What country was the first to use poison gas in World War I?

WWI was a global military conflict, and most of the European countries were involved. It resulted in 9 million fatalities. Different weapons were used in the course of this horrific war, and poison gas became one of the newest and cruelest ways of killing both military personnel and civilians. Anyway, the contribution of gas weapons to the total casualty figures was relatively minor. Even if the death from gas was always very painful and slow, the troops were specially trained to avoid such attacks and keep the damage to the minimum. The Germans became the first nation to start developing and using such weapons.

When was it used for the first time during WWI? On April 22, 1915, German forces shock Allied soldiers along the western front by firing more than 150 tons of lethal chlorine gas against two French colonial divisions at Ypres, Belgium. This was the first major gas attack by the Germans, and it devastated the Allied line. But, the Germans, perhaps as shocked as the Allies by the effects of the poison gas, failed to take full advantage of their attack. The Allies held most of their positions.