Chemical weapons were first used 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. The killing capacity of gas was limited, with four percent of combat deaths caused by gas. Gas was unlike most other weapons of the period because it was possible to develop effective countermeasures, such as
gas masks. In the later stages of the war, as the use of gas increased, its overall effectiveness diminished. The widespread use of these agents of chemical warfare, and wartime advances in the composition of
high explosives, gave rise to an occasionally expressed view of
World War I as "the chemists' war".