April 22

First Large-Scale Poison Gas Attack — Second Ypres

Germany released 168 tons of chlorine gas along a 4-mile front at Ypres, creating a yellow-green cloud that drifted into Allied trenches. French and Algerian troops fled in panic, creating a 4-mile gap in the Allied line that German infantry failed to exploit.

The gas attack at Second Ypres inaugurated chemical warfare as a major battlefield weapon and triggered an arms race in poison gas that killed 90,000 soldiers in WWI.