Germany launched its only Western Front offensive of 1915 at Ypres in Belgium, extensively using deadly chemicals for the first time to attack the Allies.
Clouds of poisonous chlorine gas were released from cylinders on April 22nd, forcing thousands of French colonial troops to flee.
The attack opened a four-mile (six kms) gap in the Allied defences, but German troops failed to exploit it.
Gas was used several more times by the Germans as they attempted to wrest control of the Ypres salient from the Allies.
Canadian troops played a prominent part in the counter-attacks, which secured the line after a month of fighting.
Posted by: Peter Alhadeff, Centenary News