Rule 13. Area Bombardment
Rule 13. Attacks by bombardment by any method or means which treats as a single military objective a number of clearly separated and distinct military objectives located in a city, town, village or other area containing a similar concentration of civilians or civilian objects are prohibited.
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
According to Additional Protocol I, an attack by bombardment by any method or means which treats as a single military objective a number of clearly separated and distinct military objectives located in a city, town, village or other area containing a similar concentration of civilians or civilian objects, so-called “area bombardments”, are indiscriminate and, as such, prohibited.[1] 
The prohibition of “area bombardment” is contained in numerous military manuals.[2]  These include manuals of States not, or not at the time, party to Additional Protocol I.[3] 
When the ICRC appealed to the parties to the conflict in the Middle East in October 1973, i.e., before the adoption of Additional Protocol I, to respect the prohibition of “area bombardment”, the States concerned (Egypt, Iraq, Israel and Syrian Arab Republic) replied favourably.[4] 
Non-international armed conflicts
The prohibition of “area bombardment” was included in the draft of Additional Protocol II but was dropped at the last moment as part of a package aimed at the adoption of a simplified text.[5]  As a result, Additional Protocol II does not contain this rule as such, even though it has been argued that it is included by inference within the prohibition contained in Article 13(2) on making the civilian population the object of attack.[6]  The prohibition is set forth in more recent treaty law applicable in non-international armed conflicts, namely Amended Protocol II to the Convention on Certain Conventional Weapons.[7]  In addition, it has been included in other instruments pertaining also to non-international armed conflicts.[8] 
Military manuals which are applicable in or have been applied in non-international armed conflicts specify the prohibition of “area bombardment”.[9] 
The conclusion that this rule is customary in non-international armed conflicts is also supported by the argument that because so-called “area bombardments” have been considered to constitute a type of indiscriminate attack, and because indiscriminate attacks are prohibited in non-international armed conflict, it must follow that “area bombardments” are prohibited in non-international armed conflicts.
No official contrary practice was found with respect to either international or non-international armed conflicts.
Interpretation
At the Diplomatic Conference leading to the adoption of the Additional Protocols, the United States specified that the words “clearly separated” in the definition of area bombardments required a distance “at least sufficiently large to permit the individual military objectives to be attacked separately”.[10]  This view was supported by some other States.[11] 

[1] Additional Protocol I, Article 51(5)(a) (adopted by 77 votes in favour, one against and 16 abstentions) (cited in Vol. II, Ch. 3, § 283).
[2] See, e.g., the military manuals of Australia (ibid., §§ 290–291), Belgium (ibid., § 292), Benin (ibid., § 293), Canada (ibid., § 294), Croatia (ibid., § 295), Germany (ibid., § 296), Israel (ibid., § 297), Italy (ibid., § 298), Kenya (ibid., § 299), Madagascar (ibid., § 300), Netherlands (ibid., § 301), New Zealand (ibid., § 302), Spain (ibid., § 303), Sweden (ibid., § 304), Switzerland (ibid., § 305), Togo (ibid., § 306), United Kingdom (ibid., § 307) and United States (ibid., § 308).
[3] See the military manuals of Israel (ibid., § 297), Kenya (ibid., § 299), United Kingdom (ibid., § 307) and United States (ibid., § 308).
[4] See ICRC, The International Committee’s Action in the Middle East (ibid., § 321).
[5] Draft Additional Protocol II submitted by the ICRC to the Diplomatic Conference leading to the adoption of the Additional Protocols, Article 26(3)(a) (ibid., § 284).
[6] Michael Bothe, Karl Joseph Partsch, Waldemar A. Solf (eds.), New Rules for Victims of Armed Conflicts, Martinus Nijhoff, The Hague, 1982, p. 677.
[7] Amended Protocol II to the Convention on Certain Conventional Weapons, Article 3(9) (cited in Vol. II, Ch. 3, § 285).
[8] See, e.g., Memorandum of Understanding on the Application of IHL between Croatia and the SFRY, para. 6 (ibid., § 288); Agreement on the Application of IHL between the Parties to the Conflict in Bosnia and Herzegovina, para. 2.5 (ibid., § 289).
[9] See, e.g., the military manuals of Australia (ibid., § 290), Benin (ibid., § 293), Croatia (ibid., § 295), Germany (ibid., § 296), Italy (ibid., § 298), Kenya (ibid., § 299), Madagascar (ibid., § 300) and Togo (ibid., § 306).
[10] United States, Statement at the Diplomatic Conference leading to the adoption of the Additional Protocols (ibid., § 315).
[11] See the statements at the Diplomatic Conference leading to the adoption of the Additional Protocols made by Canada (ibid., § 311), Egypt (ibid., § 312) and United Arab Emirates (ibid., § 314).