Rule 18. Assessment of the Effects of Attacks
Rule 18. Each party to the conflict must do everything feasible to assess whether the attack may be expected to cause incidental loss of civilian life, injury to civilians, damage to civilian objects, or a combination thereof, which would be excessive in relation to the concrete and direct military advantage anticipated.
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 duty to do everything feasible to assess whether the attack may be expected to cause excessive incidental damage is set forth in Article 57(2)(a)(iii) 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 do everything feasible to assess whether the attack may be expected to cause excessive incidental damage, the States concerned (Egypt, Iraq, Israel and Syrian Arab Republic) replied favourably.[5] 
Non-international armed conflicts
While Additional Protocol II does not include an explicit reference to the obligation to do everything feasible to assess whether the attack may be expected to cause excessive incidental damage, more recent treaty law applicable in non-international armed conflicts does so, namely the Second Protocol to the Hague Convention for the Protection of Cultural Property.[6]  In addition, this rule is contained in other instruments pertaining also to non-international armed conflicts.[7] 
The rule whereby each party must do everything feasible to assess whether the attack may be expected to cause excessive incidental damage is set forth in military manuals which are applicable in or have been applied in non-international armed conflicts.[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), which is customary in international and non-international armed conflicts, inherently requires 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.

[1] Additional Protocol I, Article 57(2)(a)(iii) (adopted by 90 votes in favour, none against and 4 abstentions) (cited in Vol. II, Ch. 5, § 325).
[2] See, e.g., the military manuals of Argentina (ibid., § 331), Australia (ibid., § 332), Belgium (ibid., § 333), Benin (ibid., § 334), Cameroon (ibid., § 335), Canada (ibid., § 336), Ecuador (ibid., § 337), France (ibid., § 338), Germany (ibid., § 339), Israel (ibid., § 340), Netherlands (ibid., § 341), New Zealand (ibid., § 342), Nigeria (ibid., § 343), Spain (ibid., § 344), Sweden (ibid., § 345), Togo (ibid., § 346), United States (ibid., §§ 347–348) and Yugoslavia (ibid., § 349).
[3] See, e.g., the statements of Indonesia (ibid., § 353), Iraq (ibid., § 354), Netherlands (ibid., § 355), United Kingdom (ibid., §§ 357–358) and United States (ibid., § 359) and the reported practice of the Syrian Arab Republic (ibid., § 356) and Zimbabwe (ibid., § 360).
[4] See, e.g., the practice of Indonesia (ibid., § 353), Iraq (ibid., § 354), Israel (ibid., § 340), United Kingdom (ibid., §§ 357–358) and United States (ibid., §§ 347–348 and 359).
[5] See ICRC, The International Committee’s Action in the Middle East (ibid., § 365).
[6] Second Protocol to the Hague Convention for the Protection of Cultural Property, Article 7 (ibid., § 326).
[7] See, e.g., Memorandum of Understanding on the Application of IHL between Croatia and the Socialist Federal Republic of Yugoslavia, § 6 (ibid., § 328); Agreement on the Application of IHL between the Parties to the Conflict in Bosnia and Herzegovina, § 2.5 (ibid., § 329); San Remo Manual, § 46(d) (ibid., § 330).
[8] See, e.g., the military manuals of Benin (ibid., § 334), Ecuador (ibid., § 337), Germany (ibid., § 339), Nigeria (ibid., § 343), Togo (ibid., § 346) and Yugoslavia (ibid., § 349).
[9] ICTY, Kupreškić case, Judgment (ibid., § 362).
[10] ICTY, Kupreškić case, Judgment (ibid., § 362).