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Approaching Custom Identification as a Conflict Avoidance Technique: Tadić and Kupreškić Revisited


International human rights law (IHRL), international humanitarian law (IHL) and international criminal law (ICL) have trouble staying faithful to the two pillars of customary international law – state practice and opinio juris. In ICL, the Tadić Interlocutory Appeal on Jurisdiction and the Kupreškić Trial Judgement have even gone as far as enunciating new models to identify customs. In this article, I show that the approaches to customs’ identification postulated in these two cases were conflict-avoidance techniques used by the International Criminal Tribunal for the Former Yugoslavia (ICTY) to bring together IHRL and IHL. The crux of the matter in the Tadić and Kupreškić cases was that the human rights of the victims of war crimes committed in internal conflicts required that a new approach to customary international law be adopted. Thus, the criminal aspect of IHL (i.e., ICL) was updated, and conceptual conflicts between IHL and IHRL were avoided.

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1 M. Wood, First Report on Formation and Evidence of Customary International Law, UN Doc. A/CN.4/663 (2013), para. 19; M. Wood, Second Report on Formation and Evidence of Customary International Law, UN Doc. A/CN.4/672 (2014), para. 28; M. Wood, Third Report on Formation and Evidence of Customary International Law, UN Doc. A/CN.4/682 (2015), para. 17; see also ILC Report, UN Doc. A/70/10 (2015), Ch. VI, para. 65; the present draft conclusions adopted by the ILC drafting committee discard the fact that the two element approach may differ according to the field of law but acknowledge that the evidence may be assessed taking into account ‘the overall context, the nature of the rule, and the particular circumstances in which the evidence in question is to be found’. Draft conclusion 3 [4], Assessment of evidence for the two constituent elements, UN Doc. A/CN.4/L.872 (2016).

2 Wouters, J. and Ryngaert, C., ‘Impact on the Process of the Formation of Customary International Law’, in Kamminga, M.T. and Scheinin, M. (eds.), The Impact of Human Rights Law on General International Law (2009), 111 at 129–30.

3 Prosecutor v. Tadić, Decision on the Defence Motion for Interlocutory Appeal on Jurisdiction, Case No. IT-94-1, A. Ch., 2 October 1995; Prosecutor v. Kupreškić et al., Judgement, Case No. IT-95-16-T, T. Ch., 14 January 2000.

4 See First Report, supra note 1, para. 70, fn 35; Second report, supra note 1, fn 48, 52.

5 Ratner, S., ‘Sources of International Humanitarian Law and International Criminal Law: War/Crimes and the Limits of the Doctrine of Sources’, in Besson, S. and d'Aspremont, J., Oxford Handbook on the Sources of International Law (2016), 912, at 921–2.

6 Prosecutor v. Kaing Guek Eav alias ‘Duch’, Appeal Judgement, Case No. 001/18-07-2007-ECCC/SC, A. Ch., 3 February 2012, para. 93.

7 Van den Herik, L., ‘The Decline of Customary International Law as a Source of International Criminal Law’, in Bradley, C. (ed.), Custom's Future: International Law in a Changing World (2016), 230 (among the various arguments Van den Herik makes, this shift is also due to the nature of ICL, and to the future of ICL as mainly residing in domestic trials).

8 Ibid.

9 Prosecutor v. Omar Hassan Ahmed Al Bashir, Decision Pursuant to the Article 87 (7) on the Failure of the Republic of Malawi to Comply with the Cooperation Requests Issued by the Court With Respect to the Arrest and Surrender of Omar Hassan Ahmed Al Bashir, ICC-02/05-01/09-139, Pre-Trial Chamber I, 12 December 2011; see also Décision rendue en application de l'article 87-7 de le Statut de Rome concernant le refus de la Republique du Tchad d'accèder aux demandes de coopération délivrées par la Cour concernant l'arrestation et la remise d'Omar Hassan Ahmad Al Bashir, ICC-02/05-01/09-140, Pre-Trial Chamber I, 13 December 2011.

10 Kress, C., ‘The International Criminal Court and Immunities under International Law for States Not Party to the Court's Statute’, in Bergsmo, M. and Yan, L. (eds.), State Sovereignty and International Criminal Law (2012), 223, at 234, 250–4.

11 Tadić Interlocutory Appeal on Jurisdiction, supra note 3, para. 99

12 See for instance, S.S. Lotus (France. v. Turkey), PCIJ Rep. Series A No. 10, Judgment, 7 September 1927, at 21; North Sea Continental Shelf (Federal Republic of Germany v. Denmark; Federal Republic of Germany v. Netherlands), Judgment of 20 February 1969, [1969] ICJ Rep. 3, at 44.

