“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 severe mustard 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.”
http://en.wikipedia.org/wiki/Chemical_weapons_in_World_War_I