By the end of 1914, the Western Front in the First World War had stabilized, and trenches ran from the North Sea to Switzerland. During the winter the armies dug in even further. A stalemate existed that both sides were desperate to break. On 22 April 1915, the Germans released chlorine gas in front of their trenches and a favorable wind blew it west into the French, Canadian, and British lines at the Ypres Salient. Mild contact with chlorine gas causes irritation to the eyes and chest, a heavy dose causes a person to drown in their own lungs. The gas cloud affected the French sector the worst and soon thousands were killed and tens of thousands more fled to the rear. A five mile gap formed in the Allied lines
Fortunately, the Germans could not exploit the gap fully due to poor staff planning and a lack of reserves to exploit the breach. (The Second Battle of Ypres is one of the great “What if? Moments of history. If they would have broken through, the First World War would have almost definitely ended in a German victory in 1915.) Also, chlorine gas is water soluble. Allied soldiers used improvised masks of cloth, usually handkerchiefs, which soldiers soaked or urinated to protect themselves from the gas. This allowed the Allies to hold the line where the gas was less prevalent. Instead of a breakthrough, the hard fought Second Battle of Ypres raged for almost a month with over 100,000 casualties from both sides.
The attack spawned a chemical warfare arms race that would last for the rest of the war.