Casus foederis

Casus foederis (or casus fœderis) 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.

Historical examples

War of the Pacific

See also: Secret treaty of alliance between Peru and Bolivia of 1873

In the War of the Pacific, Bolivia invoked casus foederis to bring Peru into the war after Chile reinvaded Bolivia's coast. 1879 when Chilean armed forces occupied the port city of Antofagasta, after a Bolivian threat to confiscate 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 defense pact, whereas Chile demanded that Peru immediately declare its neutrality. On April 5, 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 purported to require Italy and Romania to come to Austria's aid if Austria was attacked by another nation, were not honored by either Italy or Romania because, as Winston Churchill wrote, "the casus fœderis had not arisen" because the attacks on Austria had not been "unprovoked."

NATO

Article 5 of the NATO Treaty governs mutual defense in the event of an attack on a member nation. This article has only been invoked once: on September 12, 2001, in response to the 9/11 attacks on the United States.

See also

• Casus belli
• List of Latin phrases