British war crimes September 1915 and continued their usage for the remainder of the war. This was done in retaliation for the use of chlorine by Imperial German Army the preceding April. Following the Imperial German Army's use of poison gas at Ypres, the commander of II Corps, Lieutenant General Sir Charles Ferguson, had said of poison gas: Mustard gas was first used effectively in World War I by the German army against British and Canadian soldiers near Ypres, Belgium, in 1917 and later also against the French Second Army. The name "Yperite" comes from its usage by the German army near the
German Type IXA submarine The first U-boat to be laid down in the Bremen ship yards was "U-37", whose keel was laid down on 15 March 1937. The last U-boat to be laid down was , whose construction began on 15 September 1937. By November 1939, all eight submarines had been fully constructed and commissioned into the "Kriegsmarine". The Type IXA submarines were the first German submarines to have a double hull. This allowed for a greater chance of survival in the event of an attack and gave them better seaworthiness on the surface. The Type IXA submarines had a test depth of .