Casus foederis

Casus foederis alternatively spelled 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
In the War of the Pacific, Bolivia invoked casus foederis to bring Peru into the war after Chile attacked Bolivia's coast. The war began on February 14, 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 mutual defense pact, whereas Chile demanded that Peru immediately declare its neutrality. On April 5, after Peru resisted both demands, Chile declared war on both nations. The following day, Peru responded by acknowledging the casus foederis.

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."

Also the Ottoman-German Alliance involving the Ottoman Empire and German Reich in World War I worked on this basis, as the Ottomans attacked Russian Black Sea ports on 28 October 1914.

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.