Soldiers in boot camp preparing for entry into World War I had to confront a relatively new invention: poison gas. As this BBC World Service article notes, the first major gas attack ever used in warfare took place in January 1915, but actually killed comparatively few soldiers relative to the amount of fear it spread.
Still, soldiers had to be prepared:
“Gas also has become a contingency to which men must be accustomed during their training… They are passed through a dugout filled with asphyxiating gas, and the efficiency of the apparatus with which they are supplied is practically demonstrated. Gas is more dangerous to the morale of the troops than to their actual safety, and this demonstration of its harmlessness, if the right steps are taken to counteract it, through which every man going to the front must pass, does much to disarm the gas attack of its worst terrors.”
Realistic Training at Base Camp Near Front: Outline of British Methods Shows How the American Soldiers Are Being Taught — Trenches Flooded With Gas for Practice
From Sunday, August 5, 1917