Rule 12. Definition of Indiscriminate Attacks
Rule 12. Indiscriminate attacks are those:
(a) which are not directed at a specific military objective;
(b) which employ a method or means of combat which cannot be directed at a specific military objective; or
(c) which employ a method or means of combat the effects of which cannot be limited as required by international humanitarian law;
and consequently, in each such case, are of a nature to strike military objectives and civilians or civilian objects without distinction.
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
This definition of indiscriminate attacks is set forth in Article 51(4)(a) of Additional Protocol I.[1]  France voted against Article 51 at the Diplomatic Conference leading to the adoption of the Additional Protocols because it deemed that paragraph 4 by its “very complexity would seriously hamper the conduct of defensive military operations against an invader and prejudice the inherent right of legitimate defence”.[2]  Upon ratification of Additional Protocol I, however, France did not enter a reservation to this provision. At the Diplomatic Conference leading to the adoption of the Additional Protocols, Mexico stated that Article 51 was so essential that it “cannot be the subject of any reservations whatsoever since these would be inconsistent with the aim and purpose of Protocol I and undermine its basis”.[3]  A report on the work of Committee III of the Diplomatic Conference stated that there was general agreement that a proper definition of indiscriminate attacks included the three types of attack set down in this rule.[4]  With the exception of subparagraph (c), this definition of indiscriminate attacks is also contained in Protocol II and Amended Protocol II to the Convention on Certain Conventional Weapons.[5] 
A large number of military manuals contain this definition of indiscriminate attacks, in whole or in part.[6]  It has similarly been relied upon in official statements.[7]  This practice includes that of States not party to Additional Protocol I.[8] 
Non-international armed conflicts
Additional Protocol II does not contain a definition of indiscriminate attacks, even though it has been argued that subsections (a) and (b) of the definition contained in this rule are included by inference within the prohibition contained in Article 13(2) on making the civilian population the object of attack.[9]  With the exception of subsection (c), this definition has also been included in more recent treaty law applicable in non-international armed conflicts, namely Amended Protocol II to the Convention on Certain Conventional Weapons.[10]  In addition, the definition is included in other instruments pertaining also to non-international armed conflicts.[11] 
This definition of indiscriminate attacks is also set forth in military manuals which are applicable in or have been applied in non-international armed conflicts.[12]  It is supported by official statements.[13] 
The 24th International Conference of the Red Cross in 1981 urged parties to armed conflicts in general “not to use methods and means of warfare that cannot be directed against specific military targets and whose effects cannot be limited”.[14] 
Further evidence of the customary nature of the definition of indiscriminate attacks in both international and non-international armed conflicts can be found in the jurisprudence of the International Court of Justice and of the International Criminal Tribunal for the Former Yugoslavia. In its advisory opinion in the Nuclear Weapons case, the International Court of Justice stated that the prohibition of weapons that are incapable of distinguishing between civilian and military targets constitutes an “intransgressible” principle of customary international law. The Court observed that, in conformity with this principle, humanitarian law, at a very early stage, prohibited certain types of weapons “because of their indiscriminate effect on combatants and civilians”.[15]  In its review of the indictment in the Martić case in 1996, the International Criminal Tribunal for the Former Yugoslavia examined the legality of the use of cluster bombs according to customary international law, including the prohibition of indiscriminate attacks involving a means or method of warfare which cannot be directed at a specific military objective.[16] 
No official contrary practice was found. No other definition of indiscriminate attacks has officially been advanced, and the statements made with respect to indiscriminate attacks in general under Rule 11 may be based in some or more instances on an understanding of indiscriminate attacks as contained in Rule 12, especially since no other definition exists.
Interpretation
This definition of indiscriminate attacks represents an implementation of the principle of distinction and of international humanitarian law in general. Rule 12(a) is an application of the prohibition on directing attacks against civilians (see Rule 1) and the prohibition on directing attacks against civilian objects (see Rule 7), which are applicable in both international and non-international armed conflicts. Rule 12(b) is also an application of the prohibition on directing attacks against civilians or against civilian objects (see Rules 1 and 7). The prohibition of weapons which are by nature indiscriminate (see Rule 71), which is applicable in both international and non-international armed conflicts, is based on the definition of indiscriminate attacks contained in Rule 12(b). Lastly, Rule 12(c) is based on the logical argument that means or methods of warfare whose effects cannot be limited as required by international humanitarian law should be prohibited. But this reasoning begs the question as to what those limitations are. Practice in this respect points to weapons whose effects are uncontrollable in time and space and are likely to strike military objectives and civilians or civilian objects without distinction. The US Air Force Pamphlet gives the example of biological weapons.[17]  Even though biological weapons might be directed against military objectives, their very nature means that after being launched their effects escape from the control of the launcher and may strike both combatants and civilians and necessarily create a risk of excessive civilian casualties.

