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.[1][2]

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".[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.

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