Casus foederis

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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[edit]

War of the Pacific[edit]

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 a Bolivian threat 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 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[edit]

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 ignored by both Italy or Romania because, as Winston Churchill wrote, "the casus fœderis had not arisen" since the attacks on Austria had not been "unprovoked."[1]


Article 5 of the NATO Treaty governs mutual defense in the event of an attack on a member nation. It has been invoked only once: on September 12, 2001, in response to the September 11 attacks on the United States.[2]

See also[edit]


  1. ^ Winston Churchill, The World Crisis at 572 (Abridged -- Free Press 2005).
  2. ^ Invoking Article 5 NATO Review.