Gas

Gas: The use of poison gas in the war was first proposed some four [now five] centuries ago by the famous Florentine, Leonardo da Vinci, primarily for sea warfare, for the purpose of suffocating an enemy by discharging the gas when to windward at close quarters. Leonardo also designed a preventive mask in the event of a shift of wind blowing back the poison gas on its users.

Poison gas was first used in the war by the Germans at the Second Battle of Ypres, on April 22nd, 1915, against the Canadians. The British first retailiated in the following September, at the Battle of Loos.

It's use in the war had been barred by the Hague Convention, to which Gemany was a party.

See also Mustard gas and the detailed Wikipedia article.