(Main article: Chemical weapons in World War I) The First German phosgene attack on British troops took place on 19 December 1915, during World War I, at Wieltje north-east of Ypres in Belgian Flanders. German gas attacks on allied troops had begun on 22 April 1915, during the Second Battle of Ypres using chlorine gas, against French and Canadian units. The surprise led to the capture of much of the Ypres Salient, after which the effectiveness of gas as a weapon diminished, as the French and British produced anti-gas helmets. The German Nernst-Duisberg-Commission investigated the feasibility of adding the much more lethal phosgene to chlorine gas. Mixed chlorine and phosgene gas was used at the end of May 1915, in attacks against French troops on the Western Front and on the Eastern Front