Related Rule
United States of America
Practice Relating to Rule 85. The Use of Incendiary Weapons against Combatants
Section B. The use of incendiary weapons against combatants in particular
The US Field Manual (1956) states:
The use of weapons which employ fire, such as tracer flame-throwers, napalm and other incendiary agents, against targets requiring their use is not a violation of international law. They should not, however, be employed in such a way as to cause unnecessary suffering to individuals. 
United States, Field Manual 27-10, The Law of Land Warfare, US Department of the Army, 18 July 1956, as modified by Change No. 1, 15 July 1976, § 36.
The US Air Force Pamphlet (1976) states:
Incendiary weapons, such as incendiary ammunition, flame throwers, napalm and other incendiary agents have widespread uses in armed conflict. Although evoking intense international concern, combined with attempts to ban their use, state practice indicates clearly they are regarded as lawful in situations requiring their use. Conventional incendiary weapons are normally employed against materiel targets and combatants in the vicinity of such targets, such as pill boxes, tanks, vehicles, fortifications, etc. Use in ground support of friendly troops in close contact with enemy troops is an important use. Such uses are justified by the military effectiveness of incendiary weapons demonstrated during World War I, Word War II, Korea, Vietnam and other conflicts. Controversy over incendiary weapons has evolved over the years partly as the result of concern about the medical difficulties in treating burn injuries, as well as arbitrary attempts to analogise incendiary weapons to prohibited means of chemical warfare … Additionally, incendiary weapons must not be used so as to cause unnecessary suffering. 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 6-6(c).
At the Preparatory Conference on the Convention on Certain Conventional Weapons in 1979, the United States explained that it could not accept a restriction on the use of incendiaries against combatants for two reasons. First, “troops in or near the targets attacked with incendiaries would inevitably be killed, and commanding officers would risk being charged with violating the antipersonnel restriction”. Second, “the establishment of any rule embodying a comprehensive set of exceptions would not change present practices and its effect would be purely cosmetic”. 
United States, Statement at the Preparatory Conference on the Convention on Certain Conventional Weapons, UN Doc. A/CONF.95/PREP.CONF/II/ SR.28, 18 April 1979, pp. 2–3, §§ 5–6.
Course material from the US Army War College, which is also used by the US Marine Corps, states: “a) Incendiaries are lawful when utilized for the purpose(s) for which they were designed. b) There is NO prohibition on the use of napalm or flame-throwers against enemy personnel.” 
United States, Marine Corps, Reference Material for Marine Corps Law of Warfare Course, Army War College Selected Readings, Advanced Course Law for the Joint Warfighter, Vol. II, Second edition, 1989, p. 256.
(emphasis in original)