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Judicial Practice, Customary International Criminal Law and Nullum Crimen Sine Lege pp 87–116Cite as

The Formation of Customary International Law and Its Methodological Challenges

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Abstract

Developing a methodological underpinning for customary international law has been a constant challenge. Indeed, customary law has become an inspiring source for innovative legal theories. Lacking a coherent theoretical basis, critics have suggested that customary international law has an “identity crisis”, or is “situated in a theoretical minefield.” Indeed, there appears to be no common consensus, on either a catalogue of specific rules that have attained the status of customary law, or on the doctrinal and methodological approaches required to establish and determine customary international law as a source of law. When discussing customary international law as a source of international law three levels have to be distinguished: (1) the nature of customary international law, (2) the constitutive elements necessary for the formation of customary international law and (3) the evidence for the identification of customary international law.

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Notes

  1. 1.

    B. Simma and P. Alston, The Sources of Human Rights Law: Custom, Jus Cogens, and General Principles, 12 Australian Yearbook of International Law (1988–1989), 88. See also J. P. Kelly, The Twilight of Customary International Law, 40 Vanderbilt Journal of International Law (2000), 450: “[T]here is neither a common understanding of how customary international legal norms are formed, nor agreement on the content of those norms.”

  2. 2.

    M. Koskenniemi, The Pull of the Mainstream, 88 Michigan Law Review (1990), 1947.

  3. 3.

    See also L. Blutman, Conceptual Confusion and Methodological Deficiencies: Some Ways that Theories on Customary International Law Fail, 25 European Journal of International Law (2014), 536.

  4. 4.

    C. L. Lim, O. Elias, Withdrawing From Custom and the Paradox of Consensualism in International Law, 21 Duke Journal of Comparative & International law (2010), 143ff. See also C. L. Lim, O. Elias, The Paradox of Consensualism in International Law (1998).

  5. 5.

    B. D. Lepard, Customary International Law – A New Theory with Practical Applications (2010), 8: “A customary international norm arises when states generally believe that it is desirable now or in the near future to have an authoritative legal principle or rule prescribing, permitting, or prohibiting certain conduct. This belief constitutes opinio juris, and it is sufficient to create a customary law norm.”

  6. 6.

    A. Orakhelashvili, Natural Law and Customary Law, 68 Zeitschrift für ausländisches, öffentliches Recht und Völkerrecht (2008), 71.

  7. 7.

    See H. G. Cohen, Finding International Law: Rethinking the Doctrine of Sources, 93 Iowa Law Review (2007), 79ff with further references.

  8. 8.

    In the wording of the Mexico-United States General Claims Commission, North American Dredging Company of Texas (USA) v. Mexico, March 1926, 4 Reports of International Arbitral Awards, 29f: “The law of nature may have been helpful, some three centuries ago, to build up a new law of nations, and the conception of inalienable rights of men and nations may have exercised a salutary influence, some one hundred and fifty years ago, on the development of modern democracy on both sides of the ocean; but they have failed as durable foundation of either municipal or international law and cannot be used in the present day as substitutes for positive municipal law, on the one hand, and for positive international law, as recognized by nations and governments through their acts and statements, on the other hand.”

  9. 9.

    J. Austin, The Province of Jurisprudence Determined (1832), 130. Compare also S. Hall, The Persistent Spectre: Natural Law, International Order and the Limits of Legal Positivism, 12 European Journal of International Law (2001), 279.

  10. 10.

    ICJ Statute, Art. 38.

  11. 11.

    The publications on theories to custom or respectively on its elements are extensive, compare for example: A. T. Guzman, Saving Customary International Law, 27 Michigan Journal of International Law (2005), 115ff; J. Kammerhofer, Uncertainty in the Formal Sources of International Law: Customary International Law and Some of its Problems, 15 European Journal of International Law (2004), 523ff; R. Kolb, Selected Problems in the Theory of Customary International Law, 50 Netherlands International Law Review (2003), 119ff; J. L. Slama, Opinio Juris in Customary International Law, 15 Oklahoma City University Law Review (1990), 603ff; F. L. Kirgis, Custom on a Sliding Scale, 81 American Journal of International Law (1987), 146ff; R. M. Walden, The Subjective Element in the Formation of Customary International Law, 12 Israel Law Review (1977), 344ff; A. Verdross, Entstehungsweisen und Geltungsgrund des universellen völkerrechtlichen Gewohnheitsrechts, 29 Zeitschrift für ausländisches öffentliches Recht und Völkerrecht (1969), 635ff.

