('occasion for war') is an act or event that provokes or is used to justify war
A casus belli is an act or event that sparks a war or serves as its justification. It matters because nations often point to a casus belli to explain why they entered into armed conflict, making it central to how wars are initiated and understood.
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A casus belli (from Latin casus 'occasion' and belli 'for war'; pl. casus belli) is an act or an event that either provokes or is used to justify a war. A casus belli involves direct offenses or threats against the nation declaring the war, whereas a casus foederis involves offenses or threats against its ally—usually one bound by a mutual defense pact. Either may be considered an act of war. A declaration of war usually contains a description of the casus belli that has led the party in question to declare war on another party.
Terminology
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