Casus Belli Law and Legal Definition

Casus belli is a Latin phrase which means “an event or action that justifies or allegedly justifies a war or conflict.” Casus belli is used to refer to offenses or threats directly against a nation. The term came into wide usage in the seventeenth and eighteenth centuries as a result of the rise of the political doctrine of jus ad bellum or "just war theory” and the writings of Hugo Grotius. Self defense, defense of an ally under a mutual defense pact, or sanctioned by the UN are the three situations in which the international law allows any state to commence war. War for any other cause is considered illegal and will be subjected to prosecution for a war crime.