Rule 23. Location of Military Objectives outside Densely Populated Areas
Rule 23. Each party to the conflict must, to the extent feasible, avoid locating military objectives within or near densely populated areas.
Summary
State practice establishes this rule as a norm of customary international law applicable in international, and arguably also in non-international, armed conflicts. This rule is an application of the principle of distinction (see Rules 1 and 7). It is also related to the prohibition of human shields (see Rule 97), as everything feasible must be done to separate military objectives from the civilian population, but in no event may civilians be used to shield military objectives.
International armed conflicts
The duty of each party to the conflict to avoid locating military objectives within or near densely populated areas is set forth in Article 58(b) of Additional Protocol I, to which no reservations relevant to this rule have been made.[1]  It is also contained in the Israel-Lebanon Ceasefire Understanding of 1996.[2] 
A large number of military manuals include this obligation.[3]  It is also supported by official statements and reported practice.[4]  This practice includes that of States not, or not at the time, party to Additional Protocol I.[5] 
Non-international armed conflicts
Although Additional Protocol II does not explicitly require precautions against the effects of attacks, Article 13(1) stipulates that “the civilian population and individual civilians shall enjoy general protection against the dangers arising from military operations” and it would be difficult to afford such protection when military objectives are located within or near densely populated areas.[6]  The requirement to take this precaution against the effects of attacks has, moreover, been included in more recent treaty law applicable in non-international armed conflicts, namely the Second Protocol to the Hague Convention for the Protection of Cultural Property.[7]  In addition, this rule is contained 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 duty of each party to the conflict to avoid locating military objectives within or near densely populated areas.[9] 
The jurisprudence of the International Criminal Tribunal for the Former Yugoslavia in the Kupreškić case provides further evidence of the customary nature of the duty of each party to the conflict to avoid locating military objectives within or near densely populated areas in both international and non-international armed conflicts. In its judgment, the Tribunal considered that this rule was customary because it specified and fleshed out general pre-existing norms.[10]  It can be argued indeed that the principle of distinction (see Rules 1 and 7) and the principle of proportionality (see Rule 14), which are both customary in international and non-international armed conflicts, inherently require respect for this rule. The Tribunal also relied on the fact that this rule had not been contested by any State.[11]  This study found no official contrary practice either.
In 1979, in the context of the conflict in Rhodesia/Zimbabwe, the ICRC appealed to the Patriotic Front to “clearly separate civilian establishments, particularly refugee camps, from military installations”.[12] 
The rules which require that persons deprived of their liberty be held in premises which are removed from the combat zone (see Rule 121) and that in case of displacement all possible measures be taken in order that the civilian population may be received under satisfactory conditions of safety (see Rule 131), which are both applicable in international and non-international armed conflicts, are also relevant in establishing the customary nature of this rule.
Interpretation
While some practice refers to the duty to locate military bases and installations outside densely populated areas, practice in general limits this obligation to what is feasible. It is possible, as several reports on State practice point out, that demographic changes cause military bases to be located within or near cities where this was originally not the case.[13]  When such objectives involve immovable property, it is less feasible to move them than in the case of movable property. At the Diplomatic Conference leading to the adoption of the Additional Protocols, South Korea stated that this rule “does not constitute a restriction on a State’s military installations on its own territory”.[14]  Dual use installations, such as railway stations and airports, may even be located near or inside densely populated areas on purpose.

[1] Additional Protocol I, Article 58(b) (adopted by 80 votes in favour, none against and 8 abstentions) (cited in Vol. II, Ch. 6, § 70).
[2] Israel-Lebanon Ceasefire Understanding, Article 3 (ibid., § 71).
[3] See, e.g., the military manuals of Argentina (ibid., § 77), Australia (ibid., § 78), Benin (ibid., § 79), Canada (ibid., § 80), Croatia (ibid., §§ 81–82), Ecuador (ibid., § 83), Hungary (ibid., § 84), Israel (ibid., § 85), Italy (ibid., § 86), Kenya (ibid., § 87), Madagascar (ibid., § 88), Netherlands (ibid., § 89), New Zealand (ibid., § 90), Nigeria (ibid., § 91), Russian Federation (ibid., § 92), Spain (ibid., § 93), Sweden (ibid., § 94), Switzerland (ibid., § 95), Togo (ibid., § 96), United Kingdom (ibid., § 97) and United States (ibid., § 98).
[4] See, e.g., the statements of France (ibid., § 105), Iraq (ibid., § 107), Israel (ibid., §§ 105 and 108), Lebanon (ibid., §§ 105 and 113), Syrian Arab Republic (ibid., § 105), United Kingdom (ibid., § 116) and United States (ibid., §§ 105 and 117–123) and the reported practice of Botswana (ibid., § 102), Egypt (ibid., § 104), Israel (ibid., § 109), Jordan (ibid., § 110), Kuwait (ibid., § 112), Malaysia (ibid., § 114), Syrian Arab Republic (ibid., § 115), United States (ibid., § 124) and Zimbabwe (ibid., § 125).
[5] See, e.g., the practice of France (ibid., § 105), Iraq (ibid., § 107), Israel (ibid., §§ 71, 85, 105 and 108), Kenya (ibid., § 87), United Kingdom (ibid., §§ 97 and 116) and United States (ibid., §§ 98, 105 and 117–123) and the reported practice of Israel (ibid., § 109), Malaysia (ibid., § 114) and United States (ibid., § 124).
[6] Additional Protocol II, Article 13(1) (adopted by consensus) (ibid., § 2).
[7] Second Protocol to the Hague Convention for the Protection of Cultural Property, Article 8 (cited in Vol. II, Ch. 12, § 292).
[8] See, e.g., Memorandum of Understanding on the Application of IHL between Croatia and the Socialist Federal Republic of Yugoslavia, § 6 (cited in Vol. II, Ch. 6, § 74); Agreement on the Application of IHL between the Parties to the Conflict in Bosnia and Herzegovina, § 2.5 (ibid., § 75); UN Secretary-General’s Bulletin, Section 5.4 (ibid., § 76).
[9] See, e.g., the military manuals of Benin (ibid., § 79), Croatia (ibid., §§ 81–82), Ecuador (ibid., § 83), Italy (ibid., § 86), Kenya (ibid., § 87), Madagascar (ibid., § 88), Nigeria (ibid., § 91) and Togo (ibid., § 96).
[10] ICTY, Kupreškić case, Judgment (ibid., § 46).
[11] ICTY, Kupreškić case, Judgment (ibid., § 46).
[12] See ICRC, Conflict in Southern Africa: ICRC appeal (ibid., § 131).
[13] See the Reports on the Practice of the Islamic Republic of Iran (ibid., § 106), Israel (ibid., § 109), Kuwait (ibid., § 112) and Malaysia (ibid., § 114).
[14] Republic of Korea, Statement at the Diplomatic Conference leading to the adoption of the Additional Protocols (ibid., § 111).