  12. 12.

    ICJ, North Sea Continental Shelf Cases, para 77. This quotation refers to the current state of customary international law, while historically customary international law has been seen as “a manifestation of pre-existing law.” For further discussion see J. Kammerhofer, Uncertainty in International Law – A Kelsenian Perspective (2011), 78 (footnote 92), with further references.

  13. 13.

    ICJ, Continental Shelf Case (Libya v. Malta), Judgment, 3 June 1985, ICJ Reports (1985), para 27; Right of Passage over Indian Territory (Portugal v. India), Judgment, 12 April 1960, ICJ Reports (1960), 40; Asylum Case (Colombia v. Peru), Judgment, 20 November 1950, ICJ Reports (1959), 276.

  14. 14.

    For a discussion on the meaning of the term ‘source’ see G. J. H. van Hoof, Rethinking the Sources of International Law (1983), 57.

  15. 15.

    L. Kirchmair, Die Theorie des Rechtserzeugerkreises – Eine rechtstheoretische Untersuchung des Verhältnisses von Völkerrecht zu Staatsrecht am Beispiel der österreichischen Rechtsordnung (2013), 186, referring to G. G. Fitzmaurice, Some Problems Regarding the Formal Sources of International Law, in Koskenniemi (ed.), Sources of International Law (2000), 77 and M. H. Mendelson, The Subjective Element in Customary International Law, 66 British Yearbook of International Law (1995), 179 (footnote 9), who uses the metaphor that the court decides whether the fruit is ripe and does not describe its ripening process. Accepting Art. 38 ICJ Statute as the formal source of the sources of international law results, in the words of Escorihuela, in the “paradoxical exposition of the sources of law in a norm the very authority of which the sources are supposed to explain.” See A. L. Escorihuela, Alf Ross: Towards a Realist Critique and Reconstruction of International Law, 14 European Journal of International Law (2003), 730.

  16. 16.

    G. M. Danilenko; Law-Making in the International Community (1993), 33ff with further references.

  17. 17.

    de Aréchaga argues that Art. 38 ICJ Statute is “a rule about rules”, J. de Aréchaga, Custom and Treaties, in Change and Stability in International Law-Making (1988), Cassese, Weiler (eds.), 1; see also Kammerhofer (supra note 11), 541(footnote 93) with further references.

  18. 18.

    International Law Association, London Conference (2000), Committee on Formation of Customary (General) International Law, Final Report of the Committee, Statement of Principles Applicable to the Formation of General Customary International Law, Part I, Definitions, Use of terms (viii), 12. For a distinction of the formal and material sources of law, see Fitzmaurice (supra note 15), 57f, who describes material, historical and indirect sources of law as those that inspire and form “the content of the law”, and formal, legal, and direct sources of law as the method by which content “is clothed with legal validity and obligatory force”. The present thesis adheres to that distinction and thus uses a different conception of the terms “formal” and “material” sources than I. Brownlie, Principles of Public International Law (2008), 1, who understands material sources as providing “evidence of the existence of rules”. Kammerhofer (supra note 11), 541, describes a formal source of law as the source of both the validity and origin of norms. Van Hoof (supra note 14), 59, distinguishes between three layers, similar to those established above: the first being the basis of the binding force of international law, which he understands in the sense of Kelsen’s Grundnorm and which he claims to be that “State ought to behave as they have customarily behaved”; the second being the constitutive element of rules of international law, which he regards as being consent (76ff); and the third being “the relevant manifestations on the basis of which the presence or absence of the constitutive element” can be established.

  19. 19.

    That would correspond to layer (2) as identified above.

  20. 20.

    That would correspond to layer (3) as identified above.

  21. 21.

    van Hoof (supra note 14), 59–60.

  22. 22.

    Kammerhofer (supra note 11), 538ff.

  23. 23.

    van Hoof (supra note 14), 60, states that the constitutive elements come close to what Hart describes secondary rule of recognition; see also A. A. D’Amato, The Concept of Custom in International Law (1971), 41–44; R. M. Walden, Customary International Law: A Jurisprudential Analysis, 13 Israel Law Review (1978), 87.

  24. 24.

    H. L. A. Hart, The Concept of Law (1961), 92.

  25. 25.

