Casus belli is a Latin expression meaning the justification for acts of war. Casus is related to the English word "case," and can mean "case," "incident", or "rupture". Belli means bellic ("of war"). A nation's casus belli involves offenses or threats directly against it, whereas a nation's casus foederis involves offenses or threats to an ally nation or nations—usually one with which it has a mutual defense pact, such as NATO.
The term came into wide use in the seventeenth and eighteenth centuries through the writings of Hugo Grotius (1653), Cornelius van Bynkershoek (1707), and Jean-Jacques Burlamaqui (1732), among others, and due to the rise of the political doctrine of jus ad bellum or "just war theory". The term is also used informally to refer to any "just cause" a nation may claim for entering into a conflict. It is used retrospectively to describe situations that arose before the term came into wide use, as well as being used to describe present-day situations—even those in which war has not been formally declared.
In formally articulating a casus belli, a government typically lays out its reasons for going to war, its intended means of prosecuting the war, and the steps that others might take to dissuade it from going to war. It attempts to demonstrate that it is going to war only as a last resort (ultima ratio) and that it has "just cause" for doing so. Modern international law recognizes only three lawful justifications for waging war: self-defense, defense of an ally required by the terms of a treaty, and approval by the United Nations.
Proschema (plural proschemata) is the equivalent Greek term, first popularized by Thucydides in his History of the Peloponnesian War. The proschemata are the stated reasons for waging war, which may or may not be the same as the real reasons, which Thucydides called (prophasis (πρóφασις). Thucydides argued that the three primary real reasons for waging war are reasonable fear, honor, and interest, while the stated reasons involve appeals to nationalism or fearmongering (as opposed to descriptions of reasonable, empirical causes for fear).
Reasons for use
Countries need a public justification for attacking another country, both to galvanize internal support for the war, and to gain the support of potential allies.
In the post-World-War-II era, the UN Charter prohibits signatory countries from engaging in war except: 1) as a means of defending themselves - or an ally where treaty obligations require it - against aggression; 2) unless the UN as a body has given prior approval to the operation. The UN also reserves the right to ask member nations to intervene against non-signatory countries that embark on wars of aggression.
In pop-culture, the phrase "casus belli" was frequently said in the season three episode of Seinfeld, "The Cafe." Elaine states she read the phrase and enjoys saying it out loud. Elaine and Jerry then repeat the phrase multiple times. 
This section outlines a number of the more famous and/or controversial cases of casus belli which have occurred in modern times.
There have been several alternative explanations to the explosion such as that proposed by Mr. Evans, the senior editor of Newsweek. In his book, he states that the USS Maine was designed incorrectly because the boiler room was right next to the gunpowder storage room and that an overheating in the boiler room may have heated the adjacent metal wall, causing the powder to explode.
World War I
The Assassination of Archduke Franz Ferdinand of Austria provided the trigger that led to the outbreak of World War I. In June 1914, the refusal of two points of the July Ultimatum offered to Serbia, was used by Austria-Hungary as a casus belli for declaring war on Serbia. The murder at Sarajevo in Bosnia by Gavrilo Princip, a Bosnian Serb nationalist, Austrian subject and member of Young Bosnia (a secret society supported by the Serbian government), was the reason why this ultimatum was made.
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.
World War II
In his autobiography Mein Kampf, Adolf Hitler had in the 1920s advocated 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. Poland's allies, the UK and France, honoured their alliance and subsequently declared war on Germany.
The Soviet Union also employed a manufactured casus belli against Finland during World War II on its part. 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 Winter War. In 1998, Russian President Boris Yeltsin admitted that the invasion had in fact constituted a Soviet war of aggression.
A casus belli played a prominent role during the Six-Day War of 1967. The Israeli government had a short list of casūs bellorum, 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 announced a blockade of the straits, prompting Israel to cite its casus belli in opening hostilities against Egypt.
Many 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. In the documentary film "The Fog of War", then-U.S. Defense Secretary Robert McNamara concedes the attack did not happen, though he says that he and President Johnson believed it did so at the time.
Some people[who?] confuse the first Gulf of Tonkin Incident (the 2nd of August) and the second Gulf of Tonkin Incident (the 4th of August). The North Vietnamese claimed that on August 2, U.S. destroyer USS Maddox was hit by one torpedo and that one of the American aircraft had been shot down in North Vietnamese territorial waters. The PAVN Museum in Hanoi displays "Part of a torpedo boat... which successfully chased away the USS Maddox August, [sic] 2nd 1964".
The casus belli for the Vietnam War was the second incident. On August 4, USS Maddox was launched to the North Vietnamese coast in order to "show the flag" after the first incident. The U.S. authorities claimed that two Vietnamese boats tried to attack USS Maddox and were sunk. The government of North Vietnam denied the second incident completely. Deniability played favorably into the propaganda efforts of North Vietnam throughout the war, and for some years to follow.
1982 Israeli invasion of Lebanon
The casus belli cited by Israel for its June 1982 invasion of Lebanon was the attempted assassination of the Israeli Ambassador in London, which the Israeli government blamed on the Palestinian Liberation Organization. A possible invasion plan had been prepared in advance by Israel.
War on Terror
The casus belli for the Bush administration's conceptual War on Terror, which resulted in the 2001 Afghanistan war, was the September 11 attacks in 2001 on the World Trade Center in New York City, The Pentagon in Arlington, Virginia, and the intended attack on the United States Capitol in Washington, D.C.
2003 Invasion of Iraq
When the United States invaded Iraq in 2003, it cited Iraq's non-compliance with the terms of cease-fire agreement for the 1990-1991 Gulf War, as well as planning in the 1993 attempted assassination of former President George H. W. Bush and firing on coalition aircraft enforcing the no-fly zones as its stated casus belli.
Cited by the George W. Bush administration was Saddam Hussein's weapons of mass destruction (WMD) program. The administration 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 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.
- Bynkershoek, Cornelius van (2007). A Treatise on the Law of War. Lawbook Exchange. ISBN 1-58477-566-1.
- Bynkershoek, Cornelius van (1995). On Questions of Public Law. William S. Hein & Company. ISBN 1-57588-258-2.
- Russell, Frederick H. (1997). The Just War in the Middle Ages. Cambridge University Press. ISBN 0-521-29276-X.
- Childress, James F. (1978). "Just-War Theories: The Bases, Interrelations, Priorities, and Functions of Their Criteria". Theological Studies 39: 427–45.
- "McNamara asks Giap: What happened in Tonkin Gulf?". (November 9, 1995). Associated Press
- CNN Cold War - Interviews: Robert McNamara, retrieved January 23, 2007
- Sachar, Howard M.: A History of Israel from the Rise of Zionism to Our Time, Alfred A. Knopf 1996, ISBN 0-679-76563-8, page 904.
- "As early as January 1982, therefore, with Begin's approval, Sharon paid a secret visit to Beirut.... By the following month... operational plans for the offensive were well advanced. Israeli liaison officers repeatedly visited Beirut to coordinate strategy with the Phalange. In the end, the Lebanon expedition would be the most thoroughly prepared campaign in Israel's history." - Sachar, A History of Israel, p. 903.
- http://georgewbush-whitehouse.archives.gov/news/releases/2002/10/20021002-2.html http://www.milnet.com/public-law-102-1.html
- Vidal, Gore. Imperial America: Reflections on the United States of Amnesia. Hardcover ed. Avalon Group.