Since Webster 1913 is mute, I turned to the Oxford English Dictionary (OED), which says simply:
An act justifying, or regarded as a reason for, war.
I'll admit I was surprised not to find more here, because my understanding of the term included a specific doctrine or school of thought in international law and diplomacy. I figured the OED would have page after page of discussion of this use of the term, but it doesn't. I guess international law really is still in its infancy. Happily, E2 has already covered many of these meanings (without specifically mentioning casus belli!) in Just War Doctrine, a just cause, History of War, and the Geneva Convention.

"Casus Belli", Oxford English Dictionary. Ed. J. A. Simpson and E. S. C. Weiner. 2nd ed. Oxford: Clarendon Press, 1989. OED Online. Oxford University Press. 6 Oct. 2002. <>

Lometa adds, the Dictionary of Law, Oxford University Press in 1997 said: "The only legitimate casus belli now is an unprovoked attack necessitating self-defence on the part of the victim." Forgive me, but I wonder what was considered a legitimate casus prior to 1997...and I wonder about the word "unprovoked". It seems to imply that certain attacks, namely those that are "provoked", do not permit the provocateur to claim a legitimate casus belli in response to the attack...