During World War I, various forms of tear gas were used in combat and tear gas was the most common form of chemical weapon used. None of the belligerents believed that the use of irritant gases violated the Hague Convention of 1899 which prohibited the use of "poison or poisoned weapons" in warfare. Use of chemical weapons escalated during the war to lethal gases, after 1914 (during which only tear gas was used).
The US Chemical Warfare Service developed tear gas grenades for use in riot control in 1919.
Use of tear gas in interstate warfare, as with all other chemical weapons, was prohibited by the Geneva Protocol of 1925: it prohibited the use of "asphyxiating gas, or any other kind of gas, liquids, substances or similar materials", a treaty that most states have signed. Police and civilian self-defense use is not banned in the same manner.
Tear gas was used in combat by Italy in the Second Italo-Ethiopian War, by Japan in the Second Sino-Japanese War, by Spain in the Rif War and by the United States in the Vietnam War, and the Israel–Palestine conflict.
Tear gas exposure is an element of military training programs, typically as a means of improving trainees' tolerance to tear gas and encouraging confidence in the ability of their issued protective equipment to prevent chemical weapons exposure.