    Hart (supra note 24), 209; see also H. Mosler, Völkerrecht als Rechtsordnung, 36 Zeitschrift für ausländische, öffentliches Recht und Völkerrecht (1976), 32: “So rudimentär die Prozeduren der Bildung und Änderung von Recht sein mögen, sie müssen notwendigerweise existieren. Fehlt eine Institution, die von den Rechtsgenossen zur Gestaltung der Rechtsordnung gebildet ist, wie es beispielsweise innerhalb des Staates in der Regel die Volksvertretung zu sein pflegt, so ist es die Gesamtheit der Mitglieder, welche die Regeln des Zusammenlebens aufstellt. Das ist die Grundregel, die aus der internationalen Gesellschaft eine Rechtsgesellschaft macht.”

  26. 26.

    D’Amato (supra note 23), 41–44; van Hoof (supra note 14), 53–55.

  27. 27.

    Kammerhofer (supra note 11), 539–40 referring inter alia to A. Verdross, Die Quellen des universellen Völkerrechts (1973), 20, H. Mosler, The International Society as a Legal Community (1980), 16, as well as to G. M. Danilenko, The Theory of International Customary Law, 31 German Yearbook of International Law (1988), 17: “[T]he recognition of custom by States as a source of international law […] is determined by objective extralegal factors inherent in the structure of the international community.” (emphasis added); on the other hand, Danilenko (supra note 16), 27, with reference to Fitzmaurice (supra note 15) argues that the “ultimate formal source of law” does not “derive from ordinary sources: these are rules of natural law”; Kammerhofer (supra note 12), 202, is particularly critical concerning the approach to explain a meta norm of customary law creation by a customary norm itself, referring to A. Ross, A Textbook of International Law (1947), 83: “[T]he doctrine of the sources can never in principle rest on precepts contained in one among the legal sources the existence of which the doctrine itself was meant to prove.” Often scholars have invoked such a secondary rule by inferred or general consensus from States, see M. Byers, Custom, Power and the Power of Rules (1999), 143ff with further references.

  28. 28.

    Hart (supra note 24), 107: “The assertion that it [the secondary rule] exists can only be an external statement of fact. […] Its existence is a matter of fact.” See also N. Petersen, Customary Law without Custom? Rules, Principles, and the Role of State Practice in International Norm Creation, 23 American University Law Review (2008), 300: “It [Hart’s secondary rule] has to be understood as empirical recognition rather than in the normative sense of a basic norm.” See also ILA (supra note 18), Introduction, para 6, arguing that “rules about the sources of international law, and specifically this source [customary international law], are to be found in the practice of States.” Critically Kammerhofer (supra note 11), 544, in relation to Hart’s concept: “We are, in effect, asked to study which rules are in fact seen as creating rules; aptly exemplified by the English constitution where, so Hart thinks, Parliament and the courts are simply recognized as law-givers which makes them the valid law-givers” (emphasis in original).

  29. 29.

    Kammerhofer (supra note 11), 544 referring to H. Kelsen, Allgemeine Theorie der Normen (1979), 206: “Nur eine Norm kann der Geltungsgrund einer anderen Norm sein.” Furthermore: “Die Grundnorm einer positiven Moral-oder Rechtsordnung ist … keine positive, sondern eine bloß gedachte, und das heißt eine fingierte Norm … Nur wenn sie vorausgesetzt wird … können diese Sinngehalte als verbindliche Moral- oder Rechtsnormen gedeutet werden.”

  30. 30.

    Kammerhofer (supra note 11), 545. Just as the Grundnorm in Kelsen’s theory is a legal fiction that lies above the entire legal system and cannot be verified or proven, so it seems the meta-norm of customary international law as a fiction must be logically assumed.

  31. 31.

    Indeed as van Hoof stipulates that the constitutive element of rules of international law is ‘consent’ then the two elements of customary international law necessarily are not constitutive elements but merely give evidence of that consent, whereby it cannot be excluded that ‘consent’ could not be established on basis of one evidentiary element alone; see van Hoof (supra note 14), 76.

  32. 32.

