Original: International law does not prohibit the use of napalm or other incendiaries against military targets, but use against civilian populations was banned by the United Nations Convention on Certain Conventional Weapons (CCW) in 1980.
Simplified: International law does not prohibit the use of napalm or other incendiaries against military targets, but use against civilian populations was banned by the United Nations' inhumane weapons convention (often referred to as the CCW) in 1981.