[anigbrowl]

For one thing, the use among civilian populations of white phosphorus as a weapon (rather than as part of a smoke screen) is of dubious legality; see http://en.wikipedia.org/wiki/White_phosphorus

[yummyfajitas]

I think the legality is pretty clear. Protocol III of the Concentional Weapons Convention bans incendiaries under many circumstances, but the US has not adopted it.

http://en.wikipedia.org/wiki/Convention_on_Certain_Conventio...

http://www.icrc.org/ihl.nsf/NORM/C1C4BFCF736BF820C1256402003...

Some activists also wish to call it a chemical weapon, but by that logic lead bullets are also a chemical weapon (since lead is toxic).

[anigbrowl]

Britain has. Using or taking part in an action which uses American WP munitions could (theoretically) be a court-martial offence.