22 April 1915
The Germans begin the second battle of Ypres with the first use of gas on the Western Front and the first use of poisoned gas of the war. 6,000 cylinders of chlorine gas are used and the attack creates a 8,000 yard wide gap in the French lines north of Ypres. The Germans hadn't expected such a major success and aren't ready to exploit the success. Instead Smith-Dorrien is able to plug the gap with newly arrived Canadian troops.
How to cite this article: Rickard, J (28 December 2007), 22 April 1915, https://www.historyofwar.org/firstworldwar/daybyday/1915_04_22.html