At five in the afternoon on April 22, 1915, German soldiers outside Ypres opened the valves on 5,730 metal cylinders and let the wind do the rest. A hundred and sixty-eight tons of chlorine rolled toward the French and Algerian trenches. A British captain named Alfred Pollard remembered it as a “strange green cloud of death.” Along four miles of front, the line broke.

Two days later, word was moving through the Canadian trenches. If the cloud comes, pee on your handkerchief and breathe through it.