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