German forces shock Allied soldiers along the Western Front by firing 168 tons of lethal chlorine gas against two French divisions and one Canadian division at Ypres, Belgium. This was the first major gas attack by the Germans, and it devastated the Allied line.
Immediately after the attack, France and Britain began developing their own chemical weapons and gas masks. With the Germans taking the lead, an extensive number of projectiles filled with deadly substances such as mustard gas polluted the trenches of World War I. Using sophisticated gas masks and protective clothing, however, soldiers on both sides eventually negated the military importance of chemical weapons.
The Geneva Protocol of 1925 banned the use of chemical weapons in war and the belligerents of World War II largely honored this agreement, primarily for strategic military reasons. Since then, chemical weapons have only been used in a handful of conflicts, and always against forces that lacked gas masks.