    B. Cheng, United Nations Resolutions on Outer Space: “Instant” International Customary Law, in Cheng (ed.), International law: Teaching and Practice (1982), 251: “[T]he role of usage in the establishment of rules of international customary law is purely evidentiary: it provides evidence on the one hand of the contents of the rule in question and on the other hand of the opinio juris of the States concerned. Not only is it unnecessary that the usage should be prolonged, but there need also be no usage at all in the sense of repeated practice, provided that the opinio juris of the States concerned can be clearly established. Consequently, international customary law has in reality only one constitutive element, the opinio juris.”

  33. 33.

    R.B. Baker, Customary International Law in the 21st Century: Old Challenges and New Debates, 21 European Journal of international Law (2010), 173: “[T]he debate over whether consistent state practice and opinio juris are the only building blocks of customary international law is over, because clearly, for better or for worse, they no longer are.” C. Tomuschat, International Law: Ensuring the Survival of mankind on the Eve of a New Century, 281 Recueil des Cours (1999), 86.

  34. 34.

    H.W.A. Thirlway, International Customary Law and Codification (1972), 46.

  35. 35.

    van Hoof (supra note 14), 82. See in that regard in particular the position of the ILC in Sect. 4.4, The Identification of Customary International Law: The Challenge Ahead.

  36. 36.

    A. E. Roberts, Traditional and Modern Approaches to Customary International Law: A Reconciliation, 95 American Journal of International Law (2001), 757ff; Simma, Alston (supra note 1), 88.

  37. 37.

    G. Schwarzenberger, The Inductive Approach to International Law (1965), 35. According to Schwarzenberger “[i]n any field, the use of the inductive method presupposes the existence of a fair amount of case material from which plausible generalizations may be attempted”, see G. Schwarzenberger, The Inductive Approach to International Law, 60 Harvard Law Review (1947), 541.

  38. 38.

    In the words of Judge Koretsky in his Dissenting Opinion to the North Sea Continental Shelf Cases, 156: “customary international law turns its face to the past”.

  39. 39.

    Cf. Roberts (supra note 36), 758; Simma, Alston (supra note 1), 88.

  40. 40.

    D’Amato (supra note 23), 88ff. See also the dissenting opinion of Judge Read in the ICJ Fisheries Case (United Kingdom v. Norway), Judgment, 18 December 1951, ICJ Reports (1951), 191: “Customary international law is the generalization of the practice of States. This cannot be established by citing cases where coastal States have made extensive claims, but have not maintained their claims by the actual assertion of sovereignty over trespassing foreign ships. […] The only convincing evidence of State practice is to be found in seizures, where the coastal State asserts its sovereignty over the waters in question by arresting a foreign ship.” See also Thirlway (supra note 34), 51.

  41. 41.

    Petersen (supra note 28), 301.

  42. 42.

    A. Pellet, Article 38, in Zimmerman et al (eds.), The Statute of the International Court of Justice – A Commentary (2012), 817–818, MN 221.

  43. 43.

    Compare Kolb (supra note 11), 133ff; Kammerhofer (supra note 11), 530.

  44. 44.

    ICJ, North Sea Continental Shelf Cases, para 74: “Although the passage of only a short period of time is not necessarily, or of itself, a bar to the formation of a new rule of customary international law on the basis of what was originally a purely conventional rule, an indispensable requirement would be that within the period in question, short though it might be, State practice, including that of States whose interests are specially affected, should have been both extensive and virtually uniform in the sense of the provision invoked.”

  45. 45.

    ICJ, Fisheries Case, 131. However, in the Nicaragua Case the court backpedaled from this position stating that it is deemed “sufficient that the conduct of states should, in general, be consistent with such rules and that instances of state conduct inconsistent with a given rule should generally have been treated as breaches of that rule, not as indications of the recognition of a new rule.” See ICJ, Nicaragua Case, 98.

  46. 46.

    ICJ, Asylum Case, 277.

  47. 47.

    D’Amato (supra note 23), 18.

  48. 48.

    D’Amato (supra note 23), 91: “There is no metaphysically precise (such as ‘seventeen repetitions’) or vague (such as ‘in the Court’s discretion’) answer possible. States simply do not organize their behavior along absolute lines. There is no international ‘constitution’ specifying when acts become law. Rather, states resort to international law in claim-conflict situations. In such instances, counsel for either side will attempt to cite as many acts as possible. Thus we may say that persuasiveness in part depends upon the number of precedents.”

  49. 49.

