Several international agreements have sought to limit the use of chemical weapons in warfare, beginning with the 1675 Strasbourg Agreement that banned the use of poison bullets in conflict. More recently, a United Nations initiative led to the signing of the Chemical Weapons Convention in 1993. The convention, which has been signed by 182 states, enforces a comprehensive ban on developing, producing, stockpiling and using chemical weapons. Syria is not among the signatories to the Convention and thus is not legally bound by its prohibitions.
Chemical weapons include a wide variety of warfare agents developed specifically for military use, in addition to toxic industrial and commercial chemicals such as chlorine and phosgene and chemical toxins of biological origin such as ricin.
Here's a look at the ways chemical weapons have been used militarily in recent history:
World War I: Chlorine and phosgene gases were released on the battlefield. The first large scale attack with chlorine gas took place at Ieper in Belgium on April 22nd, 1915. Ninety thousand people were killed and over a million injured as a result of the use of chemical weapons in the war.
1980s: Iraq used chemical weapons against Iran in their 8-year war. Iraq also used chemical weapons against Kurdish Iraqis in Halabja in 1988.
2004: U.S. military used white phosphorus in Fallujah, Iraq. Use of white phosphorus is not banned but its use is restricted under an international agreement the U.S. has never accepted.