Chemical weapons have been used for millennia. They are among mankind’s most despicable weapons, and have never caused anything but heartbreak and devastation.
In a bizarre twist of history, invading Iranians used clouds of sodium chloride gas in AD 256 to kill Roman soldiers in the ancient city of Dura-Europos in Syria. During the American Civil War, the use of chlorine gas – which later caused hundreds of thousands of casualties in the First World War – was considered for use against Confederate soldiers by the Union Army.
America Votes for Chemical Weapons—A Hundred Years Ago 
In 1899, the Hague Conference – a multilateral meeting of the Great Powers of the time – convened to adopt rules for the conduct of warfare where they passed a proposal to prohibit artillery shells filled with poison gas. The sole dissenting vote was cast by the United States, which justified voting against the measure on the grounds that “the inventiveness of Americans should not be restricted in the development of new weapons.”
Sarin and Tabun
Before World War II, Germany discovered the deadly nerve agents Tabun and Sarin (the latter was used in the recent Syrian atrocity), and the Nazis used poison gas to murder six million Jews. Although both the Allied and Axis powers developed and stockpiled chemical weapons, there are no fully documented cases of poison gas being used for military purposes in that war (genocide was another matter). There is no doubt that if the Germans had used chemical weapons, the outcome of the war would have been different. The use of nerve gas on the Normandy beachhead would have seriously impeded the Allies and possibly caused the invasion to fail altogether.
Even the Nazis Used Restraint (in Active Warface)
So why didn’t either side use chemical weapons? Because both the Allies and the Axis treated such armaments as a 1940s version of the Mutual Assured Destruction (MAD) doctrine that later defined the Cold War. The theory was that if one side used chemical (or, later, nuclear) weapons, the retaliatory attack would be so devastating that no one could win. During the Second World War, both sides over-estimated the capabilities of the enemy, much like the US did vis-á-vis the USSR during the early days of the Cold War, and prior to the Invasion of Iraq.
It’s time for the history of chemical weapons to end here and now.
Don’t forget Biological weapons as well. The Japanese used rats to spread desease in targeted cities in China & there seems to be evidence in Europe of infected animals being used contaminate water supplies and people in castles and cities under seige.
But both will remain weapons of choice for terrorists, cults (the Japanese Subway attack) and their kind.
Cheers!