    D’Amato (supra note 23), 73; Thirlway (supra note 34), 54, goes further claiming that opinio iuris could also include “that if the practice in question was not required by the law, it was in the process of becoming so”, referring to the Dissenting Opinion of Judge Lachs to the ICJ, North Sea Continental Shelf Cases, 230–231: “[Other States] may also have been convinced that the instrument … was intended to become and would in due course become general law (the teleological element is of no small importance in the formation of law)”.

  50. 50.

    Slama (supra note 11), 605.

  51. 51.

    See for example: D’Amato (supra note 23), 66; Kammerhofer (supra note 12), 78.

  52. 52.

    H. Kelsen, Théorie du droit international coutumier, 1 Revue internationale de la théorie du droit (1939), 263; see also Cheng (supra note 32), 259; van Hoof (supra note 14), 87 (footnote 318) with further references.

  53. 53.

    O. Elias, The Nature of the Subjective Element in Customary International Law, 33 International and Comparative Law Quarterly (1995), 503; D’Amato (supra note 23), 66: “it is the difficulty of imagining that all states participating in custom-formation were erroneously advised by their legal counsel as to the requirements of prior international law.” Kelsen (supra note 52), 263.

  54. 54.

    van Hoof (supra note 14), 91ff; see also Elias (supra note 53), 508.

  55. 55.

    Simma, Alston (supra note 1), 88.

  56. 56.

    ICJ, North Sea Continental Shelf Cases, para 78.

  57. 57.

    Kolb (supra note 11), 121 referring to P. Guggenheim, Les deux éléments de la coutume en droit international, La technique et les principes du droit public: Etudes en l’honneur de Georges Scelle, Vol. 1 (1959), 275; M. H. Mendelson, The Formation of Customary International Law, 272 Recueil des Cours (1998), 289ff; Kelsen (supra note 52), 253ff who considered that opinio iuris solely disguised the law creating power of judges in the formation of customary international law; see also I. Brownlie, Principles of Public International Law (2008), 8. ILA (supra note 18), Part III: the Subjective Element, 29ff.

  58. 58.

    H. Kelsen, Principles of International Law (1952), 418.

  59. 59.

    Kammerhofer (supra note 11), 546, referring to the criticism of J. Raz, The Concept of a Legal System (1980), 67 and H. Günther, Zur Entstehung von Völkergewohnheitsrecht (1970), 81–83.

  60. 60.

    Kammerhofer (supra note 11), 547.

  61. 61.

    Kammerhofer (supra note 11), 61.

  62. 62.

    ICJ, Right of Passage over Indian Territory, 42–43; Asylum Case, 276–277; Jurisdictional Immunities of the State (Germany v. Italy: Greece Intervening), Judgment, 3 February 2012, ICJ Reports (2012) (Jurisdictional Immunities Case), para 55.

  63. 63.

    PCIJ, Lotus Case.

  64. 64.

    See the references to the jurisprudence in supra note 13.

  65. 65.

    M. Koskenniemi, From Apology to Utopia (2005), 427ff, referring to the PCIJ’s Lotus Case, the ICJ’s Asylum Case, Fisheries Jurisdiction Cases, North Sea Continental Shelf Cases and US Military and Paramilitary Activities Case. The ILA (supra note 18), Section 19, p. 41 comes to a similar conclusion.

  66. 66.

    See in this regard ICJ, North Sea Continental Shelf Cases, para 78: “[T]he general practice of States should be recognized as prima facie evidence that it is accepted as law”; Nicaragua Case, para 202: “existence in the opinio juris of States of the principle on non-intervention is backed by established and substantial practice”; cf. I. Brownlie, Principles of Public International Law (2008), 8; R. Heinsch, Die Weiterentwicklung des humanitären Völkerrechts durch die Strafgerichtshöfe für das ehemalige Jugoslawien und Ruanda (2007), 298–299; Koskenniemi, id, 427ff referring to a hypothesis of opinio iuris based on State practice. Mendelson (supra note 57), 283–293; J.-M. Henckaerts, Customary International Humanitarian Law (2005) (ICRC Custom Study), Vol. I: Rules, xlvi. In other instances the ICJ as well as the PCIJ explicitly referred to opinio iuris, however, mostly when negating the existence of customary international law, see e.g. ICJ, North Sea Continental Shelf Cases, paras 74–78; PCIJ, Lotus Case, 15–19.

  67. 67.

    D’Amato (supra note 23), 67 inter alia referring to Kelsen (supra note 52), 263; compare also Kammerhofer (supra note 12), 79.

