22 April - 25 May 1915. On the 22nd April 1915 the Germans used poison gas at Ypres. This was the first 'official' use of gas and took the Allies by surprise. After initial success capitalising on the confusion and horror of this weapon, a heroic stand, initially by the Canadians and then supported by British and Indian Battalions, held the German advance. However it became clear that the Germans had achieved a tactical advantage and eventually the British were forced to retire to more a more defendable perimeter closer to Ypres. These positions were on the last ridges before Ypres and their loss would have resulted in the loss of the town and possibly open the Channel coast to German occupation with disastrous consequences for the re-supply of the BEF.