What is Casus Foederis?


Casus foederis is derived from the Latin for “case for the alliance”. In diplomatic terms, it describes a situation in which the terms of an alliance come into play, such as one nation being attacked by another.

Refer to Casus Belli, Military Alliance, Defence Pact, Non-Aggression Pact, and Entente.

Historical Examples

War of the Pacific

In the War of the Pacific, Bolivia invoked casus foederis to bring Peru into the war after Chile reinvaded Bolivia’s coast. In 1879, Chilean armed forces occupied the port city of Antofagasta after Bolivia threatened to confiscate the Chilean Antofagasta Nitrate Company’s property. Peru attempted to mediate, but when Bolivia announced that a state of war existed, the situation deteriorated. Bolivia called on Peru to activate their secret mutual defence pact, and Chile demanded for Peru to declare its neutrality immediately. On 05 April, Chile declared war on both nations. The following day, Peru responded by acknowledging the casus foederis.

World War I

In World War I, the treaties between Italy and Austria-Hungary, and Romania, which purportedly required Italy and Romania to come to Austria’s aid if Austria was attacked by another nation, were ignored by both Italy and Romania because, as Winston Churchill wrote, “the casus fœderis had not arisen” since the attacks on Austria had not been “unprovoked.”


Refer to North Atlantic Treaty Organisation (NATO).

Article 5 of the North Atlantic Treaty governs mutual defence in the event of an attack on a member nation. It has been invoked only once, on 12 September 2001, in response to the September 11 attacks in the United States.


Where a political-military alliance pact is lacking, there is no obligation to intervene militarily alongside those asking for help, as the decision depends exclusively on the discretionary choices of foreign policy of the requested State.

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