casus belli (KAY-suhs BEL-i, rhymes with bell-eye, BEL-ee) noun plural casus belli
An action or event that causes or is used to justify starting a war. [From New Latin casus belli, from Latin casus (occasion), belli, genitive of bellum (war).]
"Education, both secondary and tertiary, remains a battleground, though the casus belli seems to be more about funding than egalitarianism."
-- Stan Heyl; Class War - The Struggle Goes On; The Independent (London, UK); May 19, 2001.