Casus belli is a modern Latin language expression meaning the justification for acts of war. Casus means "incident", "rupture" or indeed "case", while belli means "of war".

It is often misspelled and mispronounced as "causus belli" since this resembles the English "cause" (and a different Latin word, causa {cause}). "Casus belli" is also pronounced this way because the term is used with the meaning of "cause for war", instead of "case of war" (notice that "case" comes from Latin "casus").

Despite the apparent age that the use of Latin confers on it, the term did not come into wide usage until the late nineteenth century with the rise of the political doctrine of jus ad bellum or "just war theory".[citation needed] Informal usage varies beyond its technical definition to refer to any "just cause" a nation may claim for entering into a conflict. As such, it has been used both retroactively to describe situations in history before the term came into wide usage and in the present day when describing situations when war has not been formally declared.

Formally, a government would lay out its reasons for going to war, as well as its intentions in prosecuting it and the steps that might be taken to avert it. In so doing, the government would attempt to demonstrate that it was going to war only as a last resort (ultima Ratio) and that it in fact possessed "just cause" for doing so. Effectively international law today only allows two situations as legal cause to go to war. Either out of self-defense or sanctioned by the UN. Any war for another casus belli is considered illegal and as such a war crime.

Proschema (plural proschemata) is the Greek equivalent term. These stated reasons may or may not be the actual reason for waging the war (prophases). The term was first popularized by Thucydides in his History of the Peloponnesian War, who identified fear, honor, and interest as the three primary real reasons that wars are waged, while prophases commonly play up nationalism or fearmongering (as opposed to rational or reasonable fears).

Cause of use[]

Casus belli can be used to avoid loss of morale in the country or nation[citation needed] or to gain the support of the people. If a country attacked another country with no stated reason, it may cause discontent among its populace and loss of faith in their leaders and may, in extreme cases, lead to revolt or other kinds of civil uprisings.[citation needed]

In modern times casus belli may not be focused primarily on convincing the population but instead be aimed at justifying the action to the global community, which would equally affect dictatorships and militarily controlled nations who might not previously have had need of a convincing casus belli among its own people.[citation needed]

The U.S. State Department announced today 8/20/07 that a casus belli can be applied for the current situation regarding the (IRGC) Iranian Revolutionary Guard Corp who have been classified as a terrorist organisation pre-empting preparation for an attack during spring '08.[citation needed]

Historic uses[]

Spanish-American War[]

The US navy ship USS Maine sank in the Havana Harbor from an explosion whose cause remains controversial. Critics such as Gore Vidal have claimed that the explosion was a purposeful act to create a phony casus belli for the US to attack the Spanish. This gave the United States the political cover to have an excuse to attack Spain triggering the Spanish-American War because the US government accused the Spaniards of being responsible for the explosion[citation needed].

World War I[]

A political assassination provided the trigger that led to the outbreak of World War I. The assassination in June 1914 of Archduke Franz Ferdinand of Austria at Sarajevo in Austria-Hungary by Gavrilo Princip, a Yugoslav nationalist from Bosnia, Austrian subject and member of Young Bosnia, was used by Austria-Hungary as a casus belli for declaring war on Serbia.

The Russian Empire started to mobilise its troops in defence of its ally Serbia, which resulted in the German Empire declaring war on Russia in support of its ally Austria-Hungary. Very quickly, after the involvement of France, the Ottoman Empire and the British Empire, five of the six great European powers became involved in the first European general war since the Napoleonic Wars. (see Causes of World War I)

World War II[]

In his autobiography Mein Kampf, Adolf Hitler had advocated in the 1920's a policy of lebensraum ("living space") for the German people, which in practical terms meant German territorial expansion into Eastern Europe.

In August 1939, in order to implement the first phase of this policy, Germany's Nazi government under Hitler's leadership staged the Gleiwitz incident, which was used as a casus belli for the invasion of Poland the following September. Since Poland's allies Britain and France honoured their alliance and subsequently declared war on Germany, the invasion of Poland marks the start of World War II.

In 1941, acting once again in accordance with the policy of lebensraum, Nazi Germany invaded the Soviet Union, using the casus belli of pre-emptive war to justify the act of aggression.

