Since World War I, mustard gas has been used in several wars or other conflicts, usually against people who cannot retaliate in kind:
United Kingdom against the Red Army in 1919
Spain and France against the Rifian resistance in Morocco during 1921–27
Italy in Libya during 1930
The Soviet Union in Xinjiang, Republic of China, during the Soviet Invasion of Xinjiang against the 36th Division (National Revolutionary Army) in 1934, and also in the Xinjiang War (1937) during 1936–37
Italy against Abyssinia (now Ethiopia) from 1935 to 1940
The Japanese Empire against China during 1937–45
Egypt against North Yemen during 1963–67
Iraq against civilian Kurds and the Iranians during 1983–88 in the town of Halabja
Possibly Sudan against insurgents in the civil war, in 1995 and 1997
In 1943, during the Second World War, an American shipment of mustard gas exploded aboard a supply ship that was bombed during an air raid in the harbor of Bari, Italy. Eighty-three of the 628 hospitalized victims who had been exposed to the mustard gas died. The deaths and incident were partially classified for many years.
From 1943 to 1944, mustard gas experiments were performed on Australian service volunteers in tropical Queensland, Australia, by British Army and American experimenters, resulting in some severe injuries. One test site, the Brook Islands National Park, was chosen to simulate Pacific islands held by the Imperial Japanese Army.
After WWII stockpiled mustard gas was dumped by the British in the sea near Port Elizabeth, South Africa, resulting in burn cases among trawler crews.
The use of poison gases, including mustard gas, during warfare is known as chemical warfare, and this kind of warfare was prohibited by the Geneva Protocol of 1925, and also by the later Chemical Weapons Convention of 1993. The latter agreement also prohibits the development, production, stockpiling, and sale of such weapons.