Casus belli
Casus belli is a Latin expression meaning "An act or event that provokes or is used to justify war".
Casus is a 4th declension neuter noun. Related to the English word "case",
casus can mean "case", "incident", or "rupture".
Belli is the genitive singular case of
bellum, belli, a neuter noun of the 2nd declension.
Belli means
of war. A nation's
casus belli involves direct offences or threats against it, whereas a nation's
casus foederis involves offences or threats to an ally nation or nations—usually one with which it has a mutual defence pact, such as NATO. The term came into wide use in the seventeenth and eighteenth centuries through the writings of Hugo Grotius, Cornelius van Bynkershoek, and Jean-Jacques Burlamaqui, among others, and due to the rise of the political doctrine of
jus ad bellum or "just war theory". The term is also used informally to refer to any "just cause" a nation may claim for entering into a conflict.