A Latin phrase which literally translates to "occasion of war". The final insult or incident which leads to war between nations. The bombing of Pearl Harbor, the assassination of the Archduke Ferdinand, and more recently even Ariel Sharon's 2000 visit to the Temple Mount could be viewed as examples of casus belli.

Wrong clothes, wrong hair,
wrong book by your bedside there,
wrong language, wrong creed.
"Not like us". That's all we need.
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.
Sources

"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. <http://dictionary.oed.com/cgi/entry/00034315>

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...

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