is a Latin expression meaning "An act or event that provokes or is used to justify war". Casus is a 4th declension masculine noun. Related to the English word "case", can mean "case", "incident", or "rupture". Belli is the genitive singular case of bellum, belli, a neuter noun of the 2nd declension. means of war. A nation's casus belli involves direct offences or threats against it, whereas a nation's involves offences or threats to an ally nation or nations—usually one with which it has a mutual defence pact, such as NATO.