Casus foederis

From the Latin "case of the alliance". In diplomatic terms, a situation in which the terms of an alliance come into play, such as one nation being attacked by another. Thus, in World War I, the treaties between Italy and Austria, and Romania and Austria, which purported to require Italy and Romania to come to Austria’s aid if Austria was attacked by another nation was 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.” Winston Churchill, The World Crisis at 572 (Abridged -- Free Press 2005).