Casus foederis alternatively spelled Casus fœderis is derived from the Latin

Latin

Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 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

World War I

World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...

, the treaties between Italy

Kingdom of Italy (1861–1946)

The Kingdom of Italy was a state forged in 1861 by the unification of Italy under the influence of the Kingdom of Sardinia, which was its legal predecessor state...

 and Austria-Hungary

Austria-Hungary

Austria-Hungary , more formally known as the Kingdoms and Lands Represented in the Imperial Council and the Lands of the Holy Hungarian Crown of Saint Stephen, was a constitutional monarchic union between the crowns of the Austrian Empire and the Kingdom of Hungary in...

, and Romania

Romania

Romania is a country located at the crossroads of Central and Southeastern Europe, on the Lower Danube, within and outside the Carpathian arch, bordering on the Black Sea...

, 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

Winston Churchill

Sir Winston Leonard Spencer-Churchill, was a predominantly Conservative British politician and statesman known for his leadership of the United Kingdom during the Second World War. He is widely regarded as one of the greatest wartime leaders of the century and served as Prime Minister twice...

 wrote, "the casus fœderis had not arisen" because the attacks on Austria had not been "unprovoked."

Also the Ottoman-German Alliance

Ottoman-German Alliance

The Ottoman–German Alliance was established between the Ottoman Empire and the German Empire on August 2, 1914. It was this binding alliance that ultimately led the Ottoman Empire to enter the First World War on the side of the Central Powers....

 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

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

.