Quote Originally Posted by wiki
Weapons
Use of white phosphorus is not specifically banned by any treaty, however the 1980 Geneva Protocol on Prohibitions or Restrictions on the Use of Incendiary Weapons (Protocol III) prohibits the use of incendiary weapons against civilian populations or by air attack against military forces that are located within concentrations of civilians. [1] The United States is among the nations that have not signed this protocol.
Not nice, but apparently legal...