Casus belli is a Latin expression from the international law theory of Jus Ad Bellum. Formally, the expression (which can be translated as "risk of war" or "occasion for war") is the grievances section of a formal public declaration of war by a state, which lists: the grievances it has against another state which are or may become the cause of war; the intentions it has in prosecuting the war; and the actions the other state could take to avert conflict or restore peace. The declaration thus seeks to meet the Jus Ad Bellum criteria of "Just Cause", "Public Declaration", and Ultima Ratio ("Last Resort"). However formal declarations of war are rare nowadays, and casus belli is now widely used to simply mean a nation's motives for going to war, without reference to any formal documents or proposed means of redress, and sometimes without even implying that these motives are just.

The expression can however be translated in slightly different ways:

• This is the case, the opportunity for a war;
• This is how things go during a war, a typical consequence of war (casus can also mean "accident", "casual event").

Adding to the confusion, the term is widely misspelled as causus belli which is translated as "cause of war".

Like many Latin expressions, this one is sometimes used improperly by politicians or journalists for instrumental purposes. Its correct use is more often found in historical works, when describing ex post those events which effectively led to a war.

Today casus belli is also used to indicate a French role-playing game magazine and computer wargames.

See also

• Latin phrases
• Latin proverbs