The German phosgene attack (19 December 1915) was the first use of phosgene gas against British troops by the German army. The gas attack took place at Wieltje, north-east of Ypres in Belgian Flanders on the Western Front in the First World War. German gas attacks on Allied troops had begun on 22 April 1915, during the Second Battle of Ypres using chlorine 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, because the French and British introduced anti-gas measures and protective helmets. The German Nernst-Duisberg-Commission investigated the feasibility of adding the much more lethal phosgene to chlorine. Mixed chlorine and phosgene gas was used at the end of May 1915 against French