International law does not specifically prohibit the use of napalm or other incendiaries against military targets.
However, It is specifically illegal to use incenderary devices against civilians, civilain vehicles and civilian buildings or against a military target in a "heavily civilian area."
It is difficult to conjure up many situations in which the use of napalm would not put civilians at risk.
The President should direct that DOD that any use of napalm by the U,S, is prohibited, unless its use is approved by the President. And direct that the Uniform Code of Military Justice (UCMJ) provides that any unauthorized use of a prohibited weapon is a serious violation to be tried by a General Court Martial.