casus belli

sodan oikeutus, casus belli An act seen as justifying or causing a war; act of war.
1977, (w), A Savage War of Peace, New York Review Books 2006, p. 309:

Algiers seethed, and this was the casus belli for the ‘ultras’ to attempt a general strike.

Furthermore, if the French had airily waved away one potential casus belli, more than enough causes of potential conflict remained embedded in the Aix-la-Chapelle Treaty.

Had Saddam taken only the Rumaila oil field and the Bubiyan and Warba islands, there would have been no casus belli.