's iconic World War I painting:
Gassed, showing blind casualties on a battlefield after a mustard gas attack Simple chemical weapons were used sporadically throughout antiquity and into the Industrial age. It was not until the 19th century that the modern conception of chemical warfare emerged, as various scientists and nations proposed the use of asphyxiating or poisonous gases. So alarmed were nations that multiple international treaties, discussed below, were passed banning chemical weapons. This however did not prevent the extensive
use of chemical weapons in World War I. The development of
chlorine gas, among others, was used by both sides to try to break the stalemate of
trench warfare. Though largely ineffective over the long run, it decidedly changed the nature of the war. In most cases the gases used did not kill, but instead horribly maimed, injured, or disfigured casualties. Estimates for military gas casualties range from 500k to 1.3 million, with a few thousand additional civilian casualties as collateral damage or production accidents. The
interwar period saw occasional use of chemical weapons, mainly by multiple European colonial forces to put down rebellions. The Italians also used poison gas during their 1936
invasion of Ethiopia. In Nazi Germany, much research went into developing new chemical weapons, such as potent
nerve agents. However, chemical weapons saw little battlefield use in World War II. Both sides were prepared to use such weapons, but the Allied powers never did, and the Axis used them only very sparingly. The reason for the lack of use by the Nazis, despite the considerable efforts that had gone into developing new varieties, might have been a lack of technical ability or fears that the Allies would retaliate with their own chemical weapons. Those fears were not unfounded: the Allies made comprehensive plans for defensive and retaliatory use of chemical weapons, and stockpiled large quantities. Japanese forces used them more widely, though only against their Asian enemies, as they also feared that using it on Western powers would result in retaliation. Chemical weapons were frequently used against
Kuomintang and
Chinese communist troops. However, the Nazis did extensively use poison gas against civilians in
the Holocaust. Vast quantities of
Zyklon B gas and carbon monoxide were used in the
gas chambers of Nazi extermination camps, resulting in the overwhelming majority of some three million deaths. This remains the deadliest use of poison gas in history. The post-war era has seen limited, though devastating, use of chemical weapons. Some 100,000 Iranian troops were casualties of Iraqi chemical weapons during the
Iran–Iraq War. Iraq used mustard gas and nerve agents against its own civilians in the 1988
Halabja chemical attack. The
Cuban intervention in Angola saw limited use of
organophosphates. The Syrian government has used
sarin, chlorine, and mustard gas in the
Syrian civil war generally against civilians. Terrorist groups have also used chemical weapons, notably in the
Tokyo subway sarin attack and the
Matsumoto incident. During the
2026 Iran massacres, reports from 17 January indicated the Iranian government may have used chemical weapons against protesters. Footage showed security forces atop vehicles wearing hazmat suits and masks designed for hazardous chemical materials. On 23 January, chemical gas was reported to have been used on the crowds of protesters and escape routes, causing severe breathing problems, burning pain of the eyes, skin and lungs, vomiting blood, and sudden weakness and loss of movement. On 4 February, a group of human rights organizations called for an investigation into the alleged use of chemical weapons. See also
chemical terrorism. == International law==