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Chemical weapons did not become true weapons of mass destruction (WMD) until they were introduced in their modern form in World War I (1914–18). The German army initiated modern chemical warfare by launching a chlorine attack at Ypres, Belgium, on April 22, 1915, killing 5,000 French and Algerian troops and momentarily breaching their lines of defense. German use of gas and mustard was soon countered by similar tactics from the Allies. By war’s end, both sides had used massive quantities of chemical weapons, causing an estimated 1,300,000 casualties, including 91,000 fatalities. The Russian army suffered about 500,000 of these casualties, and the British had 180,000 wounded or killed by chemical arms. One-third of all U.S. casualties in World War I were from mustard and other chemical gases, roughly the ratio for all participants combined. By the war’s end, all the great powers involved had developed not only offensive chemical arms but also crude gas masks and protective overgarments to defend themselves against chemical weapon attacks. Altogether, the warring states employed more than two dozen different chemical agents during World War I, including mustard gas, which caused perhaps as many as 90 percent of all chemical casualties (though very few of these casualties were fatal) from that conflict.

Other choking gas agents used included chlorine, phosgene, diphosgene, and chloropicrin. The blood agents included hydrogen cyanide, cyanogen, chlorine, and cyanogen bromide. Arsenic-laced sneeze agents were also used, as were tear gases like ethyl bromoacetate, bromoacetone, and bromobenzyl cyanide.

The horrific casualties of World War I helped persuade many world leaders of the need to ban the use of chemical weapons. A number of proposals were made during the 1920s, and at the 1925 Geneva Conference for the Supervision of the International Traffic in Arms (see Geneva Conventions) a protocol was approved and signed by most of the world’s states. The 1925 Geneva Protocol made it illegal to employ chemical or biological weapons, though the ban extended only to those who signed the treaty. The Geneva Protocol did not ban the production, acquisition, stockpiling, or transfer of such arms, and, critically, it did not contain any verification procedure to ensure compliance.

Despite the popular reaction against this form of warfare and the international agreement banning the use of chemical weapons, chemical arms were used a number of times in the years between the two World Wars. For example, chemical weapons were employed by British forces in the Russian Civil War (1919), Spanish forces in Morocco (1923–26), Italian forces in Libya (1930), Soviet troops in Xinjiang (1934), and Italian forces in Ethiopia (1935–40).

During the Sino-Japanese War (1937–45), Japanese forces employed riot-control agents, phosgene, hydrogen cyanide, lewisite, and mustard agents extensively against Chinese targets. There is no record of chemical warfare among World War II belligerents other than that of the Japanese. The Axis forces in Europe and the Allied forces adopted no-first-use policies, though each side was ready to respond in kind if the other acted first. Indeed, all the major powers developed extensive chemical warfare capabilities as a deterrent to their use.

After World War II, chemical weapons were employed on a number of occasions. Egyptian military forces, participating in Yemen’s civil war between royalists and republicans, used chemical weapons, such as nerve and mustard agents, in 1963, 1965, and 1967. During the Soviet intervention into the Afghan War (1978–92), chemical arms, such as mustard and incapacitating agents, were used against the mujahideen rebels. In 1987 Libya used mustard munitions against rebels in Chad.

The most extensive post-World War II use of chemical weapons occurred during the Iran-Iraq War (1980–88), in which Iraq used the nerve agents sarin and tabun, as well as riot-control agents and blister agents like sulfur mustard, resulting in tens of thousands of Iranian casualties. Chemical weapons enabled Iraq to avoid defeat, though not obtain victory, against the more numerous Iranian forces. In response to Iraq’s use of chemical weapons, Iran made efforts to develop chemical weapons and may have used them against Iraq, a contention that Iran has denied. Furthermore, Iran claims to have ended its program when it signed (1993) and ratified (1997) the CWC. Iraq also used chemical weapons (thought to be hydrogen cyanide, sarin, or sulfur mustard gas) against Iraqi Kurds who were considered unfriendly to the regime of Saddam Hussein. The most notorious such attack was the killing of 5,000 Kurds, including many civilians, in the city of Halabjah in 1988.

The most notable use of chemical weapons in the 21st century was during the Syrian Civil War. Syrian troops were reported to have used chemical weapons at Homs and Aleppo on several occasions in early 2013. A larger release of chemical agents took place in the suburbs of Damascus on August 21, 2013, killing several hundred people. Over the objections of Russia and Iran, the United States dispatched five guided-missile destroyers to the eastern Mediterranean and threatened to strike military targets in Syria. U.S. Secretary of State John Kerry’s offhand remark that an attack could be avoided if Syria surrendered all its chemical weapons prompted Russian officials to arrange an agreement calling for Syria to place its chemical weapons stockpile under international control. The United States accepted the Russian initiative, averting military action, and the destruction of weapons began in October. This was by no means the end of chemical weapons use by the regime of Syrian Pres. Bashar al-Assad, however. After a chemical weapons attack was carried out in Khān Shaykhūn in April 2017, the United States barraged the Shayrat air base near Homs with 59 Tomahawk cruise missiles. A year later, after the Syrian government used chemical weapons in Douma, U.S., British, and French forces launched more than 100 strikes targeting chemical weapons facilities near Damascus and Homs.