casus belli

< /ˈkeɪ səs ˈbɛl aɪ, ˈbɛl i; Latin ˈkɑ sus ˈbɛl li/.

1. an event or political occurrence that brings about a declaration of war.

noun plural casus belli (ˈkɑːsʊs ˈbɛliː)

1. an event or act used to justify a war
2. the immediate cause of a quarrel

n.

1849, from Latin casus “case” (see case (n.1)) + belli, genitive of bellum “war” (see bellicose). An act justifying war.