Casus belli (pronounced
/'keɪsəs 'bɛlaɪ/) is a
Latin 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.
[1][2]It is sometimes misspelled and mispronounced as "causus belli" since this resembles the English "cause" (and a different Latin word, causa {cause}).[citation needed] "Casus belli" is sometimes 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"). The OED, however, gives the pronunciation above.
The term came into wide usage in the seventeenth and eighteenth centuries with the writings of Hugo Grotius (1625), Cornelius van Bynkershoek (1737), and Jean-Jacques Burlamaqui (1748), among others, and the rise of the political doctrine of jus ad bellum or "just war theory".[3][4] 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 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. Any war for another cause is considered illegal and those who engage in it subject to prosecution for a war crime.
