casus belli: Literally translated from the Latin as "an occurrence (or case) of war," casus belli is an event that starts a war or that is used as an excuse to start a war. Some famous examples: the assassination of Archduke Franz Ferdinand, WMDs in Iraq, and Simon Cowell's gruffness.
Casus belli appears to be both singular and plural, but only as it applies to a single war with multiple causes.