Soldier employing gas mask in WW I.

Germany's initiation of large-scale chemical warfare broke a previously negotiated prohibition against the use of poisons in war. (Both France and Germany experimented with the smaller-scale use of tear gas grenades and chemical irritants as early as August 1914.)  Several of  the World War I combatants had signed agreements at the 1899 International Peace Conference in the Hague to prohibit "poison or poisoned weapons" and agreed to the Hague Declaration Concerning Asphyxiating Gasses, which banned the "use of projectiles, the sole object of which is the diffusion of asphyxiating or deleterious gases."  As World War I wore on, however, both sides used mortars and artillery shells to deliver a variety of highly toxic chemical agents, including phosgene and mustard gas.

After World War I, chemical weapons were used on several occasions, including during the Russian civil war (1918-1920); by the British in Iraq in the 1920s; by Italy during its invasion of Ethiopia (1935-36); and by Japan in its war with China (1937-42). Fascist Italy’s use of chemical weapons against Ethiopia was a blatant violation of the 1925 Geneva Protocol banning the use in war of chemical and biological weapons. Although Italy was a party to the treaty, it was not held to account. During World War II, all the major powers possessed large stockpiles of chemical weapons, but mutual deterrence (the decision by the opposing sides not to launch chemical attacks against each other because of the threat of retaliation in kind) prevailed and the weapons were not used on a significant scale. Nevertheless, Germany secretly developed a new generation of highly lethal chemicals known as "nerve agents," including tabun and sarin.

After World War II, the secret of the German nerve agents came to light. Although all the major powers did intensive research and development on nerve agents, only the United States and the Soviet Union manufactured large stockpiles. New technologies for delivering chemical weapons were also developed, including aircraft-carried spray tanks, artillery rockets, and chemical warheads for ballistic missiles.

Despite the chemical arms race between the superpowers, such weapons were employed on a large scale only twice during the remainder of the 20th century. Egyptian forces used mustard, phosgene, and perhaps nerve agents during the Yemen civil war (1963-67), and Iraq employed mustard, tabun, and sarin during its eight-year war with Iran (1980-88). Saddam Hussein’s Iraq also used chemical weapons to suppress the rebellious Kurdish and Shiite populations, including the infamous gas attack on the Kurdish town of Halabja in 1988. Today, tens of thousands of Iranians and hundreds of Kurds remain chronically ill as a result of exposure to CW agents.