  68. 68.

    For a more recent proposition to use an opinio necessitatis for establishing a customary rule of humanitarian intervention see A. Cassese, A Follow-Up: Forcible Humanitarian Countermeasures and Opinio Necessitatis, 10 European Journal of International Law (1999), 797ff.

  69. 69.

    Mendelson (supra note 57), 271. Hilpold heavily criticized the approach taken by Cassese (supra note 68) in the following volume of the European Journal of International Law stating that an opinio necessitatis approach “is very dangerous for a consensus-oriented order as it introduces a unilateral element disguised as a constitutional norm, a higher ranking provision which allows no further discussion, thereby implying that the values on which this norm rests are commonly shared.” See P. Hilpold, Humanitarian Intervention: Is there a Need for a Legal Reappraisal?, 12 European Journal of International Law (2001), 461–462.

  70. 70.

    D’Amato (supra note 23), 68.

  71. 71.

    Most famously pronounced by the PCIJ in the Lotus Case, para 44: “International law governs relations between independent States. The rules of law binding upon States therefore emanate from their own free will as expressed in conventions or by usages generally accepted as expressing principles of law and established in order to regulate the relations between these co-existing independent communities or with a view to the achievement of common aims. Restrictions upon the independence of States cannot therefore be presumed.” See also ICJ, Nicaragua Case, para 135: “in international law there are no rules, other than such rules as may be accepted by the State concerned, by treaty or otherwise.”

  72. 72.

    Most prominently G.I. Tunkin, Co-Existence and International Law, 95 Recueil des Cours (1958), 13.

  73. 73.

    Walden (supra note 11), 344ff with further references and 355; Verdross (supra note 11), 636–637 with further references. Elias (surpa note 53), 501; Kammerhofer (supra note 12), 77.

  74. 74.

    J. I. Charney, The Persistent Objector Rule and the Development of Customary International Law, 56 British Yearbook of International Law (1985), 16: “It is difficult to see how the acceptance of the [persistent objector] rule does not reflect an acceptance of the consent theory of international law. If a mere objection to an evolving rule of law can prevent application of that rule to the State, then each State has the unilateral power to decide whether or not to be bound by the rule.” Contrary to that view, Kolb (supra note 11), 144: “If customary law were really voluntary, the whole doctrine of the persistent objector would be useless. For in not acquiescing in an emerging rule of international law, a state would automatically not be bound by this new rule.”

  75. 75.

    K. Wolfke, Custom in Present International Law (1964), 56.

  76. 76.

    Byers (supra note 27), 142–146 states that customary international law does not depend on explicit consent but is based on “inferred consent”; see also M. Akehurst, Custom as a Source of International Law, 47 British Yearbook of International Law (1977), 38–42.

  77. 77.

    N. Petersen, The Role of Consent and Uncertainty in the Formation of Customary International Law, in Lepard (ed.), Reexamining Customary International Law, available at: www.coll.mpg.de/pdf_dat/2011_04online.pdf (last visited 16 June 2017), 2.

  78. 78.

    See Danilenko (supra note 16), 108.

  79. 79.

    Guzman (supra note 11), 144 (footnote 125); Byers (supra note 27), 145; I. M. L. de Souza, The Role of State Consent in the Customary Process, 44 The International and Comparative Law Quarterly (1995), 533–534.

  80. 80.

    Kelsen uses the example of a State that has just gained access to the sea but finds itself already bound by the customary rules in that regard, Kelsen (supra note 58), 312; see also de Souza (supra note 79), 534.

  81. 81.

    Kammerhofer (supra note 12), 78.

  82. 82.

    A. D’Amato, Consent, Estoppel, and Reasonableness: Three Challenges to Universal International Law, 10 Virginia Journal of International Law (1969), 3 who also interprets the PCIJ’s Lotus Case in that way; A. Pellet, The Normative Dilemma: Will and Consent in International Law-Making, 12 Australian Yearbook of International Law (1991), 37; de Souza (supra note 79), 537; de Aréchaga (supra note 17), drawing that conclusion from the case law of the ICJ.

  83. 83.

    Kolb (supra note 11), 144.

  84. 84.

    Ibid.

  85. 85.

    M. C. Bassiouni, Introduction to International Criminal Law (2003), 1.

  86. 86.

