Casus belli

Casus belli is a Latin expression meaning "an act or event that provokes or is used to justify war" (literally, "a case for war"). A casus belli involves direct offenses or threats against the nation declaring the war, whereas a casus foederis involves offenses or threats against its ally—usually one bound by a mutual defense pact. Either may be considered an act of war.

The term came into wide use in the seventeenth and eighteenth centuries through the writings of Hugo Grotius (1653), Cornelius van Bynkershoek (1707), and Jean-Jacques Burlamaqui (1732), 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. It is used retrospectively to describe situations that arose before the term came into wide use, as well as being used to describe present-day situations—even those in which war has not been formally declared.

In formally articulating a casus belli, a government typically lays out its reasons for going to war, its intended means of prosecuting the war, and the steps that others might take to dissuade it from going to war. It attempts to demonstrate that it is going to war only as a last resort (ultima ratio) and that it has "just cause" for doing so. Modern international law recognizes only three lawful justifications for waging war: self-defense, defense of an ally required by the terms of a treaty, and approval by the United Nations.