In 1914 a chemist called Fritz Haber offered his knowledge to the German Army. He soon began experimenting with chlorine gas to be used in trench warfare. In April 1915, 150 tons of lethal chlorine gas was used by the German forces against the French Army at Ypres. Note: For the first attack at Ypres the Germans actually delivered the gas from cylinders using the wind, instead of shells as it looks in the video! | Simple History