13 Kupreškić Trial Judgement, supra note 3, para. 527.

14 Second Report, supra note 1, para. 28.

15 Ibid.

16 Reservations to the UN Convention on the Prevention and Punishment of the Crime of Genocide, Advisory Opinion of 28 May 1951, [1951] ICJ Rep. 16; mostly appraised for the deductive method of identifying customs, see B. Schlütter, Developments in Customary International Law: Theory and the Practice of the International Court of Justice and the International Ad hoc Criminal Tribunals for Rwanda and Yugoslavia (2010), at 142–7; the Anglo-Norwegian Fisheries Jurisdiction case is also to be noted for not requiring complete uniformity, Fisheries case (United Kingdom v. Norway), Judgment of 18 December 1951, [1951] ICJ Rep. 138. Anthea Roberts also refers to Western Sahara, Advisory Opinion of 16 October 1975, [1975] ICJ Rep. 12, at 30–57 and Legal Consequences for States of the Continued Presence of South African in Namibia (South West Africa) Notwithstanding Security Council Resolution 276 (1970), Advisory Opinion of 21 June 1971, [1971] ICJ Rep. 16, at 31–2. Roberts, A., ‘Traditional and Modern Approaches to Customary International Law: A Reconciliation’, (2001) 95 AJIL 758, fn 22.

17 Military and Paramilitary Activities in and Against Nicaragua (Nicaragua v. United States), Judgment of 27 June 1986, [1986] ICJ Rep. 14.

18 Meron, T., The Humanization of International Law (2006), 367. Meron also refers to the Military Tribunal in the Trial of Major War Criminals and the judgement in The High Command Case as initiating this trend.

19 Kirgis, F., ‘Custom on a Sliding Scale’, (1987) 81 AJIL 146.

20 Ibid., at 148.

21 Tasioulas, J., ‘In Defence of Relative Normativity: Communitarian Values and the Nicaragua Case’, (1996) 16 Oxford Journal of Legal Studies 85. For a critique see Beckett, J., ‘Behind Relative Normativity: Rules and Process as Prerequisites of Law’, (2001) 12 EJIL 627; on Dworkin interpretivism in public international law see Cali, B., ‘On Interpretivism and International Law’, (2009) 20 EJIL 805.

22 Tasioulas, supra note 21, at 113.

23 Ibid.

24 Ibid.; see also Roberts, supra note 16, at 757.

25 Milanović, M., ‘Norm Conflicts, International Humanitarian Law, and Human Rights Law’, in Ben-Naftali, O., International Humanitarian Law and International Human Rights Law (2011), 102.

26 ICJ Statute, Art. 38 (1)(b) ‘international custom, as evidence of a general practice accepted as law’.

27 Arajärvi, N., The Changing Nature of Customary International Law: Methods of Interpreting the Concept of Custom in International Criminal Tribunals (2014), 14; Roberts, supra note 16, at 758.

28 Though labeled ‘modern custom’, this method is not new to international law, see Posner, E. and Goldsmith, J., ‘A Theory of Customary International Law’, (1998) 40 VJIL 639, at 672; d'Aspremont, J., ‘The Decay of Modern Customary International Law in Spite of Scholarly Heroism’, (2015) Global Community Yearbook of International Law and Jurisprudence 672.

29 G. Abi-Saab, ‘Cours général de droit international public’, (1987) 207 RdC 177; Falkowska, M., ‘La coutume dans les statuts et la jurisprudence des juridictions pénales internationales: vers l’émergence d'une nouvelle définition de la coutume internationale?’, in Arcari, M. and Balmond, L., Diversification Des Acteurs et Dynamique Normative En Droit International: Diversity of Actors and Dynamics of International Law-Making (2013). We have seen in the introduction that it has a pedigree in the ICJ jurisprudence dating back to the 1950s, see fn 16–17. Other scholars named what I refer to as ‘modern custom’ – which I use for the sake of constancy, as Special Rapporteur Wood also adopts this terminology – as ‘instant customs’: Cheng, B., Studies in International Space Law (1997), 125 et seq; R. Bernhardt, ‘Principles and Characteristics of Customary International Law’, (1987) 205 RdC 247, at 266; or even Grotian Moments, Scharf, M. P., ‘Seizing the “Grotian Moment”: Accelerated Formation of Customary International Law in times of Fundamental Change’, (2010) 43 CILJ 439; and has been characterized by René-Jean Dupuy as early as 1974 as ‘coutume sauvage’ (wild custom) in constrast to ‘coutume sage’ (wise custom), R.-J. Dupuy, ‘Coutume sage et coutume sauvage’, in Rousseau, C., La Communauté internationale: Mélanges offerts à Charles Rousseau (1974), 7587.

