Lord Olleus said:too complicated. Anyway once armies had gas masks they became mostly useless, thats why it wasnt a battlefield weapon in WW2
Slightly off with your facts here; it wasn't that Chemical Weapons were useless, but rather they were banned under the Geneva Protocol of 1925 ... which would lead to some interesting political effects in game if ignored

http://www.olive-drab.com/od_chemical_weapons.php
Chemical weapons have been in use for centuries, but their modern form starts with World War I when large scale "gas attacks" were part of both German and Allied tactics. Those attacks used common industrial chemicals, typically delivered by artillery shells, and distributed by the wind, relatively crude technology with uncertain results. Public outcry against the use of gas (actually aerosols or vapors) led to the Geneva Protocol of 1925 under which nations agreed to not be the first to use such weapons.
Although both Nazi Germany and the Allies stockpiled chemical weapons, there was no use made of them in World War II. A few smaller conflicts have seen the use of chemical munitions, in particular Irag against Iran in 1983 through 1986 as well Iraq's use against its own Kurdish population in 1988.
Under the 1997 Chemical Weapons Convention (CWC), the United States, Russia, India, and South Korea have declared that they possess CW and have accepted an obligation to destroy these weapons. China and several other countries have declared abandoned chemical weapons on their territory, primarily left over from World War II
http://www.fas.org/nuke/intro/cw/intro.htm
Chemical weapons were never deliberately employed by the Allies or the Axis during World War II, despite the accumulation of enormous stockpiles by both sides. Instances of employment of chemical weapons in the local wars since then are arguable, although they were definitely used in the Iran-Iraq conflict of 198287.
http://www.chinadaily.com.cn/english/doc/2004-06/18/content_340357.htm
http://english.sina.com/china/1/2005/0920/46657.html
http://www.geocities.com/CapeCanaveral/Lab/4239/chemweapons/history.html