Arguably the first modern weapon of mass destruction, chemicals were first used by Germany at the Second Battle of Ypres, France, in 1915, when chlorine gas was released over French trenches on April 22.
This was followed in that year by three more uses of chlorine, produced as an industrial by-product by German chemical companies BASF, Hoechst and Bayer: On April 24 against the 1st Canadian Division, on May 2 near Mouse Trap Farm, and on May 5 against the British at Hill 60, locations in Western Belgium.
Chemicals continued to be used throughout the war, with the British using chlorine at the Battle of Loos, France, on Sept 25, 1915 – here, though, the chlorine either pooled uselessly in clouds between the trenches, or, in places, blew back across British lines.
Further developments saw phosgene gas being used in 1915, and then mustard gas in 1917. The latter caused internal and external bleeding and attacked the bronchial tubes, stripping off the mucous membrane.