The first successful use of poison gas as a weapon of warfare occurred during the Second Battle of Ypres . Gas was soon used by all major belligerents throughout the war. It is estimated that the use of chemical weapons employed by both sides throughout the war had inflicted 1.3 million casualties. For example, the British had over 180,000 chemical weapons casualties during the war, and up to one-third of American casualties were caused by them. The Russian Army reportedly suffered roughly 500,000 chemical weapon casualties in World War I. The use of chemical weapons in warfare was in direct violation of the 1899 Hague Declaration Concerning Asphyxiating Gases and the 1907 Hague Convention on Land Warfare, which prohibited their use. The effect of poison gas was not limited to combatants. Civilians were at risk from the gases as winds blew the poison gases through their towns, and they rarely received warnings or alerts of potential danger. In addition to absent warning systems, civilians often did not have access to effective gas masks. An estimated 100,000-260,000 civilian casualties were caused by chemical weapons during the conflict and tens of thousands more  died from scarring of the lungs, skin damage, and cerebral damage in the years after the conflict ended. Many commanders on both sides knew such weapons would cause major harm to civilians but nonetheless continued to use them. British Field Marshal Sir Douglas Haig wrote in his diary, "My officers and I were aware that such weapons would cause harm to women and children living in nearby towns, as strong winds were common in the battlefront. However, because the weapon was to be directed against the enemy, none of us were overly concerned at all."
Who had more chemical weapons casualties, the Russians or the British?
Russian