casus belli

noun

Etymology: From Latin cāsus (“occasion”) + bellī (“for war”, literally “of war”).

1. An act seen as justifying or causing a war; an act of war.

   “In ancient times, repeatedly failing to pay tribute by a vassal state was often used as a casus belli by the patron.”

   “The Executive, however, can do many acts which would constitute a casus belli, and thus indirectly result in war; but this does not imply in the Executive a concurrent power to declare war, and the war which would result would be one declared by a foreign power.”