<P> The first successful use of poison gas as a weapon of warfare occurred during the Second Battle of Ypres (22 April--25 May 1915). 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 . </P> <P> 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 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 (along with military personnel) 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 ." </P> <P> The ethnic cleansing of the Ottoman Empire's Armenian population, including mass deportations and executions, during the final years of the Ottoman Empire is considered genocide . The Ottomans carried out organized and systematic massacres of the Armenian population at the beginning of the war and portrayed deliberately provoked acts of Armenian resistance as rebellions to justify further extermination . In early 1915, a number of Armenians volunteered to join the Russian forces and the Ottoman government used this as a pretext to issue the Tehcir Law (Law on Deportation), which authorized the deportation of Armenians from the Empire's eastern provinces to Syria between 1915 and 1918 . The Armenians were intentionally marched to death and a number were attacked by Ottoman brigands . While an exact number of deaths is unknown, the International Association of Genocide Scholars estimates 1.5 million . The government of Turkey has consistently denied the genocide, arguing that those who died were victims of inter-ethnic fighting, famine, or disease during World War I; these claims are rejected by most historians . Other ethnic groups were similarly attacked by the Ottoman Empire during this period, including Assyrians and Greeks, and some scholars consider those events to be part of the same policy of extermination . </P> <P> Many pogroms accompanied the Russian Revolution of 1917 and the ensuing Russian Civil War . 60,000--200,000 civilian Jews were killed in the atrocities throughout the former Russian Empire (mostly within the Pale of Settlement in present - day Ukraine). </P>

When was the first war fought and why