WP is not an illegal weapon. Not strictly true. It depends on the nature of the weapon containing it and if civilian areas are being targeted.
It is forbidden for use against civilians as well as legitimate military targets that happen to be in a major civilian population center if the munition is delivered by air. This is due to its incendiary effect.( I do not fully comprehend the air bit. grenade, shell, bomb, rocket all arrive by air and if an IED kills or injures civilians it is still illegal) As white phosphorus is pyrophoric it keeps burning until used up and can cause horrendous injuries. The staus of white phosphorus is disputed.


Israeli Manual on the Rules of Warfare (2006), which explains:

A chemical weapon is a weapon intended to work on the systems of life and is constituted from a substance that causes a chemical reaction in the body expressed in such symptoms as asphyxiation, burning, weeping, etc., whereas phosphorous is an element in nature which reacts to the oxygen in the air by catching fire. In that respect, phosphorous is no different from petrol (gasoline) reacting to a lighted match, and what differentiates it from chemical weapons is that its reaction is not directed against the human physiology in particular, it will burn whatever it touches. (so that's ok then!That is one hell of a justification!)

I have no doubt water is a chemical weapon when used for waterboarding!