Chemical weapons were primarily used to demoralize, injure, and kill entrenched defenders against whom the indiscriminate and generally very 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, to lethal agents like phosgene, chlorine, and mustard gas. The killing capacity of gas was limited but its physical damages were notably present. Gas was unlike most other weapons of the period because it was possible to develop countermeasures such as gas masks. During the middle of the war during a time known as the Dark Years, the use of gas increased but its overall effectiveness diminished due to countermeasures.
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