30 Danner, A., ‘When Courts Make Law: How the International Criminal Tribunals Recast the Laws of War’, (2006) 59 Vanderbilt Law Review 27–9; Schabas, W., An introduction to the International Criminal Court (2011), 124; Darcy, S., ‘The Reinvention of War Crimes by the International Criminal Tribunals’. in Darcy, S. and Powderly, J. (eds.), Judicial Creativity at the International Criminal Tribunals (2010), 118.

31 The Appeals Chamber held that Art. 3 ICTY Statute functioned as a ‘residual clause’. This ‘residual clause’, according to the Appeals Chamber, endowed the ICTY to prosecute all serious violations of IHL, including violations committed in internal armed conflicts, Tadić Interlocutory Appeal on Jurisdiction, supra note 3, paras. 89–93.

32 Tadić Interlocutory Appeal on Jurisdiction, supra note 3, para. 94: These four requirements are: (i) a rule of IHL must have been violated; (ii) the rule must have been of a customary character, or if it was a conventional rule it must have been binding on the parties to the conflict; (iii) the violation must have been serious; and (iv) the violation must entail individual criminal responsibility.

33 See Section 6.3 infra.

34 Arajärvi, N., ‘Role of the International Criminal Judge in the Formation of Customary International Law’, (2007) 1 EJLS 90, at 100. This refers to ‘the willingness of the court to find customary rules aiming for a broader protection of human beings’.

35 Tadić Interlocutory Appeal on Jurisdiction, supra note 3, para. 99.

36 Arajärvi, supra note 27, at 91: Nonetheless, some have also accepted that these elements may count as state practice, e.g., Crawford, J., Brownlie's Principles of Public International Law (2012), at 24.

37 Tadić Interlocutory Appeal on Jurisdiction, supra note 3, para. 99; this may be read in line with the Nicaragua case, supra note 17, para. 186.

38 Murphy, S., Principles of International Law (2012), 92101.

39 Meron, T., ‘Is International Law Moving Towards Criminalization?’, (1998) 9 EJIL 18, at 28.

40 Wouters and Ryngaert, supra note 2, at 116, according to them ‘The Tadić method takes the Nicaragua method one step further: not only does it play down inconsistent state practice, notably on the battlefield, in the face of more humane verbal state practice and opinio juris; it even seems to deem battlefield practice methodologically irrelevant because of its untrustworthiness.’

41 Greenwood, C., ‘International Humanitarian Law and the Tadic Case’, (1996) 7 EJIL 265, at 277–9: See also Zahar, A. and Sluiter, G., International Criminal Law: A Critical Introduction (2008), 93. This discusses how the ICTY came to find that many of the provisions of AP II were rules established under CIL.

42 See Nicaragua case, supra note 17, para. 218.

43 Tadić Interlocutory Appeal on Jurisdiction, supra note 3, para. 119.

44 Schlütter, supra note 16, at 225; Sivakumaran, S., ‘Re-envisaging the International Law of Internal Armed Conflict’, (2011) 22 EJIL 219, at 230.

45 References were also made to the 1967 conflict in Yemen, the statement of the Prime Minister of Congo in 1964 during the civil war in Congo, the 1967 Operational Code of Conduct for Nigerian Armed Forces, and the 1988 statement by the rebels in El Salvador, Tadić Interlocutory Appeal on Jurisdiction, supra note 3 paras. 105–7.

46 Schlütter, supra note 16, at 225.

47 See Jurisdictional Immunities of the State (Germany v. Italy: Greece Intervening), Judgment of 3 February 2012, [2012] ICJ Rep. 99, para. 73: ‘For the purposes of the present case the most pertinent State practice is to be found in . . . national judicial decisions . . .’.

48 See Nicaragua case, supra note 17, paras. 184–8.

49 ICRC, Preliminary Remarks on the Setting-up of an International Tribunal for the Prosecution of Persons Responsible for Serious Violations of International Humanitarian Law Committed in the Territory of the Former Yugoslavia, DDM/JUR/442 b, 25 March 1993, para. 4; Final Report of 27 May 1994 of the Commission of Experts established pursuant to Security Council resolution 780 (1992), para. 54. See also Report of the Secretary-General Pursuant to Paragraph 5 of Security Council Resolution 955 (1994), UN Doc. S/1995/134 (1995), para. 12.

50 Tadić Interlocutory Appeal on Jurisdiction, supra note 3, para. 129.

51 Ibid, para. 128; see also Darcy, S., Judges, Law and War: The Judicial Development of International Humanitarian Law (2014), 77.

52 Meron, supra note 18, at 367.

53 Schlütter, supra note 16, at 225. This is scrutinized in detail in Section 6.3 infra.

54 Ronen, Y., ‘International Humanitarian Law’, in Kammerhofer, J. and D'Aspremont, J. (eds.), International Legal Positivism in a Post-Modern World (2014), 489.

