The use of poison gas was a major military innovation of the First World War. The gases used ranged from tear gas to disabling chemicals such as mustard gas and killing agents like phosgene. The killing capacity of gas was limited — only 3% of combat deaths were due to gas — however, the proportion of non-fatal casualties was high and gas remained one of the soldier's greatest fears. Unlike most other weapons of the period, it was possible to develop effective countermeasures to gas and hence in the latter stages of the war, as the use of gas increased, in many cases its effectiveness was diminished.
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