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Lookups spiked on September 5, 2013.
Use of the word increased during debates over whether the U.S. and other powers should conduct military strikes against Syria.
For example, according to one columnist,
"In 1986, the Reagan administration launched air strikes at the homes of Libyan dictator Moammar Gadhafi, but only after an incident in which Libyan agents had bombed a disco in Berlin, killing two American servicemen. No such casus belli exists with Syria today." Peter Bergen, CNN.com, September 1, 2013
Casus belli means "an event or action that justifies or allegedly justifies a war or conflict." It comes from the Latin for "occasion of war."