1915 April 22nd. First use of poison gas.

• 1915 – The use of poison gas in World War I escalates when chlorine gas is released as a chemical weapon in the Second Battle of Ypres.
• Chemical weapons in World War I were primarily used to demoralize, injure, and kill entrenched defenders, against whom the indiscriminate and generally 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 and the severemustard gas, to lethal agents like phosgene and chlorine. 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, however, was limited – only four percent of combat deaths were caused by gas. Gas was unlike most other weapons of the period because it was possible to develop effective countermeasures, such asgas 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 chemists' war".
  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.