55 Kupreškić Trial Judgement, supra note 3, para. 511.

56 The tu quoque argument posits that breaches of IHL being committed by the enemy justify similar breaches by the other party to the conflict: Ibid., para. 515

57 Darcy, supra note 30, at 258: ‘Customary international law, however, has long established a number of requirements governing the use of reprisals, such as proportionality and last resort’. See also Hampson, F., ‘Belligerent Reprisals and the 1977 Protocols to the Geneva Conventions of 1949’, (1988) 37 ICLQ 818, at 821–4; Bílková, V., ‘Belligerent Reprisals in Non-International Armed Conflicts’, (2014) 63 ICLQ 31, at 34–5.

58 Kupreškić Trial Judgement, supra note 3, paras. 515–20.

59 Ibid., para. 515.

60 Also, the Geneva Conventions I-IV prohibit reprisals against protected persons under the Conventions: Arts. 46, 47, 13, and 33, respectively.

61 Kupreškić Trial Judgement, supra note 3, para. 527.

62 ICRC Rule 148 apparently conceives this silence as implying the non-applicability of reprisals in internal armed conflicts; C.f. see Kalshoven, F, Reflections on the Law of War: Collected Essays (2007), 826, 828, who argues that AP II is purposely silent on reprisals.

63 Bílková, supra note 57.

64 Kupreškić Trial Judgement, supra note 3, para. 527.

65 Ibid., para. 527.

66 Ibid., para. 527.

67 Ibid., para. 531, we could even argue that these are not examples of state practice but of opinio juris, see Murphy, supra note 38, at 92–101.

68 Ibid., paras. 532–4.

69 Schlütter, supra note 16, at 235. The Chamber, however, held that ‘elements of a widespread opinio juris sive necessitates are discernible in international dealings’. Ibid., para. 532.

70 Da Corrias, L. and Gordon, G., ‘Judging in the Name of Humanity International Criminal Tribunals and the Representation of a Global Public’, (2015) 13 JICJ 97, at 101.

71 AP I, Art. 1(2). The Martens Clause was first adopted in the Preamble of Hague Convention II (1899) and reaffirmed in subsequent treaties. See, for example, Convention Respecting the Laws and Customs of War on Land (1907), Preambles H IV R; Additional Protocol II (1977); United Nations Convention on Certain Conventional Weapons (1980); Geneva Convention I (1949), Art. 63, Geneva Convention II (1949), Art. 62; Geneva Convention III, Art. 142; Geneva Convention IV (1949), Art. 158; Legality of the Threat or Use of Nuclear Weapons, Advisory Opinion of 8 July 1996, [1996] ICJ Rep. 226, para. 78.

72 See Prosecutor v. Hadzihasanovic, Alagic and Kubura, Interlocutory Appeal on Decision on Joint Challenge to Jurisdiction, Case No. IT-01-47-PT, A. Ch., 27 November 2002, para. 82.

73 Meron, T., ‘The Martens Clause, Principles of Humanity, and Dictates of Public Conscience’, (2000) 94 AJIL 78, at 87–8; Cassese, A., ‘The Martens Clause: Half a Loaf or Simply Pie in the Sky’, (2000) 11 EJIL 187, at 212.

74 Cassese, supra note 73, at 214; Meron, supra note 73, at 88.

75 Salter, M., ‘Reinterpreting Competing Interpretations of the Scope and Potential of the Martens Clause’, (2012) 17 JCLS 403.

76 Tomuschat, C., General Course on Public International Law, (1999) 281 RdC 356.

77 Tadić Interlocutory Appeal on Jurisdiction, supra note 3, para. 119.

78 Schlütter, supra note 16, at 225.

80 Ibid., para. 531; see also Salter, supra note 75, at 418.

81 Kupreškić Trial Judgement, supra note 3, para. 527.

82 Ibid., para. 531: ‘Due to the pressure exerted by the requirements of humanity and the dictates of public conscience, a customary rule of international law has emerged on the matter under discussion.’

83 See Nuclear Weapons case, supra note 71, Dissenting Opinion of Judge Shahabuddeen, at 408.

84 Kupreškić Trial Judgement, supra note 3, para. 525.

85 Darcy, S., Collective Responsibility and Accountability under International Law (2007), 163–4.

86 See Cassese, supra note 73, at 207, 212; Meron, supra note 73, at 84; Odello, M., ‘Fundamental Standards of Humanity: A Common Language of IHL and HRL’, in Arnold, R. and Quénivet, N. (eds.), International Humanitarian Law and Human Rights Law: Towards a New Merger in International Law (2008), 50–1. For instance, in its oral submissions in the Nuclear Weapons case, Australia emphasized the role of human rights in shaping the ‘dictates of public conscience’, arguing that ‘international standards of human rights must shape conceptions of humanity and have an impact on the dictates of public conscience’: International Court of Justice, Request for Advisory Opinions on the Legality of Nuclear Weapons- Australian Statement, [1996] AYBIL 685, at 699. Judge Weeramantry, dissenting in the same case, recognized the role of the human rights movement in shaping ‘the dictates of public conscience’. See Nuclear Weapons case, supra note 71, Dissenting Opinion of Judge Weeramantry, at 490. In the same vein the Conseil de Guerre de Bruxelles, in the K.W. case, noted that ‘in its search for the principles of international law resulting from the principles of humanity and dictates of public conscience, it was to be guided by the Universal Declaration of Human Rights’.

