World War I became known as "the chemists' war," for the deadly gases it introduced to combat. At the Second Battle of Ypres, the German army released thousands of cylinders of yellow-green chlorine gas across the battlefield—the first major use of a chemical weapon in modern warfare. (Small amounts of tear gas had been used earlier by the French and Germans as an irritant.)
The gas, a choking agent that causes fluid to build up in victims' lungs, killed hundreds of French soldiers—at least; accounts vary—but didn't give the Germans an immediate advantage. It's been suggested that they themselves were so shocked by chlorine's effects that they failed to make an advance.