Chemical weapons are extremely difficult to use effectively because of their potential for collateral damage and friendly fire. The first issue is that the most minor errors in the manufacture or transport of chemical weapons can result in the death of your workers or the pollution of your own territory. For airborne chemical weapons, since they are diffused over a wide area, they cannot be targeted precisely. As happened on occasion in WW I, with mustard gas, a shift in the wind can cause them to be blown over your own troops. The other major problem is that they are sufficiently horrible that use of them may actually strengthen your enemy`s resolve and cause other nations to ally with them because you, by using them, will be seen as a rogue or evil state.