Poison gas in World War I

Although the use of toxic chemicals as weapons dates back thousands of years, the first large scale use of chemical weapons was during World War I. They were primarily used to demoralize, injure, and kill entrenched defenders, against whom the indiscriminate and generally very slow-moving or static nature of gas clouds would be most effective. The types of weapons employed ranged from disabling chemicals, such as tear gas, to lethal agents like phosgene, chlorine, and mustard gas. This chemical warfare was a major component of the first global war and first total war of the 20th century. The killing capacity of gas was limited, with only about 90 thousand fatalities from a total of some 1.2 million casualties caused by gas attacks. Gas was unlike most other weapons of the period because it was possible to develop effective countermeasures, such as gas masks. In the later stages of the war, as the use of gas increased, its overall effectiveness diminished. The widespread use of these agents of chemical warfare, and wartime advances in the composition of high explosives, gave rise to an occasionally expressed view of World War I as "the chemist's war" and also the era where "weapons of mass destruction" were created.

The use of poison gas performed by all major belligerents throughout World War I constituted war crimes as its use violated the 1899 Hague Declaration Concerning Asphyxiating Gases and the 1907 Hague Convention on Land Warfare, which prohibited the use of "poison or poisoned weapons" in warfare.

The earliest military uses of chemicals were tear-inducing irritants rather than fatal or disabling poisons. During World War I, the French army was the first to employ gas, using 26 mm grenades filled with tear gas (ethyl bromoacetate) in August 1914. The small quantities of gas delivered, roughly 19 cm³ per cartridge, were not even detected by the Germans. The stocks were rapidly consumed and by November a new order was placed by the French military. As bromine was scarce among the Entente allies, the active ingredient was changed to chloroacetone.

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