Casus belli

Europeans had access to Chinese ports as outlined in the Treaty of Nanking from the First Opium War. France used the execution of Auguste Chapdelaine as a casus belli for the Second Opium War. On February 29, 1856, Chapdelaine, a French missionary, was killed in the province of Guangxi, which was not open to foreigners. In response, British and French forces quickly took control of Guangzhou (Canton).

While long-term conflict between the Northern and Southern States (mainly due to moral questions caused by slavery, as well as socio-economic disparities) was the cause of the American Civil War, the Confederate attack on Fort Sumter (April 12–14, 1861) served as casus belli for the Union.[13] Historian David Herbert Donald (1996) concluded that President Abraham Lincoln's "repeated efforts to avoid collision in the months between inauguration and the firing on Ft. Sumter showed he adhered to his vow not to be the first to shed fraternal blood. But he also vowed not to surrender the forts. The only resolution of these contradictory positions was for the confederates to fire the first shot; they did just that."[14] Confederate veteran William Watson opined in 1887 that up until that point, U.S. Secretary of State William H. Seward had not been able to find 'a just cause to declare war against the seceded States', but Sumter gave him 'the casus belli he had sought'.[13] Watson lamented how Jefferson Davis and other Confederate leaders were 'vainglorious[ly]' celebrating the victory at Sumter, while forgetting that making the first move had given the Confederacy the immediate internationally negative reputation of being the aggressor, and had granted Seward 'the undivided sympathy of the North'.[13]

Cartoon of belligerent Uncle Sam placing Spain on notice, c. 1898

The Maine was a United States Navy ship that sank in Havana Harbor, Spanish Cuba on February 15, 1898. While the destruction of the Maine did not result in an immediate declaration of war with Spain, it did create an atmosphere that precluded a peaceful solution.[15] The Spanish investigation found that the explosion had been caused by spontaneous combustion of the coal bunkers, but the US Sampson Board's Court of Inquiry ruled that the explosion had been caused by an external explosion from a torpedo. The McKinley administration did not cite the explosion as a casus belli, but others were already inclined to go to war with Spain over perceived atrocities and loss of control in Cuba.[16] Advocates of war used the rallying cry, "Remember the Maine! To hell with Spain!"[17][18]

Austria-Hungary's casus belli against Serbia in July 1914 was based upon Serbia's refusal to investigate the involvement of Serbian government officials in the equipping, training and paying the assassins who murdered Archduke Franz Ferdinand of Austria at Sarajevo. The Serbian government refused the Austrian Démarche, and Austria-Hungary declared war.

For Britain, the direct cause of entering the war was the German invasion and occupation of Belgium, violating Belgian neutrality which Britain was bound by treaty to uphold.

In 1917, the German Empire sent the Zimmermann Telegram to Mexico, in which they tried to persuade Mexico to join the war and fight against the United States, for which they would be rewarded Texas, New Mexico, and Arizona, all former Mexican territories. This telegram was intercepted by the British, then relayed to the U.S., which led to President Woodrow Wilson then using it to convince Congress to join World War I alongside the Allies. The Mexican president at the time, Venustiano Carranza, had a military commission assess the feasibility, which concluded that this would not be feasible for a number of reasons.

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.[19]

Alfred Naujocks, who organized and led the Gleiwitz incident on the orders of Heydrich.

In August 1939, 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. Nazi forces used concentration camp prisoners posing as Poles on 31 August 1939, to attack the German radio station Sender Gleiwitz in Gleiwitz, Upper Silesia, Germany (since 1945: Gliwice, Poland) on the eve of World War II. Poland's allies, the UK and France, subsequently declared war on Germany in accordance with their alliance. The United States would declare war on Japan after the attack on Pearl Harbor on December 7th, 1941.

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

Many historians have suggested that the Second Gulf of Tonkin Incident was a manufactured pretext for the Vietnam War. North Vietnamese Naval officials have publicly stated that during the second incident the USS Maddox was never fired on by North Vietnamese naval forces.[20][21] In the documentary film "The Fog of War", then-US Defense Secretary Robert McNamara concedes the attack during the second incident did not happen, though he says that he and President Johnson believed it did so at the time.[22]

The first Gulf of Tonkin Incident (the 2nd of August) should not be confused with the second Gulf of Tonkin Incident (4 August). The North Vietnamese claimed that on August 2, US 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".[23]

The casus belli for the Vietnam War was the second incident. On August 4, USS Maddox was launched to the North Vietnamese coast to "show the flag" after the first incident. The US 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.

A casus belli played a prominent role during the Six-Day War of 1967. The Israeli government had a short list of casūs 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 announced a blockade of the straits, prompting Israel to cite its casus belli in opening hostilities against Egypt.

During the 1979 Sino-Vietnamese War, China's leader Deng Xiaoping told the United States that its plan to fight the Vietnamese was revenge for Vietnam's toppling of the Khmer Rouge regime of Cambodia, an ally of China. However Chinese nationalists have argued that the real casus belli was Vietnam's poor treatment of its ethnic Chinese population, as well as suspicion of Vietnam trying to consolidate Cambodia with Soviet backing.[24]

February 5, 2003 – U.S. Secretary of State Colin Powell holding a model vial of anthrax while giving the presentation to the UN Security Council.

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.[25][26]

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.[27] Since-declassified National Intelligence Estimates (NIE's) indicate that any certainty may have been overstated in justification of armed intervention; the extent, origin and intent of these overstatements cannot be conclusively determined from the NIE.[28]

After the annexation of Crimea by the Russian Federation in 2014, Russian President Vladimir Putin argued that Crimea and other regions "were not part of Ukraine" after it was taken in the 18th century. The ethnic Russian population in Crimea and eastern Ukraine has been seen as a casus belli for Russia's annexation.[29] The Foreign Ministry claimed that Ukraine tried to seize Crimean government buildings, citing this as a casus belli.[30]

Prior to the 2022 Russian invasion of Ukraine, Russia recognized the separatist republics in Donetsk and Luhansk, and the alliance between them was ratified in their parliaments, thus creating a usable casus belli.[31] Russia also claimed a genocide was being committed against Russian speakers in Ukraine by neo-Nazi groups and that the Ukrainian government were neo-Nazis.[32] A false-flag operation was also considered by Russia, according to US, UK, and Ukrainian intelligence.[33]

On October 7th, 2023, Palestinian militant groups, led by Hamas, launched a major attack into Israeli territory from the Gaza Strip. This operation was called 'Operation Al-Aqsa Flood' by Hamas. In response, the Israel Defense Forces launched a counteroffensive, officially named 'Operation Iron Swords'.[34][35][36] Following reports that Egypt had given Israel a three day heads-up that "something big" and "an explosion of the situation is coming",[37] questions emerged on how the IDF wasn't prepared,[38] and whether Israel had ignored the information, in order to establish a casus belli on Hamas once they had attacked.[39]