87 Skordas, A., ‘Hegemonic Custom?’, in Byers, M. and Nolte, G. (eds.), United States Hegemony and the Foundation of International Law (2003), 325–30.

88 For a comprehensive overview of the different definitions in doctrinal writings see Pauwelyn, J., Conflict of Norms in Public International Law: How WTO Law Relates to Other Rules of International Law (2003) 166–74.

89 Prost, M., The Concept of Unity in Public International Law (2012), 61–6.

90 De Wet, E. and Vidmar, J., Hierarchy in International Law: The Place of Human Rights (2012), 2.

91 Kelsen, H., ‘Derogation’, in Klecatsky, H., Marcic, R. and Schambeck, H. (eds.), Die Wiener Rechtstheoretische Schule (1968), 1438. Pauwelin also defines conflict of norms ‘as a situation where one norm breaches, has led to or may lead to breach of, another norm’ in Pauwelyn, supra note 88, at 199; even Wilfrid Jenks considered that we might have a conflict in situations where norms ‘have repercussions upon one another in any other way’, Jenks, Wilfred, ‘The Conflict of Law-Making Treaties’, (1953) 30 BYIL 404, however, note that Jenks adopts a stricter definition requiring the impossibility of joint compliance.

92 M. Koskenniemi, Fragmentation of International Law: Difficulties Arising from the Diversification and Expansion of International Law, UN Doc. A/CN.4/L.682, 13 April 2006, para. 25.

93 Prost, supra note 89, at 61–6.

94 We may also situate this wide definition within the literature on ‘regime interaction’ and ‘norm collision’; see also Blome, K. et al., Contested Regime Collisions: Norm Fragmentation in World Society (2016), 34–5.

95 Wolfrum, R. and Matz, N., Conflicts in International Environmental Law (2003), 7.

96 Report on Fragmentation, supra note 92, para. 23

97 Ibid., paras. 24–5; see also Dworkin's interpretative theory of adjudication, Dworkin, R., Law's Empire (1986).

98 Report on Fragmentation, supra note 92, para. 25: ‘Focusing on a mere logical incompatibility mischaracterizes legal reasoning as logical subsumption’. The definition of conflict adopted in this article is not in order to make legal practice more responsive to conflicting goals underlying hard ICL cases but to decipher why the Tadić and Kupreškić Chambers decided to formulate alternative rules on custom identification.

99 Pauwelyn, supra note 88, at 170–1.

100 When the conflict cannot be avoided, priority rule will be available to the adjudicator, such as lex specialis, lex posterior, lex prior, and lex superior. For further explanation of these concepts see Report on Fragmentation, supra note 92.

101 Case concerning the Right of Passage over Indian Territory (Portugal v. India), Preliminary Objections, Judgment of 26 November 1957, [1957] ICJ Rep. 125, at 142.

102 Krieger, H., ‘A Conflict of Norms: The Relationship Between Humanitarian Law and Human Rights Law in the ICRC Customary Law Study’, (2006) 11 JCLS 265, at 266; L. Hill-Cawthorne, ‘Just Another Case of Treaty Interpretation? Reconciling Humanitarian Law and Human Rights Law in the ICJ’, in Andenas, M. and Bjorge, E., A Farewell to Fragmentation: Reassertion and Convergence in International Law (2015), 274–6; Nuclear Weapons case, supra note 71, para. 25.

103 Nuclear Weapons case, supra note 71, at para 25.

104 Hill-Cawthorne, supra note 102, at 279.

105 Legal Consequences of the Construction of a Wall in the Occupied Palestinian Territory, Advisory Opinion of 9 July 2014, [2004] ICJ Rep. 136, para. 106.

106 Case Concerning Armed Activities on the Territory of the Congo (Democratic Republic of the Congo v. Uganda), Judgment of 19 December 2005, [2005] ICJ Rep. 168, para. 216. See Hill-Cawthorne, supra note 102, at 281–2, the author notes that in this case the lex specialis rule was not quoted.

