From Wikipedia

Ypres - Gas Inferno

The Second Battle of Ypres was the first time Germany used poison gas on a large scale on the Western Front in World War I and the first time a former colonial force (Canadians) pushed back a major European power (Germans) on European soil, which occurred in the battle of St. Julien-Kitcheners' Wood.

The Second Battle of Ypres consisted of four separate engagements:

• The Battle of Gravenstafel: Thursday 22 April – Friday 23 April 1915
• The Battle of St Julien: Saturday 24 April – 4 May 1915
• The Battle of Frezenberg: 8 May – 13 May 1915
• The Battle of Bellewaarde: 24 May – 25 May 1915

The scene of the battles was the Ypres salient where the Allied line which followed the canal bulged eastward around the town of Ypres, Belgium. North of the salient were the Belgians; covering the northern part of the salient itself were two French divisions (one Metropolitan and one Algerian) The eastern part of the salient was defended by one Canadian division and two UK divisions.

In total during the battles, the British Commonwealth forces were the II and V Corps of the Second Army made up of the 1st, 2nd and 3rd Cavalry divisions, and the 4th, 27th, 28th, 50th, Lahore and 1st Canadian Divisions.