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