The Soviet Union also employed a manufactured casus belli during World War II. In November 1939, shortly after the outbreak of hostilities between Germany, Britain and France, the Soviet Union staged the shelling of the Russian village of Mainila, which it blamed on the Finns. This manufactured incident was then used as a casus belli for the invasion of Finland. In 1998, Russian President Boris Yeltsin admitted that the invasion had in fact constituted a Soviet war of aggression. There is indirect evidence that President Franklin Roosevelt knew of the Japanese plans to attack Pearl Harbor because the Japanese Code Purple had been deciphered by British signals intelligence and US Naval Intelligence. This attack unified the US which was an isolationist nation to support the president's call to go to war.

Six-Day War[]

A casus belli played a prominent role during the Six-Day War of 1967. The Israeli government had a short list of casus belli, acts that it would consider provocations justifying armed retaliation. The most important was a blockade of the Straits of Tiran leading into Eilat, Israel's only port to the Red Sea, through which Israel received much of its oil. After several border incidents between Israel and Egypt's allies Syria and Jordan, Egypt expelled UNEF peacekeepers from the Sinai Peninsula, established a military presence at Sharm el-Sheikh, and blockaded the straits, prompting Israel to cite its casus belli in opening hostilities against Egypt.

Vietnam War[]

Some historians have suggested that the Gulf of Tonkin Incident was a manufactured pretext for the Vietnam War. North Vietnamese Naval officials have publicly stated that the USS Maddox was never fired on by North Vietnamese naval forces[1][2].

Israeli Invasion of Lebanon[]

The casus belli behind the 1982 Lebanon War was the attempted assassination of the Israeli Ambassador in London. Some have claimed that the rogue Mossad officers were responsible for this "fake assassination attempt" eventhough the ambassador was not involved in the plan.[citation needed]

Turkey and Greece[]

In 1995, the Turkish parliament issued a casus belli against Greece in reaction to an enacted extension of Greek territorial waters from 6 to 12 nautical miles from the coast.[citation needed] Turkey has not removed this casus belli despite initiation of preliminary negotiations in order for it to join the European Union.[citation needed]

War on Terror[]

The casus belli for the Bush administration's concept War on Terror, which resulted in the 2001 Afghan war and the 2003 Iraq war, was allegedly the involvement in the September 11, 2001 attacks on the World Trade Center in New York City, The Pentagon in Arlington, Virginia and the apparently intended attack on the United States Capitol in Washington, D.C..

2003 Invasion of Iraq[]

The main casus belli cited by the Bush Administration for its 2003 invasion of Iraq was the imminent threat posed by Saddam Hussein's purported weapons of mass destruction programme. They erroneously claimed that Iraq had not conformed with its obligation to disarm under past UN Resolutions, and that Saddam Hussein was actively attempting to acquire a nuclear weapons capability as well as enhance an alleged existing arsenal of chemical and biological weapons. Secretary of State Colin Powell addressed a plenary session of the United Nations Security Council on February 5, 2003 citing these reasons as justification for military action.[1]

A more comprehensive enumeration of factors used to justify the administration's choice to select military action is included in Public Law 107-243, (AUTHORIZATION FOR USE OF MILITARY FORCE AGAINST IRAQ RESOLUTION OF 2002), commonly noted as the Congressional resolution on Iraq signed into law in Oct 2002. This resolution lists Iraq's alleged:

  • WMD program;
  • links to terrorism;
  • failure to co-operate with weapons inspectors;
  • suppression of its own population;
  • threats to neighbouring countries;
  • firing on allied aircraft enforcing the no-fly zone;
  • failure to return all Kuwaiti property or all POW's after the 1991 Gulf war;
  • violation of UN Resolutions pertaining to all of the above;

- as factors in the Congressional vote to authorize military action. [2]


  • Vidal, Gore. Imperial America: Reflections on the United States of Amnesia. Hardcover ed. Avalon Group.
  1. "McNamara asks Giap: What happened in Tonkin Gulf?". (November 9, 1995). Associated Press
  2. CNN Cold War - Interviews: Robert McNamara, retrieved January 23, 2007

See also[]

  • Command responsibility
  • Jus ad bellum
  • UN Charter
  • War of aggression