107 Human Rights Committee, General Comment No. 31, CCPR/C/21/Rev.1/Add.13 (26 May 2004), para. 11.

108 Mapiripan Massacre v. Colombia, Judgment, Inter-American Court of Human Rights, 15 September 2005, para. 115.

109 Hassan v. the United Kingdom, Judgment, European Court of Human Rights, Application No. 29750/09, 16 September 2014, para. 106. In this case it was held that internee review in IACs could be conducted by a ‘competent body’, as provided by Geneva Conventions III and IV, instead of a ‘court’ as required by Art. 5 of the European Convention on Human Rights. The Court nonetheless considered that the competent body ‘should provide sufficient guarantees of impartiality and fair procedure to protect against arbitrariness’, as is generally provided under IHRL. The Court, in addition to Art. 31(3)(c), referred to subsequent practice (Art. 31(3)(b) Vienna Convention on the Law of Treaties).

110 Prost, supra note 89, at 64, referring to Krieger, supra note 102, at 266–8; Cassimatis, A., ‘International Humanitarian Law, International Human Rights Law, and Fragmentation of International Law’, (2007) 56 ICLQ 623.

111 Cassese, A. et al., Cassese's International Criminal Law (2013), 2.

112 Ambos, K., Treatise on International Criminal Law: Volume I (2013), 55.

113 Ibid.

114 Tadić, para. 72, referring to the Preamble of Resolution 827.

115 Schabas, W., The UN International Criminal Tribunals: The Former Yugoslavia, Rwanda, and Sierra Leone (2006), 157.

116 Cryer, R., ‘The Interplay of Human Rights and Humanitarian Law: The Approach of the ICTY’, (2009) 14 JCLS 513–14.

117 Bassiouni, M.C., ‘The Proscribing Function of International Criminal Law in the Processes of International Protection of Human Rights’, (1982) 9 YJWPO 193. He also includes other transnational crimes in his list.

118 Robinson, D., ‘The Identity Crises of International Criminal Law’, (2008) 21 LJIL 925–63.

119 Ambos, supra note 112, at 55.

120 Rome Statute, Art. 21.

121 Sliedregt, E., Individual Criminal Responsibility in International Law (2012), 8.

122 Schabas, W., The UN International Criminal Tribunals: The Former Yugoslavia, Rwanda, and Sierra Leone (2006), 75.

123 Ambos, supra note 112, at 55.

124 Bassiouni, M.C., ‘The Normative Framework of International Humanitarian Law: Overlaps, Gaps and Ambiguities’, (1998) 8 TCLB 199; Cryer, R. et al., An Introduction to International Criminal Law and Procedure (2010), 7.

125 Bassiouni, supra note 124.

126 For other reasons concerning the nature of state practice see d'Aspremont, J., ‘Théorie des sources’, in van Steenberghe, R. (ed.), Droit international humanitaire: un régime spécial de droit international? (2013), 73.

127 Robinson, supra note 118.

128 See Section 2.

129 Tadić Interlocutory Appeal on Jurisdiction, supra note 3, paras. 96–9.

130 Hill-Cawthorne, L., ‘Humanitarian Law, Human Rights Law and the Bifurcation of Armed Conflict’, (2015) 64 ICLQ 293, 294–307. This significant change also developed with AP I and AP II.

131 Hellestveit, C., ‘The Geneva Conventions and the Dichotomy between International and Non-International Armed Conflict: Curse or Blessing for the “Principle of Humanity”?', in Larsen, K., Cooper, C. and Nystuen, G. (eds.), Searching for a ‘Principle of Humanity’ in International Humanitarian Law (2013), 88–9.

132 Tadić Interlocutory Appeal on Jurisdiction, supra note 3, para. 96.

133 Ibid., para. 97.

134 Ibid.

135 Ibid.

136 It is Theodor Meron that came first with the concept of ‘humanization’ of IHL in 2000, observing the developments at the ad hoc tribunals. See Meron, T., ‘The Humanization of Humanitarian Law’, (2000) 94 AJIL 239.

137 Hakimi argues that, primarily, it was the human rights challenge posed to the Appeals Chamber that made it assert that IHL appeared to be ‘outmoded’, see Hakimi, M., ‘Custom's Method and Process: Lessons from Humanitarian Law’, in Bradley, C. (ed.), Custom's Future: International Law in a Changing World (2016), 148.

138 Hill-Cawthorne, supra note 130, at 294–307.

139 Seibert-Fohr, A., Prosecuting Serious Human Rights Violations (2009), 199; the duty to prosecute, however, may not be well settled.

140 See Meron, T., ‘International Criminalization of Internal Atrocities’, (1995) 89 AJIL 554, at 559–60; detailing the view of the ICRC and the ILC on this issue.

