The Second Battle of Ypres marked a terrifying turning point in military history: the first large-scale use of lethal chemical weapons. German forces released approximately 168 tons of chlorine gas from thousands of cylinders toward French and Algerian colonial troops. The yellowish-green cloud settled into the trenches, causing victims to choke as their lungs filled with fluid. The gas created a massive four-mile gap in the Allied lines, but the Germans, surprised by the weapon's effectiveness and lacking their own protection, failed to fully exploit the breakthrough. This event sparked a chemical arms race; soon, both sides were deploying phosgene and the dreaded mustard gas. The use of gas necessitated the invention of the gas mask, which became an iconic and haunting piece of equipment for every soldier on the front.