[1] Additional Protocol I, Article 51(4)(a) (adopted by 77 votes in favour, one against and 16 abstentions) (cited in Vol. II, Ch. 3, § 164).
[2] France, Statement at the Diplomatic Conference leading to the adoption of the Additional Protocols (ibid., § 73).
[3] Mexico, Statement at the Diplomatic Conference leading to the adoption of the Additional Protocols (ibid., §§ 228 and 268).
[4] Report on the work of Committee III of the Diplomatic Conference leading to the adoption of the Additional Protocols (ibid., § 200).
[5] Protocol II to the Convention on Certain Conventional Weapons, Article 3(3)(a) (ibid., § 165); Amended Protocol II to the Convention on Certain Conventional Weapons, Article 3(8)(a) (ibid., § 166).
[6] See, e.g., the military manuals of Australia (ibid., §§ 170, 212 and 256), Belgium (ibid., §§ 170, 212 and 256), Benin (ibid., § 171), Canada (ibid., §§ 170, 212 and 256), Ecuador (ibid., §§ 172 and 213), Germany (ibid., §§ 170, 212 and 256), Israel (ibid., §§ 173, 214 and 257), Kenya (ibid., § 174), Netherlands (ibid., §§ 170, 212 and 256), New Zealand (ibid., §§ 170, 212 and 256), Nigeria (ibid., § 175), South Africa (ibid., § 176), Spain (ibid., §§ 170, 212 and 256), Sweden (ibid., §§ 170, 212 and 256), Togo (ibid., § 177), United Kingdom (ibid., § 178), United States (ibid., §§ 179–180, 215–217 and 258) and Yugoslavia (ibid., § 259); see also the draft legislation of El Salvador (ibid., §§ 181, 218 and 260) and Nicaragua (ibid., §§ 182, 219 and 261).
[7] See, e.g., the statements of Canada (ibid., § 221), Colombia (ibid., § 184), Federal Republic of Germany (ibid., § 222), German Democratic Republic (ibid., § 223), India (ibid., §§ 185 and 224), Iraq (ibid., § 225), Italy (ibid., § 226), Jordan and United States (ibid., §§ 186 and 227), Mexico (ibid., §§ 188 and 228–229), Nauru (ibid., § 230), Rwanda (ibid., § 190), Sri Lanka (ibid., § 231), United Kingdom (ibid., §§ 191 and 232) and United States (ibid., §§ 192–195 and 233–237).
[8] See, e.g., the practice of India (ibid., §§ 185, 224 and 265) and United States (ibid., §§ 186, 227 and 267).
[9] Michael Bothe, Karl Joseph Partsch, Waldemar A. Solf (eds.), New Rules for Victims of Armed Conflicts, Martinus Nijhoff, The Hague, 1982, p. 677.
[10] Amended Protocol II to the Convention on Certain Conventional Weapons, Article 3(8)(a) (cited in Vol. II, Ch. 3, § 166).
[11] See, e.g., Memorandum of Understanding on the Application of IHL between Croatia and the Socialist Federal Republic of Yugoslavia, § 6 (ibid., §§ 167, 209 and 253); Agreement on the Application of IHL between the Parties to the Conflict in Bosnia and Herzegovina, § 2.5 (ibid., §§ 168, 210 and 254); San Remo Manual, § 42(b) (ibid., §§ 169, 211 and 255).
[12] See, e.g., the military manuals of Australia (ibid., §§ 170, 212 and 256), Benin (ibid., § 171), Ecuador (ibid., §§ 172 and 213), Germany (ibid., §§ 170, 212 and 256), Kenya (ibid., § 174), Nigeria (ibid., § 175), Togo (ibid., § 177) and Yugoslavia (ibid., § 259).
[13] See, e.g., the statements of India (ibid., §§ 185, 224 and 265), Jordan (ibid., §§ 186, 227 and 267) and United States (ibid., §§ 186, 195, 227, 236 and 267); see also the draft legislation of El Salvador (ibid., §§ 181, 218 and 260) and Nicaragua (ibid., §§ 182, 219 and 261).
[14] 24th International Conference of the Red Cross, Res. XIII (ibid., §§ 242 and 279).
[15] ICJ, Nuclear Weapons case (ibid., § 243).
[16] ICTY, Martić case, Review of the Indictment (ibid., § 246).
[17] United States, Air Force Pamphlet (ibid., § 258).