RE: US now admits it used chemical weapons against Iraqis
Take some time to educate yourself--so-called "chemical weapons" fall into four basic categories (called "agents").
Mustard gas is a "blistering agent"--it causes severe irritation of internal membranes/skin and fluid production as a result. A mustard gas casualty drowns in his own internal secretions or dies of dehydration/infection much as a burn victim does. Nasty stuff but hard to use effectively on a battlefield. Also archaic--other agents kill more people faster.
Chlorine/phosphine are "choking agents"--they kill by depriving the victim of breathable air (we'll come back to that shortly) and often combine characteristics of blistering agents. I remember hearing about some soldiers who died on a cleaning detail--they mixed up clorox bleach with ammonia in an enclosed space. That produces chlorine gas. In the open they might have survived, but not in a closed room. They're killers but must must be highly concentrated to work on an open battlefield--that makes them impractical.
Hydrogen Cyanide is a "blood agent". It robs the blood's ability to transfer oxygen via hemoglobin. You can breathe all you want, but if oxygen doesn't get to your brain you die. It's that simple. It's an effective agent on the battlefield but hard to use effectively, as it dissipates quickly. The latter makes most blood agents "non-persistent" agents. You can't contaminate an area with them.
VX is a "nerve agent"--basically an airborne poison. It interferes chemically with basic neural functions, including involuntary ones governing breathing and heartbeat. They're insecticides writ large: the first true nerve gas, Tabun, was accidentally discovered by a German chemist on an insecticide project who barely survived his discovery. Nerve agents are "Raid" for people. They also come in "persistent" (which can be used to contaminate an area) and non-persistent versions. When most laymen talk about "chemical weapons", they're referring to nerve gases. The others don't factor in.
Right--back to "choking agents". A choking agent kills with a combination of asphyxiation and toxin. So too does smoke from an ordinary house fire--and any fireman will tell you smoke kills many more people than fire itself. People in burning buildings die most often from smoke inhalation, yet those watching the fire outside are not physically affected by the same smoke. Why?
They're not in an enclosed area, and the smoke isn't concentrated.
Got all of that? Good. Now on to white phosphorus (WP), aka "Willy Peter".
It's a chemical, but an incendiary one--it burns things (fire itself is a chemical reaction, and no, I'm not going to teach Chemistry 101 here). In munitions it's used as a catalyst/accelerant for other chemicals; the reaction produces either illumination or smoke.
If a white phosphorous round goes off next to you you'll be burnt--just like if you jumped into a bonfire. But if you're in a bunker/house and a WP smoke round goes off nearby, the smoke will choke you unless you get out in the open--where you can and will be shot. That's what "shake-and-bake" is: "shake" them out of their holes with with smoke and then "bake" them with high explosives, shrapnel and bullets. And that's how we use it.
Fire and smoke are not illegal weapons, but they are the results of chemistry. WP is a catalyzing agent for both. Beyond that, it's not a chemical agent in any educated sense....