In World War I , the treaties between Italy and Austria-Hungary , and Romania , which purportedly required Italy and Romania to come to Austria 's aid if Austria was attacked by another nation , were ignored by both Italy and Romania because , as Winston Churchill wrote , " the casus fœderis had not arisen " since the attacks on Austria had not been " unprovoked . " Article 5 of the North Atlantic 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
September 11 attacks
September 11 attacks
Casus foederis