Casus belli 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 ( ) . 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 . Historical examples . This section outlines a number of the more famous and / or controversial cases of " casus belli " which have occurred in modern times . Spanish - American War . The " casus belli " in the eyes of the United States for the Spanish – American War was the sinking of the USS " Maine " . 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 " " for the invasion of Poland the following September . Poland ' s allies , the UK and France , honoured their alliance and subsequently declared war on Germany . 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 " 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 . 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 " " , 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 . Vietnam War . 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 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 , 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 " " 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 " 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 " . 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 .