Rule 84. The Protection of Civilians and Civilian Objects from the Effects of Incendiary Weapons
Rule 84. If incendiary weapons are used, particular care must be taken to avoid, and in any event to minimize, incidental loss of civilian life, injury to civilians and damage to civilian objects.
Summary
State practice establishes this rule as a norm of customary international law applicable in both international and non-international armed conflicts.
International armed conflicts
The discussions in the 1970s at the UN General Assembly and during the diplomatic conferences that led to the adoption of the Additional Protocols and the Convention on Certain Conventional Weapons show that the use of incendiary weapons is a sensitive issue. The controversy was occasioned in particular by the effects of these weapons during the Vietnam War, and a large number of States advocated a total prohibition of their use.[1]  The majority of those that did not subscribe to a total ban did urge strict restrictions in order to avoid civilian casualties.[2] 
The treaty provisions finally adopted by consensus in Protocol III to the Convention on Certain Conventional Weapons reflect the latter trend, not only by repeating the principle of distinction applicable to the use of all weapons, but also by prohibiting the use of air-delivered incendiary weapons against military objectives located within a concentration of civilians and by restricting the use of other incendiary weapons within such a concentration.[3]  Less than half of all States are party to this treaty. However, many States do not stock incendiary weapons, and such weapons have rarely been used since the adoption of the Protocol.
Furthermore, most military manuals either refer to the rules in Protocol III to the Convention on Certain Conventional Weapons or state the requirement to avoid, or at least to minimize, civilian casualties.[4]  This includes manuals of several States not, or not at the time, party to the Protocol.[5] 
While the rule in Article 2(1) of Protocol III, which is a mere application of the principle of distinction (see Rules 1 and 7), is undoubtedly part of customary international law, it is more difficult to conclude that the detailed rules in Article 2(2)–(4) of Protocol III are also customary international law, but they may be seen as guidelines for the implementation of the customary rule that particular care must be taken to avoid civilian casualties.[6]  Furthermore, military manuals, official statements and other practice stress that incendiary weapons may only be used for certain legitimate purposes.[7]  Combined with the fact that incendiary weapons are far less frequently used than other conventional weapons, this indicates that the general opinion of States is that their use should be avoided, if militarily feasible (see also Rule 85).
Non-international armed conflicts
Protocol III to the Convention on Certain Conventional Weapons, until the amendment of the Convention in December 2001,[8]  applied only to international armed conflicts. Most developments in relation to the application of international humanitarian law in non-international armed conflicts have occurred over the last two decades, and the fact that incendiary weapons have generally not been used during this period means that there has been no reason for the international community to address the issue. However, given the controversy that the use of incendiary weapons occasioned in the 1970s and the clear opinion that has developed since then in the international community that civilians need to be protected with particular care against the effects of armed conflict, it can be concluded that this rule is equally valid for non-international armed conflicts. The fact that the extension of the scope of application of Protocol III to non-international armed conflicts in 2001 was not controversial during the negotiations and has meanwhile entered into force further supports this conclusion.[9] 

[1] See, e.g., the statements (cited in Vol. II, Ch. 30, §§ 9–73).
[2] See, e.g., the statements of Australia (ibid., §§ 141 and 143–144), Austria (ibid., § 146), Denmark (ibid., §§ 148–149), Egypt (ibid., § 146), Ghana (ibid., § 146), Indonesia (ibid., § 154), Jamaica (ibid., § 146), Japan (ibid., §§ 155–156), Mexico (ibid., § 146), Netherlands (ibid., §§ 142–144 and 158), New Zealand (ibid., § 159), Norway (ibid., §§ 149 and 160), Romania (ibid., § 146), Sweden (ibid., § 146), Syria (ibid., § 162), USSR (ibid., § 163), United Kingdom (ibid., § 164), United States (ibid., §§ 165–166), Venezuela (ibid., § 146) and Yugoslavia (ibid., § 146); see also the reported practice of the United States (ibid., § 167).
[3] Protocol III to the Convention on Certain Conventional Weapons, Article 2 (ibid., § 110).
[4] See, e.g., the military manuals of Argentina (ibid., § 117), Australia (ibid., §§ 118–119), Belgium (ibid., § 120), Cameroon (ibid., § 121), Canada (ibid., § 122), Ecuador (ibid., § 123), France (ibid., §§ 124–125), Germany (ibid., § 126), Israel (ibid., § 127), Kenya (ibid., § 128), Netherlands (ibid., § 129), New Zealand (ibid., § 130), Russian Federation (ibid., § 131), Spain (ibid., § 132), Sweden (ibid., § 133), Switzerland (ibid., § 134) and United States (ibid., §§ 136–137).
[5] See the military manuals of Argentina (ibid., § 117), Belgium (ibid., § 120), Cameroon (ibid., § 121), Israel (ibid., § 127), Kenya (ibid., § 128) and United States (ibid., § 136).
[6] Protocol III to the Convention on Certain Conventional Weapons, Article 2(2)–(4) (ibid., § 110).
[7] See, e.g., the military manuals of Argentina (ibid., § 117), Australia (ibid., §§ 118–119), Belgium (ibid., § 120), Cameroon (ibid., § 121), Canada (ibid., § 122), Ecuador (ibid., § 123), France (ibid., §§ 124–125), Germany (ibid., § 126), Israel (ibid., § 127), Kenya (ibid., § 128), Netherlands (ibid., § 129), New Zealand (ibid., § 130), Russian Federation (ibid., § 131), Spain (ibid., § 132), Sweden (ibid., § 133), Switzerland (ibid., § 134) and United States (ibid., §§ 136–137), the statements of Austria (ibid., § 146), Australia (ibid., §§ 141 and 143–144), Denmark (ibid., §§ 148–149), Egypt (ibid., § 146), Ghana (ibid., § 146), Indonesia (ibid., § 154), Jamaica (ibid., § 146), Japan (ibid., §§ 155–156), Mexico (ibid., § 146), Netherlands (ibid., §§ 142–144 and 158), New Zealand (ibid., § 159), Norway (ibid., §§ 149 and 160), Romania (ibid., § 146), Sweden (ibid., § 146), Syrian Arab Republic (ibid., § 162), USSR (ibid., § 163), United Kingdom (ibid., § 164), United States (ibid., §§ 165–166 and 168), Bolivarian Republic of Venezuela (ibid., § 146) and Yugoslavia (ibid., § 146) and the reported practice of the United States (ibid., § 167).
[8] See Convention on Certain Conventional Weapons, amended Article 1 (ibid., § 115).
[9] The amendment entered into force on 18 May 2004. To date, 29 States have ratified the amended Convention on Certain Conventional Weapons: Argentina, Australia, Austria, Belgium, Bulgaria, Burkina Faso, Canada, China, Croatia, Estonia, Finland, France, Holy See, Hungary, Japan, Latvia, Liechtenstein, Lithuania, Mexico, Netherlands, Norway, Republic of Korea, Romania, Serbia and Montenegro, Slovakia, Spain, Sweden, Switzerland and United Kingdom.