Chemical warfare is at its base the use in wartime of a substance (a chemical) against human targets which relies on that chemical's deleterious
direct effects on the human organism. The most internationally famous agreement forbidding such warfare is the "Protocol for the prohibition of the use in war of asphyxiating, poisonous or other gases, and of bacteriological methods of warfare," signed at Geneva
. It is more commonly known as the 'Geneva Protocol
Unfortuantely for our purposes, it offers no definition of what chemical warfare is other than pretty much what's in the title, when it says:
Whereas the use in war of asphyxiating, poisonous or other gases, and of all analogous liquids, materials or devices, has been justly condemned by the general opinion of the civilized world; and
Whereas the prohibition of such use has been declared in Treaties to which the majority of Powers of the World are Parties; and
To the end that this prohibition shall be universally accepted as a part of International Law, binding alike the conscience and the practice of nations[...]
The Geneva Protocol
...and goes on to basically say 'we the undersigned nations agree not to do it.' Well, that seems simple enough. But is it? The problem is that there are all manner of ways you might use chemicals during warfare, and the prohibition (rightly) was written to serve as much as a blanket forbiddance as possible. Let's have a look.
In the first case, there's what everyone expects - there is a direct attack by a nation's declared armed forces with a lethal agent. This is the use of chemical agents (the substances themselves) directly on humans, be they soldiers or civilians. The important bit here is that the humans are targets, and the effects on those humans are the objective of the action. Next, there's suppressive use, usually with irritants rather than something like a nerve agent. For example, if you're fighting an enemy who is entrenched in a sunken position, you might consider using something like tear gas to flush them out into the open where you could engage them directly and conventionally. This begins to display some of the subtleties. Is this chemical warfare? You're using a chemical against human targets, but you're not trying to harm them permanently with the agent. All you want is for them to run away to avoid the agent! If none of them were affected by the chemical, but still ran out, you'd be just as happy. Furthermore, you might argue, this is a tactic that your law enforcement uses all the time against your very own citizens. Is this, therefore, chemical warfare? Something which we're forbidden to do to enemy combatants but can do to our own civliians?
Well, technically, yes. There's no provision in that definition above for exceptions based on domestic use, and 'trying to make people throw up and have trouble breathing' sure sounds like it falls into 'asphyxiating or poisonous gas.' So bzzzt, you can't do that.
Except, of course, you can. It just requires interpreting the text in a specific manner, and lawyers and politicians are very good at that.
What if you're facing someone who isn't a nation? This wasn't something the original writers really considered, but has become quite important in modern times and the rise of non-state-actor combat. Well, you can interpret that clause. Israel did in 1969 for just that purpose:
The said protocol is only binding on Israel as regards the States which have signed and ratified it or which may accede to it.
The said Protocol shall ipso facto cease to be binding on the State of Israel in regard to any enemy State whose armed forces or the armed forces of its allies or the regular or irregular forces or the groups or individuals operating from its territory fail to respect the prohibitions which are the object of this Protocol.
CICR translation of original Israeli reservation
In other words, Israel reserves the right to use chems on any state which did not itself ratify the Protocol, or in fact any state combatants fighting from the territory of another state who use chemicals on Israel targets. In other words, if you use 'em first, you're a completely legitimate target as far as they're concerned. While it's debatable whether or not this provision is intended to actually give Israel freedom to use chems in anger or whether it's primarily a pointed means of deterring Israel's enemies from resorting to chemicals first is up for debate.
There are other gray areas which make the definition problematic. One of the most famous is the use of chemicals not directly against humans, but against the environment. Agent Orange, the infamous Vietnam War defoliant, springs to mind. Although it was later found to be very toxic to humans in the long term, at the time, it was thought to be (and used as) primarily a means of killing vegetation. It was in fact even deployed over areas where friendly troops were operating, because it was thought that it wasn't terribly harmful to humans. The removal of leaves and other vegetation from an area was considered to be advantageous to U.S. and allied forces, as it denied the guerrilla enemy means of concealment. Is this chemical warfare? This is bitterly debated to this day, by a U.S. that denies it was in fact chemical warfare since it was aimed at leaves and in fact denies that Agent Orange is harmful; and by Vietnam, whose delegates exhibit vials of 80 grams of TCCD during conferences, explaining that the amount in the vial is enough to kill the population of a major metropolitan area if delivered into its water supply and reminding their listeners that the U.S. sprayed between 150 and 200 kgs of the substance over Vietnam as a component of Agent Orange.
In the original protocols, intent matters. But intent is very hard to prove in legal fora. This distinction is even embedded in the various definitions of chemical weapons. Types of Chemical Weapons (from the FAS) notes: "A chemical agent is a substance which is intended for use in military operations to kill, seriously injure or incapacitate people because of its physiological effects. Excluded from this definition are riot control agents, herbicides, smoke and flame."
Iron Noder 2010