    See the position of a member of the Judge Advocate’s Office of the US as cited by Cassese in Cassese and Weiler (eds.), Change and Stability in International Law-Making (1988), 24: “State practice on humanitarian law and the laws or warfare is the practice of the battlefield; it is the members of the army who make practice.”

  87. 87.

    Heinsch (supra note 65), 298.

  88. 88.

    M. Akehurst, Custom as a Source of International Law, 47 British Yearbook of International Law (1977), 10; Mendelson (supra note 57), 207; H. Spieker, Völkergewohnheitsrechtlicher Schutz der natürlichen Umwelt im internationalen bewaffneten Konflikt (1992), 80ff; Kammerhofer (supra note 11), 529 with further references.

  89. 89.

    Kammerhofer (supra note 11), 529; Danilenko (supra note 27), 28; Heinsch (supra note 65), 298.

  90. 90.

    Spieker (supra note 88), 83ff, in relation to the customary prohibition of warfare that is damaging to the environment, analysis that omissions can only then be coined as State practice if States possess the technological premises to use weapons that are damaging to the environment. See also M. E. Villiger, Customary International Law and Treaties (1997), 39; D’Amato (supra note 23), 81, 82: “surely one cannot draw any conclusion from the fact that a state did not do what it was not capable of doing.”

  91. 91.

    PCIJ, Lotus Case, 28; A similar approach is taken by the ICRC in its Custom Study (supra note 66), Vol. I: Rules, xxxviii.

  92. 92.

    Heinsch (supra note 65), 299: “Die einzige Methode, um ein Unterlassen auch als relevante Staatenpraxis zu akzeptieren, besteht darin, dass das Unterlassen von einer entsprechenden opinio iuris begleitet wird.“; ICRC Custom Study (supra note 66), Vol. I: Rules, xlvi Elias (surpa note 53), 502 referring to the role of opinio iuris in “qualifying State practice”. See also Byers (supra note 27), 148 who considers that opinio iuris can be used to distinguish between legally relevant and legally irrelevant State practice.

  93. 93.

    Spieker (supra note 88), 81.

  94. 94.

    See supra note 66.

  95. 95.

    Spieker (supra note 88), 82–83 with further references.

  96. 96.

    One might consider that for these reasons the ICTY, Tadić Appeals Chamber Jurisdiction Decision, at para 99 disclaimed “the actual behavior of the troops” as relevant State practice and prefers that “on account of the inherent nature of this subject-matter, reliance must primarily be placed on such elements as official pronouncements of States, military manuals and judicial decisions.”

  97. 97.

    ICJ, Nuclear Weapons Advisory Opinion, para 64.

  98. 98.

    ICJ, Nuclear Weapons Advisory Opinion, para 73.

  99. 99.

    ILA (supra note 18), Part II: The Objective Element: State Practice, pp. 13–20, the ILA however opens up the possibility that verbal practice, while being able to evince State practice, could be attributed less weight than “actual” practice; M.N. Shaw, International Law (2008), 82; I. Brownlie, Principles of Public International Law (2008), 6; Simma, Alston (supra note 1), 89 referring to a “dubious metamorphosis” of State practice.

  100. 100.

    Cf. Akehurst (supra note 88), 10, see also 53.

  101. 101.

    ILA (supra note 18), Section 11 at p. 19. This view is also shared by Akehurst (supra note 88), 53.

  102. 102.

    ICTY, Tadić Appeals Chamber Jurisdiction Decision, para 99.

  103. 103.

    See ICRC Custom Study (supra note 66), Vol. I: Rules, xxxviii. However, the ICRC has been criticized by the US expressing its concern about the methodology stating that the State practice delivered by the ICRC Custom Study is “insufficiently dense to meet the ‘extensive and virtually uniform’ standard generally required to demonstrate the existence of a customary rule” and that it “places too much emphasis on written materials, such as military manuals and other guidelines published by States, as opposed to actual operational practice by States during armed conflict”. See further J. B. Bellinger and W. J. Haynes, A US government response to the International Committee of the Red Cross study Customary International Humanitarian Law, 89 Review of the Red Cross (2007), 444–445.

  104. 104.

    ICRC Custom Study (supra note 66), Vol. I: Rules, xlvi.

  105. 105.

