Casus belli

A casus belli is an act or event invoked as justification for starting or declaring war. It can be a genuine cause, a public rationale, or both.

Gulf of Tonkin incident: A pair of reported naval encounters in 1964 that prompted the United States Congress to authorize expanded military action in Vietnam. The disputed second encounter helped justify a major escalation of U.S. involvement in Vietnam.
Just war theory: A tradition of ethical thought that sets conditions for morally justified war and governs conduct during war. Its criteria distinguish a morally adequate cause from a mere public pretext.
War guilt: Responsibility assigned to a state or party for causing or initiating a war. A casus belli is a stated justification; war guilt is a broader judgment of responsibility.
Ultimatum: A final demand that sets conditions and usually threatens consequences for noncompliance. An ultimatum can turn a disputed incident into an immediate diplomatic crisis.
Mukden Incident: A 1931 explosion on a Japanese-owned railway in Manchuria, staged by Japanese officers as a pretext for invasion. Japan used the staged explosion to justify its occupation of Manchuria.
Declaration of war: A formal statement by a political authority that a state of war exists with another polity. A casus belli may be presented as grounds for this formal act, but need not accompany one.
War of aggression: A state's use of armed force against another state's sovereignty or political independence in violation of international law. An asserted casus belli does not by itself prevent an invasion from being aggression.
Mobilization: The organization and deployment of a state's armed forces for military operations. Mobilization can follow an asserted casus belli and make escalation harder to reverse.
Gleiwitz incident: A staged 1939 attack on a German radio station, carried out by Nazi Germany and blamed on Poland. Nazi Germany presented the false attribution as evidence for its invasion of Poland.
War propaganda: Persuasive communication designed to shape public support for or opposition to war. Publicizing a casus belli can build support by framing an enemy's act as intolerable.

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