running high after the Abh annex the Hyde System. Ultimately subverted in that it is revealed that UM has been planning the whole thing for decades and on the Abh side Empress Ramaj sees right through it and refuses to play their games. She basically says "If you want a war then I will give you a war."
 * Mocked in the Star Trek: The Next Generation novel I, Q. The Q Continuum has a mortal enemy, the M Continuum. The Ms decided they wanted to go to war with the Qs. Why? Because there is something about them that pisses them off (their exact words). The Q Continuum requested a more eloquent reason. So one of the Ms insulted the mother of one of the Qs. This horrific affront (Despite the fact that this Q, like all other Qs, didn't have a mother) could only be answered by a full scale war.
 * Eh, the Your Mom taunt can infuriate many immortal, parentless beings. Amon fell for it.
 * In the third Dresden Files book, Grave Peril, this turns out to be the cause of most of the problems of the books:  Harry sees that he's being maneuvered into the trap and knows what the consequences will be, but since the alternatives would be the death of him and several innocents, along with the unmaking of a Holy Sword, he does it anyway.
 * Discussed in the Tom Clancy novel The Bear and the Dragon. China is considering initiating a war of aggression against Russia, and Russian observation planes are staying well within Russian air space, but examining the Chinese preparations. The Chinese war minister recommends shooting down one of the spy planes and stating that it had violated Chinese air space, and then using that as casus belli for the war. This is never mentioned again, mostly because, thanks to a well-placed spy, China's opponents know exactly what they're doing.
 * In the Star Wars Expanded Universe, the Chiss are only allowed to go to war if provoked (i.e. the other guy has to shoot first), so developing a pretext for preemptive strikes becomes a veritable art form for the Chiss Expansionary Defense Force. The CEDF wanted to take out the marauding Vagaari for decades but were never directly attacked, so in Survivors Quest they, then into.

Live Action TV

 * The Silurian captured by the humans in the recent Doctor Who two-parter hopes to be killed and tries to get killed, just to start the war.
 * And several seasons earlier, the Slitheen try to start World War Three so they can sell the radioactive remains of the planet as fuel.

Radio

 * In The Hitchhiker's Guide to the Galaxy it says that the history of warfare is divided into three phases: retribution, anticipation, and diplomacy.
 * Retribution: “I’m going to kill you because you killed my brother.”
 * Anticipation: “I’m going to kill you because I killed your brother.”
 * Diplomacy: “I’m going to kill my brother and then kill you on the pretext that your brother did it.”
 * There was also the incident where two very tiny alien races were on the brink of war due to a Your Mom joke, but what really pushed both parties over the edge was a random comment made by Arthur Dent that traveled through space and time.

Theatre

 * In Knickerbocker Holiday, Stuyvesant, Glorious Leader of New Amsterdam, believing that national greatness lies more in guns than butter, suggests that war with Connecticut could be imminent because, he alleges, the Connecticutans have had the cheek to build a fort on the Connecticut River.

Video Games

 * Used in Modern Warfare 2, where Vladimir Makarov, a major Russian extremist terrorist, perpetuates a massacre in an airport in the middle of Moscow while Private Joeseph Allen, an American CIA agent, is planted in his inner circle. However, Makarov knows about Allen, and the Private is killed and dumped in the airport, and his body used as a pretext by the war-happy Ultranationalists to give them an excuse to invade the United States. The fact that as of Modern Warfare 3  helped sell this.
 * This trope is actually active on both the Russian and American sides in different ways. The Russians have been looking for an excuse to go to war with America for years, and are just waiting for a catalyst. Even if the US was completely uninvolved, there's a fairly decent chance they would have blamed the CIA anyway. Indeed, they may have done just that. A well known internationally wanted terrorist is seen, plain as day, strolling into an airport with a machinegun. Even though Allen's corpse is left there, the only thing that would identify him as a CIA agent (or even an American for that matter) is information being fed to the Russian government by a terrorist organization or the game's villian (who are both less than reputable). On the American side, Allen is there for the sole purpose of BEING the catalyst, and his mission was made from the start.
 * In Europa Universalis you can manufacture claims on another country as a pretext for war using the 'obscure documents' Casus Belli. And in Vicky and Europa occasional border incidents like the pig one occur
 * In Dragon Age II, the centerpiece of
 * Galactic Civilizations 2 has a random event where people from one civ automatically assassinate a very high-ranking politician of another civ, forcing war between the two.

