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Chemical warfare is a type of warfare where chemicals are used in usually gaseous or aerosolized liquid form to injure, incapacitate, and kill enemy troops as well as for related purposes, such as clearing vegetation from regions which may be used as hideouts and ambush locations. The history of chemical warfare can be traced back 2,000 years in time, but the greatest large-scale use of chemical weapons as of 2011 took place during World War I. The horrific results of the use of such weapons eventually resulted in an international treaty in 1929 known as the Geneva Protocol, banning their use, which, as of 2010, has been agreed to or signed by 137 countries worldwide.
Agents used in chemical warfare are broken down into four distinct classes of compounds. Nerve agents are among the deadliest, and can kill in as little as 15 minutes with very minute exposure levels. They work by inhibiting the function of the human nervous system, often by disabling enzymes that are responsible for transmitting nerve impulses in the body. Blister agents like mustard gas widely used in World War I have corrosive effects on the skin, as well as internal body surfaces such as mucous membranes, the respiratory tract, and organs. They often don't kill immediately, but incapacitate troops over 12 to 24 hours and make it impossible for them to fight or otherwise function in a normal manner.
Blood agents cause severe abnormal responses in the body such as seizures, heart attacks, and respiratory failure. They are often based on cyanide compounds and are extremely deadly. Pulmonary chemical warfare agents act more slowly like blister agents, and cause respiratory failure in about four hours, usually resulting in death. They include such widely-used compounds in World War I as phosgene gas.
One of the very first broad uses of chemical gas warfare was the German use of chlorine gas, a pulmonary agent, in 1915, at Ypres, Belgium. The German military dispersed 168 tons of the gas from canisters that blew downwind against allied troops, exploiting a loophole in international law that allowed them to kill 5,000 soldiers. At the time, the Hague Treaty of 1899 had already banned the use of poison gas in war through projectile dispersal, such as from artillery shells. The Germans later replied to international condemnation by stating that, since they hadn't used shells to deploy the gas, it was legal. The British later responded by using chlorine gas themselves, as well as the French launching phosgene gas attacks against the Germans.
Numerous other instances exist for chemical gas warfare. A researcher in the UK, Simon James, in 2009, traced chemical warfare history back to A.D. 256 during excavations of a battle at a Roman fortress at the city of Dura-Europos in Syria. The Persian attackers gassed Roman defenders with a sulfur-based gas that they pumped into tunnels that the Romans had built as a defensive measure. In the 20th century, Saddam Hussein is known to have attacked citizens of his own country, Iraq, with chemical weapons, and they were widely used during the eight-year-long Iran-Iraq war, from 1980 to 1988.
Since chemical warfare weapons are fairly easy to produce, they are also a weapon of choice for terrorist groups. The Aum Shinrikyo cult in Japan has released the nerve agent sarin on the Japanese population twice as of 2011, first in 1994 in the city of Matsumoto, and second in 1995 in the subway system of Tokyo. Conventional armies also see other uses for chemical weapons, as the US found for Agent Orange and related compounds, types of high-grade defoliants used in the Vietnam conflict from 1962 to 1971. An estimated 12,000,000 to 19,000,000 gallons (45,420,000 to 71,920,000 liters) of the compound was sprayed on jungle vegetation, and had the unanticipated side effect of causing at least 400,000 deaths and another 500,000 children later born in Vietnam with birth defects from contamination by the chemicals, which contained highly carcinogenic dioxin derivatives.