Casus belli
Casus belli is a Latin phrase for an event or a situation that directly provokes a war. The plural of the phrase is the same as its singular.
In an aggression in the sense of international law, a country is allowed to start a war in self-defence. It is common to provoke an aggressor to be able to start a war legally.
Suspected examples are the Arrow incident, which started the Second Opium War, and the Maine incident, which led the United States to take Puerto Rico and the Philippinesand to occupy Cuba from the Spanish Empire. The Iraq War was started by the US and its allies on the pretext that Saddam Hussein had weapons of mass destruction. Years after the war, it turned out that there were none in Iraq.
Another example is the assassination of Archduke Franz Ferdinand of Austria starting World War I.