Real Life

 * That pig thing, in the trope discription? It was real. It wasn't much of a war though: "The pig was the only casualty of the war, making the conflict otherwise bloodless." from The Other Wiki.
 * The War of Jenkins' Ear. The British had managed to get themselves exclusive rights to trade slaves in the Spanish colonies in America, but at the cost of Spanish crews being allowed to board British ships and search their cargo. Relations became rather strained, and then a one-eared merchant captain by the name of Jenkins showed up in Parliament with a severed ear and a story of Spanish brutality that sparked the above war. It is doubtful as to whether the ear exhibited in Parliament was actually Jenkins' lost ear, as historians today and his contemporaries believe that he lost his ear in a bar fight years before.
 * World War I. The assassination of the heir to the Austro-Hungarian throne, Archduke Franz Ferdinand, was just an excuse for destroying Serbia. Which sounds a bit unreasonable until you realise that the very existence of Serbia, an independent nation of ethnic Serbs who aimed to someday unite all of Greater Serbia, was a threat to the unity of the Habsburgs' multi-national empire. Right from the start they thought that, with Germany backing them all the way, Russia would back down rather than risk war with them 'and' Germany at the same time, as happened 6 years before over the formal annexation of Habsburg-occupied, technically Ottoman Bosnia.
 * Furthermore, right from the start the German High Command's universally accepted plan of action was the Schlieffen plan, which involved invading France through Belgium and taking her out of the equation before the Russian steamroller could mobilise and flatten the Allies from the east. So when Russia mobilised her armies Germany delivered an ultimatum to France as a pretext for war, demanding her neutrality despite being an ally of Russia and asking her to hand over fortresses integral to France's defences along their shared border within 24 hours. The French were only too happy to refuse with indignation; the Revanche-ist movement, a movement seeking revenge for France's defeat in the Franco-Prussian War and the re-annexation of the province of Elsaß-Lothringen (Alsace-Lorraine) was a force not to be denied.
 * In other words, the true causes were jingoism, militarism, alliances, imperialism, commercialism, and nationalism.
 * Or to make a long story short, a bunch of people wanted some stuff and threatened to go to war if they didn't get it but only mostly wanted a war. Some other people didn't want to give it to them but kind of wanted a war too. One thing led to another and there was a war. A really, really big war. Then about 6 months later they all thought to themselves, 'Why the hell did we do that?'
 * The Mainila incident, which the Soviet Union used as a pretext for invading Finland during the Winter War. It has been established that there is no way the Finns could have been responsible, as none of their artillery was in range at the time. See False-Flag Operation.
 * Similarly, the Gulf of Tonkin incident, in which an American ship on patrol claimed to have been attacked by Vietnamese gunboats, serving as a pretext for the Vietnam War; but the reports may have been fabricated to gain popular support for escalating military operations in Southeast Asia.
 * To make a trifecta, the sinking of the USS Maine in Havana Harbor provided America with the perfect excuse to start the Spanish-American War and take over Cuba and Spain's Pacific possessions. Investigations since then seem to be split on whether it was a deliberate act of war by the Spanish, a False-Flag Operation by Cuban rebels, or a genuine accident caused by a fire in a coal bunker.
 * The independence of the Republic of Colombia was supposedly triggered by "El Florero de Llorente" (the vase of Llorente) which was a trick by the supporters of the independence to upset a spanish merchant so they will have an excuse to arise the crowd against the spanish people. This lead to the popular expression used when you need a excuse to start a fight.
 * The Gleiwitz Incident was used as a pretext for Germany's invasion of Poland in 1939. An SS commando in Polish uniforms attacked a radio station near the Polish border and broadcast anti-German propaganda. They even dressed a prisoner in Polish uniform and shot him to add authenticity.