Casus Belli

Casus belli is a Latin expression meaning the justification for acts of war. Casus means "incident", "rupture" or indeed "case", while belli means bellic ("of war"). It is usually distinguished from casus foederis, where casus belli refers to offenses or threats directly against a nation, and casus foederis refers to offenses or threats to a fellow allied nation with which the justifying nation is engaged in a mutual defense treaty, such as NATO.

The term came into wide usage in the seventeenth and eighteenth centuries with the writings of Hugo Grotius (1653), Cornelius van Bynkershoek (1707), and Jean-Jacques Burlamaqui (1732), among others, and the rise of the political doctrine of jus ad bellum or "just war theory". Informal usage varies beyond its technical definition to refer to any "just cause" a nation may claim for entering into a conflict. As such, it has been used both retroactively to describe situations in history before the term came into wide usage and in the present day when describing situations when war has not been formally declared.

Formally, a government would lay out its reasons for going to war, as well as its intentions in prosecuting it and the steps that might be taken to avert it. In so doing, the government would attempt to demonstrate that it was going to war only as a last resort (ultima Ratio) and that it in fact possessed "just cause" for doing so. In theory international law today allows only three situations as legal cause to go to war: out of self-defense, defense of an ally under a mutual defense pact, or sanctioned by the UN.

Proschema (plural proschemata) is the Greek equivalent term. The stated reasons may or may not be the actual reason for waging the war (prophasis, πρóφασις). The term was first popularized by Thucydides in his History of the Peloponnesian War, who identified fear, honor, and interest as the three primary real reasons that wars are waged, while proschemata commonly play up nationalism or fearmongering (as opposed to rational or reasonable fears).