Casus belli
Etymology
A Latin phrase meaning literally "case for war," from "casus," an occurrence or case, and "belli," the genitive of "bellum," war. It entered diplomatic and legal usage to name the specific grievance invoked to legitimize armed conflict.
noun phrase · /KAH-soos BEL-ee/
An act or event that a state offers as the justification or provocation for going to war.
What it names
The precise term for the stated reason for a war — useful for separating the official trigger from the deeper motives, which are often quite different.
Example
The border skirmish was slight, but it was seized upon as the casus belli for an invasion long in the planning.
Related idea
Often manufactured by a false flag; the casus belli is the justification presented to the world, while the false flag is the deceptive incident sometimes staged to produce it.