141 See Mégret, F., ‘The Anxieties of International Criminal Justice’, (2016) 29 LJIL 197, at 204–5.

142 Kupreškić Trial Judgement, supra note 3, para. 513.

143 Ibid., para. 528.

144 Ibid., para. 528.

145 Ibid., para. 529.

146 Ibid.

147 Darcy, supra note 85, at 179–80.

148 Ibid., at 181–2.

149 Ibid., at 180–1.

150 S. Sivakumaran, ‘How to Improve upon the Faulty Legal Regime of Internal Armed Conflicts’, in A. Cassese (ed.), Realizing Utopia (2012), 525.

151 Ibid., at 533; Clapham, A., Human Rights Obligations of Non-State Actors (2006); Henckaerts, J-M. and Wiesener, C., ‘Human rights obligations of non-state armed groups: a possible contribution from customary international law?’, in Kolb, R. and Gaggioli, G. (eds.), Research Handbook on Human Rights and Humanitarian Law (2013), 146, at 157. However, see also Murray, D., Human rights obligations of non-state armed groups (2016).

152 See Nuclear Weapons case, supra note 71, para. 25.

153 Schmitt, M., Essays on Law and War at the Fault Lines (2011), 112.

154 Darcy, supra note 51, at 165.

155 Pauwelyn, supra note 88, at 245; I am not implying that interpretation in an (international) criminal trial is governed by the Vienna Convention. On this matter see Grover, L., Interpreting Crimes in the Rome Statute of the International Criminal Court (2014).

156 Secretary General Report, supra note 49, para. 34.

157 Wood, First Report, supra note 1, para. 18. However, he also wrote ‘as in any legal system, there must in public international law be rules for identifying the sources of law. These can be found for present purposes by examining in particular how states and courts set about the task of identifying the law’. Ibid, at 17.

158 Note that the ILC had a debate on whether it should issue a guideline on custom identification or a set of rules. See ILC, UN Doc. A/CN.4/SR.3225 (18 September 2014); and UN Doc. A/CN.4/SR.3226 (17 July 2014).

159 Milanović, M., ‘A Norm Conflict Perspective on the Relationship between International Humanitarian Law and Human Rights Law’, (2009) 14 JCSL 459, at 468–70.

160 See Nicaragua case, supra note 17.

161 Roberts, supra note 16.

162 Henckaerts, J.-M. and Doswald-Beck, L., Customary International Humanitarian Law, (2005), Volume I: Rules and Volume II: Practice.

163 See, e.g., Bellinger, J.B. and Haynes, W.J., ‘A US Government Response to the International Committee of the Red Cross's Customary International Humanitarian Law Study’, (2007) 89 IRRC 443.

164 See, e.g., United States – Standards for Reformulated and Conventional Gasoline case, WT/DS2/AB/R, 20 May 1996.

165 See Cassese et al., supra note 111, at 2.

166 It is, indeed, with these concerns in mind that Pauwelyn wrote his seminal book Conflicts of Norms in Public International Law, see Pauwelyn, supra note 88.

167 Canada – Administration of the Foreign Investment Review Act, 7 February 1984, L/5504, BISD 30S/140; United States – Imports of Sugar from Nicaragua, 13 March 1984, BISD 31S/67, L/5607; Canada – Measures Affecting Exports of Unprocessed Herring and Salmon, 22 March 1988, BISD 35S/98, L/6268, para. 5.3. See also the panel report on United States – Taxes on Petroleum and Certain Imported Substances, 17 June 1987, BISD 34S/136, para. 5.2.6.

168 See Report on Fragmentation, supra note 92, paras. 422–3.

169 Ibid., para. 479.

170 See generally Marschik, A., ‘Too Much Order? The Impact of Special Secondary Norms on the Unity and Efficacy of the International Legal System’, (1998) 9 EJIL 212.

171 It may be argued that the ICTY operated in a sub-system of international law, as it was governed by its Statute and rules. However, the fact that a crime was listed in the Statute was not sufficient; for a crime to fall within its jurisdiction it had also to be established under customary international law. Thus, there was a fall back to general international law. E.g., see Cassese commenting on Art. 8 Rome Statute: ‘the Statute would not provide a self-contained legal regime, but would rather presuppose a mandatory examination by the Court, on a case by case basis, of the current status of general international law’. This reasoning would apply to the ICTY Tribunals Statute as well.

172 Venzke, I., How Interpretation Makes International Law: On Semantic Change and Normative Twists (2012), 198205.

173 Ibid.

174 Wagner, N., ‘The Development of the Grave Breaches Regime and of Individual Criminal Responsibility by the International Criminal Tribunal for the former Yugoslavia’, (2003) 85 IRRC 351, at 352; Swart, M., ‘Is There a Text in This Court? The Purposive Method of Interpretation and the Ad Hoc Tribunals’, (2010) 70 Zeitschrift für ausländisches öffentliches Recht und Völkerrecht 767, at 781–2; Tadić Interlocutory Appeal on Jurisdiction, supra note 3, paras. 71–78 and para. 72, in particular referring to the Preamble of Resolution 827.

