''Casus foederis'' (or ''casus fœderis'') is derived from the

Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...

for "case for the alliance". In

diplomatic Diplomatics (in American English, and in most anglophone countries), or diplomatic (in British English), is a scholarly discipline centred on the critical analysis of documents: especially, historical documents. It focuses on the conventions, p ...

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

War of the Pacific

In the

War of the Pacific The War of the Pacific ( es, link=no, Guerra del Pacífico), also known as the Saltpeter War ( es, link=no, Guerra del salitre) and by multiple other names, was a war between Chile and a Bolivian–Peruvian alliance from 1879 to 1884. Fought ...

Bolivia , image_flag = Bandera de Bolivia (Estado).svg , flag_alt = Horizontal tricolor (red, yellow, and green from top to bottom) with the coat of arms of Bolivia in the center , flag_alt2 = 7 × 7 square p ...

invoked ''casus foederis'' to bring

Peru , image_flag = Flag of Peru.svg , image_coat = Escudo nacional del Perú.svg , other_symbol = Great Seal of the State , other_symbol_type = Seal (emblem), National seal , national_motto = "Fi ...

into the war after

Chile Chile, officially the Republic of Chile, is a country in the western part of South America. It is the southernmost country in the world, and the closest to Antarctica, occupying a long and narrow strip of land between the Andes to the east a ...

reinvaded Bolivia's coast. In 1879, Chilean armed forces occupied the port city of

Antofagasta Antofagasta () is a port city in northern Chile, about north of Santiago. It is the capital of Antofagasta Province and Antofagasta Region. According to the 2015 census, the city has a population of 402,669. After the Spanish American wars ...

after Bolivia threatened to confiscate the 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 secret mutual defense pact, and Chile demanded for Peru to declare its neutrality immediately. On April 5, Chile declared war on both nations. The following day, Peru responded by acknowledging the ''casus foederis''.

World War I

World War I World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, the United States, and the Ottoman Empire, with fightin ...

, the treaties between

Italy Italy ( it, Italia ), officially the Italian Republic, ) or the Republic of Italy, is a country in Southern Europe. It is located in the middle of the Mediterranean Sea, and its territory largely coincides with the homonymous geographical re ...

and

Austria-Hungary Austria-Hungary, often referred to as the Austro-Hungarian Empire,, the Dual Monarchy, or Austria, was a constitutional monarchy and great power in Central Europe between 1867 and 1918. It was formed with the Austro-Hungarian Compromise of ...

, and

Romania Romania ( ; ro, România ) is a country located at the crossroads of Central Europe, Central, Eastern Europe, Eastern, and Southeast Europe, Southeastern Europe. It borders Bulgaria to the south, Ukraine to the north, Hungary to the west, S ...

, 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 Sir Winston Leonard Spencer Churchill (30 November 187424 January 1965) was a British statesman, soldier, and writer who served as Prime Minister of the United Kingdom twice, from 1940 to 1945 Winston Churchill in the Second World War, dur ...

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

NATO

Article 5 of the

North Atlantic Treaty The North Atlantic Treaty, also referred to as the Washington Treaty, is the treaty that forms the legal basis of, and is implemented by, the North Atlantic Treaty Organization (NATO). The treaty was signed in Washington, D.C., on 4 April 1949 ...

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 The September 11 attacks, commonly known as 9/11, were four coordinated suicide terrorist attacks carried out by al-Qaeda against the United States on Tuesday, September 11, 2001. That morning, nineteen terrorists hijacked four commercia ...

in the

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...

Exceptions

Where a political-military alliance pact is lacking, there is no obligation to intervene militarily alongside those asking for help, as the decision depends exclusively on the discretionary choices of foreign policy of the requested State.For example, on 18 December 2021 the UK's defence secretary Ben Wallace told the

Spectator ''Spectator'' or ''The Spectator'' may refer to: *Spectator sport, a sport that is characterized by the presence of spectators, or watchers, at its matches *Audience Publications Canada * ''The Hamilton Spectator'', a Hamilton, Ontario, Canada, ...

magazine that Ukraine was "not a member of

Nato The North Atlantic Treaty Organization (NATO, ; french: Organisation du traité de l'Atlantique nord, ), also called the North Atlantic Alliance, is an intergovernmental military alliance between 30 member states – 28 European and two No ...

, so it is highly unlikely that anyone is going to send troops into

Ukraine Ukraine ( uk, Україна, Ukraïna, ) is a country in Eastern Europe. It is the second-largest European country after Russia, which it borders to the east and northeast. Ukraine covers approximately . Prior to the ongoing Russian inv ...

to challenge Russia"
''UK troops will not be deployed to Ukraine to defend against Russia'', BBC news, 18 December 2021

References