Chlorine
Chlorine was a chemical used by the German dye industry. Today, we still use chlorine bleach for industrial uses, to clean in our homes and to whiten our laundry; however, on the battlefield, this chemical had a significantly greater impact. Chlorine was, in fact, the first major chemical weapon, deployed on the battlefields of World War I on April 22, 1915. The Germans had experimented with chemical weapons previously, but had been less successful.
Chemist Fritz Haber had used chlorine in an industrial setting, and recognized several key attributes that made it an ideal chemical weapon. Chlorine remains gaseous at cold temperatures and is heavier than air, so sank into enemy trenches. When inhaled, chlorine causes the lungs to fill with fluid, killing by drowning.
Even the Germans were surprised by their own success in April 1915. The gas attack decimated entire divisions of French and Algerian troops at the Second Battle of Ypres. The use of chemical weapons by the Germans continued throughout the war. While poison gas was defended as an effective strategy, defensive strategies, like gas masks kept pace with the development of new gases.
Chlorine was a significant ingredient in most of the chemical weaponry of the period, used by both sides in the conflict. Phosgene, for instance, was a chlorine-based, nearly odorless gas. This gas was responsible for 80 percent of gas deaths in World War I. In total, some 100,000 pounds of poison gases were used in World War I, causing almost 30,000 deaths.
The 1925 Geneva Protocol banned the use of chemical weapons, but not their production or stockpiling. While chemical weapons were not in use in combat in World War II, they were, of course, used in the death camps of the Holocaust.