Rule 21. Target Selection
Rule 21. When a choice is possible between several military objectives for obtaining a similar military advantage, the objective to be selected must be that the attack on which may be expected to cause the least danger to civilian lives and to civilian objects.
Summary
State practice establishes this rule as a norm of customary international law applicable in international, and arguably also in non-international, armed conflicts.
International armed conflicts
The requirement that, when a choice is possible, the military objective to be selected be that which may be expected to cause the least danger to civilian lives and to civilian objects is set forth in Article 57(3) of Additional Protocol I, to which no relevant reservations have been made.[1] 
This obligation is included in numerous military manuals.[2]  It is also supported by official statements and reported practice.[3]  This practice includes that of States not, or not at the time, party to Additional Protocol I.[4]  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 requirement that, when a choice is possible, the military objective to be selected be that which may be expected to cause the least danger to civilian lives and to civilian objects, the States concerned (Egypt, Iraq, Israel and Syrian Arab Republic) replied favourably.[5] 
Non-international armed conflicts
While Additional Protocol II does not contain an explicit reference to the requirement that, when a choice is possible, the military objective to be selected be that which may be expected to cause the least danger to civilian lives and to civilian objects, it has 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.[6]  In addition, it is specified in other instruments pertaining also to non-international armed conflicts.[7] 
Military manuals which are applicable in or have been applied in non-international armed conflicts specify the requirement that, when a choice is possible, the military objective to be selected be that the attack on which may be expected to cause the least danger to civilian lives and to civilian objects.[8] 
The jurisprudence of the International Criminal Tribunal for the Former Yugoslavia in the Kupreškić case provides further evidence of the customary nature of this rule 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.[9]  It can be argued indeed that the principle of proportionality (see Rule 14) and the obligation to take all feasible precautions to avoid, and in any event to minimize, incidental loss of civilian life, injury to civilians and damage to civilian objects (see Rule 15), which are customary in both 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.[10]  This study found no official contrary practice either.
There is only one instance of apparently contrary practice. In response to an ICRC memorandum on the applicability of international humanitarian law in the Gulf region, the United States denied that this rule was customary but then restated the rule and recognized its validity,[11]  consistent with its other practice referred to above.[12] 
This rule should also be seen as a further specification of Rule 17 on the precautions to be taken in the choice of means and methods of warfare. Some States indicate that target selection is a means of complying with that requirement, and this rule describes a way in which target selection can operate as a precautionary measure.
Interpretation
The United States has emphasized that the obligation to select an objective the attack on which may be expected to cause the least danger to civilian lives and to civilian objects is not an absolute obligation, as it only applies “when a choice is possible” and thus “an attacker may comply with it if it is possible to do so, subject to mission accomplishment and allowable risk, or he may determine that it is impossible to make such a determination”.[13] 

[1] Additional Protocol I, Article 57(3) (adopted by 90 votes in favour, none against and 4 abstentions) (cited in Vol. II, Ch. 5, § 502).
[2] See, e.g., the military manuals of Australia (ibid., § 506), Benin (ibid., § 507), Canada (ibid., § 508), Croatia (ibid., §§ 509–510), France (ibid., § 511), Germany (ibid., § 512), Hungary (ibid., § 513), Italy (ibid., § 514), Kenya (ibid., § 515), Madagascar (ibid., § 516), Netherlands (ibid., § 517), New Zealand (ibid., § 518), Nigeria (ibid., § 519), Spain (ibid., § 520), Sweden (ibid., § 521), Togo (ibid., § 522), United States (ibid., § 523) and Yugoslavia (ibid., § 524).
[3] See, e.g., the practice of Indonesia (ibid., § 528), Jordan (ibid., § 531), Netherlands (ibid., § 533) and United States (ibid., § 535, but see ibid., § 536) and the reported practice of the Islamic Republic of Iran (ibid., § 529), Israel (ibid., § 530), Malaysia (ibid., § 532), Syrian Arab Republic (ibid., § 534) and Zimbabwe (ibid., § 537).
[4] See, e.g., the practice of France (ibid., § 511), Indonesia (ibid., § 528), Kenya (ibid., § 515) and United States (ibid., §§ 523 and 535) and the reported practice of the Islamic Republic of Iran (ibid., § 529), Israel (ibid., § 530) and Malaysia (ibid., § 532).
[5] See ICRC, The International Committee’s Action in the Middle East (ibid., § 541).
[6] Second Protocol to the Hague Convention for the Protection of Cultural Property, Article 6 (cited in Vol. II, Ch. 12, § 21).
[7] 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. 5, § 504); Agreement on the Application of IHL between the Parties to the Conflict in Bosnia and Herzegovina, § 2.5 (ibid., § 505).
[8] See, e.g., the military manuals of Benin (ibid., § 507), Croatia (ibid., §§ 509–510), Germany (ibid., § 512), Italy (ibid., § 514), Kenya (ibid., § 515), Madagascar (ibid., § 516), Nigeria (ibid., § 519), Togo (ibid., § 522) and Yugoslavia (ibid., § 524).
[9] ICTY, Kupreškić case, Judgment (ibid., § 539).
[10] ICTY, Kupreškić case, Judgment (ibid., § 539).
[11] See the practice of the United States (ibid., § 536).
[12] See the practice of the United States (ibid., §§ 523 and 535).
[13] See the practice of the United States (ibid., § 536).