Casus belli is a Latin phrase for an event or a situation that directly provokes a war. Note that the plural of the phrase is also casus belli. If this was an aggression in the sense of international law, the country is allowed to start a war in self-defence. It is common to provoke an aggressor, so as to be able to legally start a war. Examples of this are the Arrow incident that started the Second Opium War, or the Maine incident, which led the United states to annex Cuba and the Philippines 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 he didn't have any. Another example: the Assassination of Archduke Franz Ferdinand of Austria was the casus belli for the First World War.