Casus belli

Casus belli is a modern Latin language expression meaning the justification for acts of war. Casus means "incident", "rupture" or indeed "case", while belli means "of war".

It is often misspelled and mispronounced as "causus belli" since this resembles the English "cause" (and a different Latin word, causa {cause}). "Casus belli" is also pronounced this way because the term is used with the meaning of "cause for war", instead of "case of war" (notice that "case" comes from Latin "casus").

Despite the apparent age that the use of Latin confers on it, the term did not come into wide usage until the late nineteenth century with the rise of the political doctrine of jus ad bellum or "just war theory".citation needed] 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. Effectively international law today only allows two situations as legal cause to go to war. Either out of self-defense or sanctioned by the UN. Any war for another casus belli is considered illegal and as such a war crime.

Proschema (plural proschemata) is the Greek equivalent term. These stated reasons may or may not be the actual reason for waging the war (prophases). 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 prophases commonly play up nationalism or fearmongering (as opposed to rational or reasonable fears).