The first (and most famous) example of a chemical weapon used in military combat that I could find is mustard gas, so-called because of it's smell of mustard or horseradish or garlic - not because it is made from the oil of a plant of the mustard family (such as rapeseed.) The blistering and often fatal inhalation of mustard gas comes from sulfur. Sulfur mustard is made by treating sulfur dichloride with ethylene.
Mustard gas was first produced in mass quantities by the Germans in WWI, used mostly by the Germans in that war against the British and their allies. Mustard gas can be delivered by several means. Artillery shells were a primary method of choice in WWI. Since the chemical lingers on the ground for days and weeks, it was used as a "territory denial" agent as well as being fired directly at and over troops on battlefields. Ict should be noted that a gas mask is insufficient protection against mustard gas, even when used properly, because the gas attacks unprotected skin as well. Although chemical warfare is primarily a province of the military, it has also become a favorite of terrorists and crackpot groups (especially in subways.)
Chemical warfare also includes the nerve agents as well as the blister death methods. The category includes all poison gasses, really. Here are notable instances in history where chemical weapons (as we know them today) have been used on a large scale:
(Wikipedia)
1. In WWI by the Germans; by both sides towards the end of the war.
2. By the UK against the Red Army in 1919.
3. By Spain and France against insurgents in Morocco 1921-1927.
4. By Italy in Libya in 1930.
5. By the Soviet Union in China in 1934, and again in 1936-1937.
6. By Italy against Abyssinia (Ethiopia) 1935-1940.
7. Nazi Germany against Poland and the Soviet Union in a few instances in WWII. (And Poland against Germany in one isolated incident.)
8. By the Japanese against China 1937-1945.
9. By Egypt against North Yemen 1963-1967.
10. By Iraq against Iran 1983-1988.
11. By Iraq against the Kurds, 1988.
12. Probably by Sudan against the civil war insurgents in 1995 and 1997.
