Chemical weapons in World War I

Chemical warfare dates back thousands of years, but the first large-scale use of chemical weapons was during World War I. Austria-Hungary, France, the German Empire, the United Kingdom, and the United States used chemical weapons on the battlefield. 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 chlorine, phosgene, and mustard gas. In survivors, gas attacks caused medical problems and a strong psychological impact. This chemical warfare was a major component of the first global war and first total war of the 20th century.

Estimates go up to about 90,000 fatalities and a total of about 1.3 million casualties. However, this amounts to only 3–3.5% of overall casualties, and gas was unlike most other weapons of the period because it was possible to develop 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, 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 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. After the war, the 1925 Geneva Protocol was signed by all major powers, however it only prohibited state-to-state chemical warfare, not internal use or stockpiling, and Japan and the United States did not ratify it until the 1970s. Chemical weapons in World War II saw widespread use by Germany during the Holocaust and by Japan against China. However, extensive battlefield use was not seen outside China, i.e. in Europe or the Pacific, due to Western Allied stockpiles, and a stronger and transnational repugnance for poison gases following World War I.