Casus Belli
The stated justification a state offers for going to war, which is often assembled after the decision to fight has already been made.
Casus belli is Latin for the case, or occasion, for war: the specific act a state points to when it says this is why we are fighting. It is not the same as the cause of a war. The cause is why the war happened, and it is usually structural, some mix of interest, fear, and opportunity. The casus belli is the incident nominated to carry the argument in public. The two sometimes coincide. Very often the casus belli is chosen, or arranged, to fit a decision already taken.
The distinction matters because a state at war needs a story that will satisfy three audiences at once: its own population, whose sons will be conscripted and whose taxes will pay; its allies, whose treaty obligations may be triggered only by particular kinds of attack; and neutrals, whose trade and goodwill are worth having. A pretext is cheap, and it does work that armies cannot do.
Manufacturing the occasion
The classic modern case is the Ems Dispatch of 1870. Bismarck received a telegram describing a courteous conversation between the Prussian king and the French ambassador, edited it to read as a mutual insult, released it to the press, and watched France declare war on Prussia within days. He wanted the southern German states to enter the war as allies of an attacked Prussia rather than as auxiliaries of an aggressor, and the edited telegram delivered exactly that.
Twenty-eight years later the United States found its own. When the battleship Maine blew up in Havana harbor in February 1898, killing 266 men, a naval board concluded that a mine had done it and the American press, already campaigning for intervention in Cuba, supplied the culprit. Remember the Maine became the slogan of a war that took the Philippines, Puerto Rico, and Guam. Later investigations have pointed to a coal bunker fire igniting a magazine, and no one has ever produced evidence of Spanish sabotage.
The twentieth century industrialized the technique. In September 1931 Japanese officers detonated a small charge on their own railway line near Mukden, blamed Chinese troops, and used it to seize all of Manchuria. In November 1939 Soviet artillery shelled the Soviet village of Mainila and blamed Finland, opening the Winter War. On the night of August 31, 1939, SS men in Polish uniforms staged an attack on a German radio station at Gleiwitz and left a murdered concentration camp prisoner behind as evidence. Hitler invaded Poland the next morning announcing that fire was being returned. In August 1964 a confused radar and sonar contact in the Gulf of Tonkin, in which no attack on American ships almost certainly occurred, produced a congressional resolution that carried the United States into a decade of war in Vietnam.
After 1945
Before the twentieth century, war was a legal instrument of policy, and the casus belli was mostly about legitimacy and alliance mechanics. A related term, casus foederis, means the event that activates a treaty of alliance, and states drafted those clauses with care precisely so that a partner's adventurism would not automatically become their war.
The United Nations Charter changed the frame. Article 2(4) prohibits the threat or use of force against another state's territorial integrity, and Article 51 preserves an inherent right of self-defense against armed attack. In principle the only lawful unilateral war is a defensive one. In practice this did not abolish the pretext, it standardized it. Every modern intervention is now announced in the vocabulary of self-defense, protection of nationals, humanitarian rescue, or a request from a legitimate government. When Russia invaded Ukraine in February 2022 it cited genocide against Russian speakers in the Donbas and the need for denazification, alongside a claim that Ukraine was not a real state at all.
Reading the pretext
The practical value of the concept is diagnostic. When a state's stated grievance is much smaller than the war it launches, when the incident is unverifiable, when it occurs after the mobilization rather than before, or when the demands attached to it are written so that no government could accept them, as Austria-Hungary's ultimatum to Serbia was in July 1914, the grievance is not the reason. It is the paperwork. Watching for that gap is one of the few reliable ways to tell a state that has been provoked from a state that has decided.