United States chemical weapons program

The United States chemical weapons program began in 1917 during World War I with the creation of the U.S. Army's Gas Service Section and effectively ended 73 years later in 1990 with the country's adoption of the Chemical Weapons Convention (signed 1993; entered into force, 1997). Destruction of stockpiled chemical weapons began in 1986 and was completed on July 7, 2023.

As part of the widespread use of chemical weapons in World War I, the U.S. manufactured and used phosgene and mustard gas on the Western Front although these stockpiles represented 4% of the World War I total. In World War II, chemical warfare by or against U.S. forces was practically nonexistent, however the Western Allies, Nazi Germany, and Empire of Japan possessed large chemical stockpiles. In the 1943 Germany air raid on Bari, Italy, the SS John Harvey was hit, unintentionally releasing its secret cargo of mustard gas. Operation Downfall, the unrealized U.S. plan to invade the core islands of Japan in 1945, considered U.S. and Japanese use of chemical weapons.

During the Cold War, the U.S. stockpiled large amounts of mustard and the nerve agents sarin and VX. The Central Intelligence Agency's MKUltra program investigated psychochemical warfare. The Vietnam War saw widespread U.S. use of tear gas and defoliants, including the controversial Agent Orange. Vietnam operations were also alleged to have used sarin in Operation Tailwind, and BZ incapacitating agent. The Dugway sheep incident and the anti-war movement prompted president Richard Nixon to unilaterally renounce the first use of chemical weapons and renounce all offensive use and development of the United States biological weapons program in 1969. The U.S. also forward-deployed chemical weapons, including in U.S.-administered Okinawa and West Germany. The weapons were withdrawn to Johnston Atoll in the 1971 Operation Red Hat and 1990 Operation Steel Box respectively.

As the Cold War ended, the U.S. began destruction of its stockpile in 1986 and signed the Chemical Weapons Convention at its opening in 1993. The chemical stockpile, declared in 1997 and destroyed by 2023, included over 30,000 tons of chemical agents stored within almost 3.5 million munitions, and well over 22,500 tons of agents in other containers. The U.S. Army Medical Research Institute of Chemical Defense (USAMRICD), at Aberdeen Proving Ground, Maryland, continues to operate. The U.S. Army's Chemical Corps has since transitioned to CBRN defense, and the employment of smoke, obscurant, and flame capabilities.