The use of poisons that could be considered chemical weapons (CW) dates to antiquity. During the Peloponnesian War (431-404 BC), for example, the Spartans used arsenic smoke. A millennium later at the siege of Constantinople (637 AD), the Byzantine Greeks employed "Greek Fire" - a mixture of petroleum, pitch sulfur and resins. The first modern use of CW, however, occurred during the First World War. At the second battle of Ypres in April 1915, the German army released hundreds of tons of chlorine gas. Thousands of Allied troops were killed or wounded in the gas cloud attacks, including nearly 7,000 Canadians (1,000 dead and 5,975 injured). Throughout the war, both the Allied and the Central Powers developed toxic chemical warfare agents and the means to deliver them. They also refined their tactical doctrines to take into account the new reality of chemical warfare.
By 1917, the development of chemical agent artillery munitions and other means of delivery (e.g. the Livens Projector, a large-scale mortar developed for delivering large amounts of chemical warfare agent), coupled with the use of new persistent agents such as mustard gas, made cloud tactics obsolete. By 1918, the World War I battlefield was saturated with a variety of persistent and non-persistent chemicals, which caused casualties among troops and increased the danger and difficulty of military operations. By the end of the First World War, approximately 125,000 tons of toxic chemicals had been used, causing more than 1.3 million casualties, including more than 100,000 deaths. To this day, live rounds of World War I chemical munitions remain buried beneath the battlefields of Europe.
While the consequences of the use of gas during the war - images of wounded and blinded men waiting in long lines to be given medical assistance - created in the general public a visceral loathing of chemical weapons, the development and use of CW continued throughout the twentieth century. Italian troops employed chemical weapons during their invasion of Ethiopia (1935-1936) while Japan used CW during its war with China (1937-1945). During the Second World War, both the Allied countries (including Canada) and Axis powers developed a significant inventory of chemical weapons; the lack of effective large-scale delivery systems played a part in the decision of both sides not to use them (another powerful constraint was the fear of retaliation). Chemical weapons were used by Egypt in North Yemen (1963-1967); they were also used extensively during the Iran-Iraq War (1983-1988). Despite the conclusion and entry into force of the Chemical Weapons Convention (CWC), some states maintain offensive CW programmes and/or CW stockpiles.