From the Wiki article above:

According to the field manual on the Rule of Land Warfare, "The use of weapons which employ fire, such as tracer ammunition, flamethrowers, napalm and other incendiary agents, against targets requiring their use is not violative of international law."[109] The 1999 ST 100-3 Battle Book, a student text published by the US Command and General Staff College at Fort Leavenworth, states that "It is against the law of land warfare to employ WP against personnel targets."[110] At the same time, other field manuals discuss the use of white phosphorus against personnel