n occurrence giving rise to or justifying war.
1849, from Latin casus "situation" (see situation (n.1)) + belli, genitive of bellum "war" (view bellicose). An act justifying war.
a meeting regularly justify beginning a war
casus belli [Lat.: whatever requires or warrants war]
To Athens no formal declaration of war but rather sought first to create some specious casus belli by sending requisitions to Athens.