Rule 20. Advance Warning
Rule 20. Each party to the conflict must give effective advance warning of attacks which may affect the civilian population, unless circumstances do not permit.
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 obligation to give effective advance warning prior to an attack which may affect the civilian population is a long-standing rule of customary international law already recognized in the Lieber Code, the Brussels Declaration and the Oxford Manual.[1]  It was first codified in the Hague Regulations and is restated in Article 57(2)(c) of Additional Protocol I, to which no relevant reservations have been made.[2] 
This obligation is included in a large number of military manuals.[3]  Some national legislation incorporates it.[4]  The obligation to give advance warning is also supported by official statements and other practice, including several accounts of advance warning.[5]  Practice includes that of States not, or not at the time, party to Additional Protocol I.[6]  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 give effective advance warning prior to an attack which may affect the civilian population, the States concerned (Egypt, Iraq, Israel and Syrian Arab Republic) replied favourably.[7] 
Non-international armed conflicts
While Additional Protocol II does not include an explicit reference to the obligation to give effective advance warning prior to an attack which may affect the civilian population, more recent treaty law applicable in non-international armed conflicts does so, namely Amended Protocol II to the Convention on Certain Conventional Weapons.[8]  While this rule deals with the requirement to give warning of attacks which may affect the civilian population, it is nevertheless relevant to point out that the concept of warnings has also been extended to non-international armed conflicts in the context of the protection of cultural property.[9]  In addition, this rule is contained in other instruments pertaining also to non-international armed conflicts.[10] 
Military manuals which are applicable in or have been applied in non-international armed conflicts specify this obligation.[11]  There are, in addition, several accounts of warnings that were issued in the context of non-international armed conflicts.[12] 
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.[13]  It can be argued indeed that respect for the principle of distinction (see Rules 1 and 7) and the principle of proportionality (see Rule 14), both of which are customary in international and non-international armed conflicts, requires respect for this rule by inference. The Tribunal also relied on the fact that this rule had not been contested by any State.[14]  This study found no official contrary practice either. Instead, it found accounts of warnings given in the context of both international and non-international armed conflicts.[15] 
Interpretation
As the rule indicates, State practice considers that a warning is not required when circumstances do not permit, such as in cases where the element of surprise is essential to the success of an operation or to the security of the attacking forces or that of friendly forces.[16]  Necessary speed of response is another consideration cited in practice as relevant to determining the feasibility of warnings.[17] 
Furthermore, the rule provides that warnings must only be given of attacks which may affect the civilian population. Hence, the UK Military Manual considers that no warning is required if no civilians are left in the area to be attacked.[18]  The US Air Force Pamphlet states that no warning is required if civilians are unlikely to be affected by the attack.[19] 
Some practice was found to interpret the requirement that a warning be “effective”. The United States, in particular, has stated that a warning need not be specific and may be general in order not to endanger the attacking forces or the success of their mission. It has also stated that such a general warning can consist of a blanket alert delivered by broadcast advising the civilian population to stay away from certain military objectives.[20] 
State practice indicates that all obligations with respect to the principle of distinction and the conduct of hostilities remain applicable even if civilians remain in the zone of operations after a warning has been issued. Threats that all remaining civilians would be considered liable to attack have been condemned and withdrawn.[21] 

[1] Lieber Code, Article 19 (cited in Vol. II, Ch. 5, § 424); Brussels Declaration, Article 16 (ibid., § 425); Oxford Manual, Article 33 (ibid., § 426).
[2] Hague Regulations, Article 26 (ibid., §§ 420–421); Additional Protocol I, Article 57(2)(c) (adopted by 90 votes in favour, none against and 4 abstentions) (ibid., § 423).
[3] See, e.g., the military manuals of Argentina (ibid., § 430), Australia (ibid., § 431), Belgium (ibid., § 432), Benin (ibid., § 433), Cameroon (ibid., § 434), Canada (ibid., § 435), Croatia (ibid., § 436), Ecuador (ibid., § 437), France (ibid., § 438), Germany (ibid., § 439), Italy (ibid., §§ 440–441), Kenya (ibid., § 442), Madagascar (ibid., § 443), Netherlands (ibid., §§ 444–445), New Zealand (ibid., § 446), Nigeria (ibid., § 447), South Africa (ibid., § 448), Spain (ibid., § 449), Sweden (ibid., § 450), Switzerland (ibid., § 451), Togo (ibid., § 452), United Kingdom (ibid., §§ 453–454), United States (ibid., §§ 455–457) and Yugoslavia (ibid., § 458).
[4] See, e.g., the legislation of Ireland (ibid., § 460), Italy (ibid., § 461–462) and Norway (ibid., § 463).
