Chemical weapons were used regularly on the Western Front in World War One. Trench warfare was an ideal battleground for chemical attacks due to the static nature of the gas, which could be easily avoided in other theatres. Despite its widespread use, only four percent of combat deaths were caused by gas weaponry. The low mortality rate was due, in part, to the availability of effective precautions such as the gas masks the troops are wearing in this video.
Although all major belligerents of World War One used poison gas as a weapon, it was considered a war crime and a direct violation of the 1899 Hague Declaration Concerning Asphyxiating Gases.