The village of Saint Julien and a section of forested land called St Julien Wood was at a pronounced bend in the north east sector of the Ypres Salient prior to the Second Battle of Ypres. The area was also the junction between the British and French sectors of responsibility. The Canadian First Division was assigned the most northern section of the British line and to their left, the 45th (Algerian) Division held the southernmost end of the French line. The German Army had brought forward 168 tons of chlorine gas deployed in 5,730 cylinders buried in front of their trenches, opposite Langemark-Poelkapelle, north of Ypres. The Canadians, who had been moved into their positions only a few days earlier were manning the lines for several hundred metres along a front to the southwest of St. Julien when the German Army unleashed the first poison gasattack on the Western Front on 22 April 1915.
Pushed towards the Allied lines by a wind from the north, the initial gas attack largely drifted to the north and west of the Canadian lines, into the trenches of the French colonial troops of the French 45th (Algerian) and 87th (Territorial) Divisions, of 26th Reserve Corps. The gas drifted across positions largely held French colonial troops who broke ranks and abandoned their trenches after witnessing the early casualties, creating an 8,000 yard (7 km) gap in the Allied line. The German infantry were also wary of the gas and, lacking reinforcements, failed to exploit the break before the First Canadian Division and assorted French troops reformed the line in scattered, hastily prepared positions 1,000 to 3,000 yards apart. In actions at Kitcheners Wood, Mauser Ridge, Pilkem Ridge and Gravenstafel Ridge the Canadians held the line and prevented a German breakthrough until they were relieved by reinforcements on the 24 April.
In the 48 crucial hours that they held the line, 6,035 Canadians – or one man in every three who went into battle – became casualties; of that number, approximately 2,000 (or one man in every nine) were killed.