Casus belli, Latin for "occasion for war", is an incident which ultimately leads to war between two or more countries, or within a country.
The action may be a real aggression (such as the Confederate attack on Fort Sumter which led to the American Civil War, or the Japanese attack on Pearl Harbor which led to America's entry into World War II) or a false flag operation (such as the Gleiwitz Incident, a Nazi attack disguised as a Polish one, leading to Germany's invasion of Poland and ultimately leading to World War II).