A chemical weapon is a weapon which relies on the chemical properties of a given substance for its primary effect - this therefore includes poisons and asphyxiants but does not include other types of weapons that have a toxic chemical effect incidental to their function1.
The most (in)famous applications of chemical warfare are probably the heavy application of toxic gases during WW1 and in the inter-war period. These gases were mainly asphyxiants such as chlorine and phosgene or blister agents such as the various mustard gases. Nerve agents - of which VX is probably the most widely known - were developed during and after WW2 and remain a significant worry in international relations.
As a result of this kind of profiling, chemical weapons are classed as weapons of mass destruction, despite having been shown to be relatively ineffective on the battlefield. Arguably they make far better terror weapons for use against civilians and/or irregular troops than they do in open warfare2.
Chemical weapons are also prohibited by various customs and usages of warfare (such as the Hague Conventions).
Ironically this makes it harder for most armed forces to deploy less lethal agents such as sedatives, hallucinogens and incapacitants (e.g. CS or CR) which in some circumstances might well lead to fewer casualties all around. Less lethal agents are however frequently used by various police and security agencies worldwide, who are not governed by rules related to open warfare. Counter insurgency campaigns are a grey area and often see some degree of flexibility.
In the pre-modern era, chemical warfare was usually limited to the fumes of various burning compounds, often used against fortifications or tunnels and to poisons applied to blades and weapon points. Quicklime, deployed as a dust cloud to attack the eyes of enemy troops was also popular. In most warrior cultures the use of poisons will generally be considered 'dishonourable' and will be frowned upon in personal combat and subject to reprisal in warfare - that does not mean that they will not be used, especially in siege warfare which has historically been far more "professional" than open battle. Poison used in assasination work also qualifies as a chemical weapon, albiet not a battlefield one.
Chemical weapons also have a reputation for being dangerous to their users - once released the agent doesn't discriminate between the side that deployed it and the side that it was aimed at and may be hard to remove from the battlefield once used3. Due to their aggressive nature they can even be dangerous to store, being prone to attack their packaging and/or diffuse out over time. A popular solution is to generate the agent in situ by chemical reaction (including the medieval burning of sulphur and the like) - as in the case of modern binary agents which consist of two less hazardous substances which combine on mixing to give the desired agent. Another example of this is the (largely exaggerated) rules in various RPGs for accidentally poisoning oneself when applying poison to a blade.
Game and Story Use
- For a modern game, a cache of elderly, unstable chemical munitions might be an unwelcome part of the salvage found on occupying a location - something NATO forces deployed to the former Yugoslavia often happened upon.
- "Terrorist with chemical weapon" makes a fairly usable villain in a modern game.
- Use of chemical weapons in a pre-modern context might well be a nasty surprise for PCs - but then most PCs are likely to hi-jack them, use them on a dungeon and try and claim experience points for all the monsters they just killed.
- PCs who use less-lethal chemical agents may be surprised to find themselves charged with violations of "International Law".
- Use of chemical weapons in WW2 is an interesting "what if" … historically the UK prepared to meet a German invasion with agents including mustard gas and Lewisite and the US deployed a shipload of mustard gas to Bari in 1943 ready to retaliate against a rumoured German gas attack that was being prepared. That didn't end well. The Japanese also employed various gas agents against the Chinese during their ongoing conflict. The obvious gap would be German failing to use nerve agents against the Red Army - who would have been hard put to equip their forces to resist them. Allegedly this was averted because the Germans weren't aware that they were the only power to possess nerve agents at the time and feared retribution in kind.