175 Ibid., para. 91.

176 Kupreškić Trial Judgement, supra note 3, para. 530.

177 Bassiouni, for instance, argues that the specific crimes, except for persecution per se, provided in the London Charter's definition of crimes against humanity existed before 1945 in the world's major legal systems and were thus prohibited as general principles of law. See Bassiouni, C., Crimes Against Humanity (2011), 469–70.

178 E.g., ICCPR, Art. 15(2) and ECHR, Art. 7(2), which provide that any act or omission can be punished if it was ‘criminal according to the general principles of law recognised by civilised nations’. Though it may be claimed that they would not meet the principle of specificity, which arguably is also a defect of CIL. See also Simma, B. and Alston, P., ‘The Sources of Human Rights Law: Custom, Jus Cogens and General Principles’, (1992) AYIL 8.

179 Cryer, R., ‘Of Custom, Treaties, Scholars and the Gavel: The Influence of the International Criminal Tribunals on the ICRC Customary Law Study’, (2006) 11 JCSL 239, at 254–5; Greenwood, supra note 41, at 278.

180 Tadić was not accused of having used prohibited weapons in an NIAC, nor of an attack directed at the civilian population or cultural heritage. Furthermore, the Appeals Chamber could have simply found, as the Trial Chamber did, that the norms existed in CIL without proposing a new method to identify CIL. Finally, it would have been easier to classify the conflict as an IAC. Kupreskić did not formally raise the defence of reprisals which would have allowed the Chamber to decided that reprisals are prohibited because they were not foreseen in the AP I, see Section 1.2.

181 Lauterpacht, H, The Development of International Law by the International Court (1958).

182 See Berman, F., ‘The International Court of Justice as an ‘Agent’ of Legal Development?’, in Tams, C. and Sloan, J., The Development of International Law by the International Court of Justice (2013), at 721.

183 Barcelona Traction, Light and Power Company, Limited, Judgment of 5 February 1970, [1970] ICJ Rep. 3, at 64, para. 2, Judge Fitzmaurice.

184 Cassese, A., ‘The International Court of Justice: Is it High Time to Restyle the Respected Old Lady’, in Cassese, A., Realizing Utopia: The Future of International Law (2012), 239, 240.

185 Ronen, supra note 54, at 497: The strict regulation of the applicable law before the ICC are examples of where thwarting CIL theories lead.

186 Ronen, supra note 54, at 480.

187 Knottnerus, A.S., ‘The AU, the ICC, and the Prosecution of African Presidents’, in Clarke, K. et al., Africa and the ICC (2016), 152–84; Tladi, D., ‘The ICC Decisions on Chad and Malawi: On Cooperation, Immunities, and Article 98’, (2013) 11 JICJ 199221.

188 Scobbie, I., ‘The Approach to Customary International Law in the Study’, in Whimshurst, E. and Breau, S., Perspectives on the ICRC Study on Customary International Law (2007), 29.

189 Kalshoven, F., ‘Reprisals and the Protection of Civilians: Two Recent Decisions of the Yugoslavia Tribunal’, in Vohrah, L.C. et al., Man's Inhumanity to Man: Essays in Honour of Antonio Cassese (2003), 504.

190 United Kingdom Ministry of Defence, The Manual of Law of Armed Conflict (2004), 421, fn 62. For a summary of criticisms see Cryer, supra note 170, at 256.

191 Henckaerts and Doswald-Beck, supra note 162, at 523.

192 However, see Ronen, supra note 54, at 497. It is set out here that states have not objected because it is concerned only with criminal responsibility (possibility of low level perpetrators). With regard to state responsibility the situation might change.

193 Rome Statute, Art. 8(c)(e).

194 See Rome Statute, Arts. 21, 22; nonetheless customary international law may be applied inter alia when there is a lacuna in the Statute. It may also be of relevance for defences or mens rea questions and has been explored within the framework of customary criminal procedural law, see Van den Herik, supra note 7, at 230.

195 Pellet, A., ‘Applicable Law’, in Cassese, A. et al., The Rome Statute of the International Criminal Court: A Commentary (2002), 1077

196 Ibid.

* Newton Post Doctoral Researcher (Center for Global Public Law, Koç University); Ph. D. (EUI) []. This article was written as part of a project on the effects of international human rights law on other branches of international law, co-ordinated by Professor Başak Çalı and Professor Lorna McGregor, and supported by a British Academy Advanced Newton Fellowship Grant. The author would like to thanks Başak Çalı and Valentina Azarova for their comments on earlier drafts.

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