Ypres occupied a strategic position during World War I because it stood in the path of Germany's planned sweep across the rest of Belgium and into France. In the Second Battle of Ypres (22 April to 25 May 1915), the Germans used poison gas for the first time on the Western Front (they had used it earlier at the Battle of Bolimov on 3 January 1915) and captured high ground east of the town. The first gas attack occurred against Canadian, British, and French soldiers including both metropolitan French soldiers as well as colonial tirailleurs. The gas used was chlorine gas. Mustard gas, also called Yperite from the name of this city, was also used for the first time near Ieper in the autumn of 1917.