[5] See, e.g., the statements of Netherlands (ibid., § 476) and United States (ibid., §§ 482–484), the practice of France (ibid., § 467) and Israel (ibid., §§ 471–472) and the reported practice of Indonesia (ibid., § 468), Islamic Republic of Iran (ibid., § 469), Iraq (ibid., § 470), Israel (ibid., §§ 473 and 489), Jordan (ibid., § 474), Syrian Arab Republic (ibid., § 478), United Kingdom (ibid., § 479), United States (ibid., §§ 480–481 and 485) and Zimbabwe (ibid., § 486).
[6] See, e.g., the military manuals of France (ibid., § 438), Kenya (ibid., § 442), United Kingdom (ibid., §§ 453–454) and United States (ibid., §§ 455–457); the statements of the United States (ibid., §§ 482–484); the practice of France (ibid., § 467) and Israel (ibid., §§ 471–472) and the reported practice of Indonesia (ibid., § 468), Islamic Republic of Iran (ibid., § 469), Iraq (ibid., § 470), Israel (ibid., §§ 473 and 489), United Kingdom (ibid., § 479) and United States (ibid., §§ 480–481 and 485).
[7] See ICRC, The International Committee’s Action in the Middle East (ibid., § 495).
[8] Amended Protocol II to the Convention on Certain Conventional Weapons, Article 3(11).
[9] See Second Protocol to the Hague Convention for the Protection of Cultural Property, Articles 6(d) and 13(2)(c).
[10] 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, § 428); Agreement on the Application of IHL between the Parties to the Conflict in Bosnia and Herzegovina, § 2.5 (ibid., § 429).
[11] See, e.g., the military manuals of Benin (ibid., § 433), Croatia (ibid., § 436), Ecuador (ibid., § 437), Germany (ibid., § 439), Italy (ibid., §§ 440–441), Kenya (ibid., § 442), Madagascar (ibid., § 443), Nigeria (ibid., § 447), South Africa (ibid., § 448), Togo (ibid., § 452) and Yugoslavia (ibid., § 458).
[12] See, e.g., the reported practice of China (ibid., § 465), Malaysia (ibid., § 475), Russian Federation (ibid., § 477) and two other States (ibid., §§ 487–488).
[13] ICTY, Kupreškić case, Judgment (ibid., § 492).
[14] ICTY, Kupreškić case, Judgment (ibid., § 492).
[15] See, e.g., the practice and reported practice of China (ibid., § 465), Islamic Republic of Iran (ibid., § 469), Iraq (ibid., § 470), Israel (ibid., §§ 471–473 and 489), Malaysia (ibid., § 475), Russian Federation (ibid., § 477), United Kingdom (ibid., § 479), United States (ibid., §§ 480–481 and 485) and two other States (ibid., §§ 487–488).
[16] See, e.g., Hague Regulations, Article 26 (ibid., §§ 420–421); Additional Protocol I, Article 57(2)(c) (adopted by 90 votes in favour, none against and 4 abstentions) (ibid., § 423); Brussels Declaration, Article 16 (ibid., § 425); Oxford Manual, Article 33 (ibid., § 426); Memorandum of Understanding on the Application of IHL between Croatia and the Socialist Federal Republic of Yugoslavia, § 6 (ibid., § 428); Agreement on the Application of IHL between the Parties to the Conflict in Bosnia and Herzegovina, § 2.5 (ibid., § 429); practice of Australia (ibid., § 431), Belgium (ibid., § 432), Benin (ibid., § 433), Cameroon (ibid., § 434), Canada (ibid. § 435), Croatia (ibid., § 436), Ecuador (ibid., § 437), France (ibid., §§ 438 and 467), Germany (ibid., § 439), Italy (ibid., §§ 440–441), Kenya (ibid., § 442), Madagascar (ibid., § 443), Netherlands (ibid., §§ 444–445), New Zealand (ibid., § 446), South Africa (ibid., § 448), Spain (ibid., § 449), Switzerland (ibid., § 451), Togo (ibid., § 452), United Kingdom (ibid., §§ 453–454), United States (ibid., §§ 455–457 and 483–484) and Yugoslavia (ibid., § 458) and the reported practice of Israel (ibid., § 473).
[17] See, e.g., the reported practice of Israel (ibid., § 473).
[18] United Kingdom, Military Manual (ibid., § 453).
[19] United States, Air Force Pamphlet (ibid., § 456).
[20] See the practice of the United States (ibid., §§ 456, 483 and 485); see also the reported practice of Israel (ibid., § 473).
[21] See the practice of Israel (ibid., § 489) and Russian Federation (ibid., § 477).