Casus foederis alternatively spelled Casus fœderis is derived from the Latin for "case of 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.
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."[1]
Also the Ottoman-German Alliance involving the Ottoman Empire and German Reich in World War I[2] worked on this basis, as the Ottomans attacked Russian Black Sea ports on 28 October 1914.[3]
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.[4]