It is often said the world lost its innocence during the first world war. Empires fell. The social order changed. Women won more political and social freedom. The war set into motion a series of events that would make the twentieth century what it became -- a second world war, the cold war, the end of European colonialism, the women's rights movement, civil rights movement, the sexual revolution.
If there is a certain date innocence was lost it is April 22, 1915, the date of the first chlorine gas attack.
Prior to this date both sides had used tear gas, but tear gas is meant to distract the enemy not harm them. On April 22, the Germans fired several thousand canisters of chlorine gas at the French line at Ypres in Belgium. The line broke. The Germans advanced but the Allied line was soon secured again by the Canadians and British. Two days later, the Germans fired chorine gas again, this time at the Canadians.
Exposure to chlorine gas causes irritation to the eyes, lungs, nose and throat with prolonged exposure leading to death.
The gas attacks established a new era of warfare, one in which armies would do whatever it takes to ensure victory.