During World War I, France, not Germany as commonly believed, was actually the first nation to use chemical weapons though this was notably a nonlethal tear gas attack (xylyl bromide) carried out in August 1914 against invading German troops. Once the war had degenerated into trench warfare and new methods to attain an advantage were sought, the German Army initiated a chlorine gas attack against the French Army at Ypres on 15 April 1915, initiating a new method of warfare but failed that day to exploit the resulting break in the French line. In time, the more potent phosgene replaced chlorine in use by armies on the Western Front, including France, leading to massive casualties on both sides of the conflict however the effects were mitigated by development of protective clothing and masks as the war progressed.