    See for instance De Witte in relation to the discussion “The Classical “Sources” of International Law Revisited in Cassese, Weiler (eds.), Change and Stability in International Law-Making (1988), 14; contrary Wolfke states that “customs arise from acts of conduct and not from promises of such acts.” He would thus distinguish between physical acts and verbal acts. While verbal acts deliver evidence of the opinio iuris by their content they can only be used as State practice in a narrow sense as State practice on making verbal statements, but they do not provide for State practice in relation to their content. See Wolfke (supra note 75), 42–43; for a strict separation of State practice as physical acts and opinio iuris see D’Amato (supra note 23), 88: “[A] claim is not an act. […] But the claims themselves, although they may articulate a legal norm, cannot constitute the material component of custom.”

  106. 106.

    See further: Kammerhofer (supra note 11), 525;. R. Müllerson, The Interplay of Objective and Subjective Elements in Customary Law, in K. Wellens (ed.), International Law: Theory and Practice: Essays in Honour of Eric Suy (1998), 162; see also I. Brownlie, Principles of Public International Law (2008), 8, who does not differentiate between State practice and opinio iuris in his list of evidence for customary international law: “diplomatic correspondence, policy statements, press releases, the opinions of official legal advisers, official manuals on legal questions, e.g. manuals of military law, executive decisions and practices, orders to naval forces etc., comments by governments on drafts produced by the International law Commission, state legislation, international and national judicial decisions, recitals in treaties and other international instruments, a pattern of treaties in the same form, the practice of international organs, and resolutions relating to legal questions in the United Nations General Assembly.”

  107. 107.

    Mendelson (supra note 57), 206–7, who considers however that opinio iuris needs to be established only in cases of doubt, see ibid, 292.

  108. 108.

    Kammerhofer (supra note 12), 66–67.

  109. 109.

    See P. Haggenmacher, La doctrine des deux éléments du droit coutumier dans la pratique de la Cour internationale, 90 Revue générale de droit international public (1986), 114: “Les deux prétendus éléments n’ont en réalité aucune individualité propre; ils se trouvent inextricablement mêlés au sein d’une ‘pratique’ unitaire. Cette pratique forme pour ainsi dire un seul ‘élément’ complexe, fait d’aspects ‘matériels’ et ‘psychologiques’.”

  110. 110.

    Wolfke (supra note 75), 42–45 states that there is a “notorious confusion of that element [referring to opinio iuris] with its evidence.” Zemanek considers that “separating material recording ‘State practice’ from material recording opinio juris, though theoretically perhaps desirable, is practically impossible because the first may, through its language, evidence the second.”, K. Zemanek, What is State Practice and Who Makes it?, in Beyerlin et al. (eds.), Recht zwischen Umbruch und Bewahrung: Völkerrecht, Europarecht, Staatsrecht: Festschrift für Rudolf Bernhardt (1995), 292; see also Kammerhofer (supra note 12), 69–70, who considers that the distinction of element and evidence would only shift the problem and reveal an “epistemological uncertainty”.

  111. 111.

    Cheng (supra note 32), 251; see also Simma, Alston (supra note 1), 89 and Guzman (supra note 11), 149.

  112. 112.

    Roberts (supra note 36), 758ff; Simma, Alston (supra note 1), 89.

  113. 113.

    R. Y. Jennings, The Identification of International Law, in Cheng (ed.), International law: Teaching and Practice (1982), 5: “[W]hat we perversely persist in calling customary international law is not only not customary law: it does not even faintly resemble a customary law.” Wolfke (supra note 75), 40–41: “Without practice (consuetudo), customary international law would obviously be a misnomer, since practice constitutes precisely the main differentia specifica of that kind of international law.” (emphasis in original).

  114. 114.

    ICJ, Nicaragua Case, paras 183–4.

  115. 115.

    See e.g. T. Meron, The Humanization of International Law (2006), 102. See also G. Werle, Principles of International Criminal Law (2009), 368.

  116. 116.

    ICJ, Nicaragua Case, paras 188–195 (“this opinio juris may be deduced from, inter alia, the attitude of the Parties and States towards certain General Assembly resolutions”); see also Simma, Alston (supra note 1), 97.

  117. 117.

    Roberts (supra note 36), 758; Simma, Alston (supra note 1), 96; V. Gowlland-Debbas, Judicial Insights into the Fundamental Values and Interests of the International Community, in Muller et al (eds.), The International Court of Justice, Its future role after fifty years (1997), 348. The approach of the ICJ has heavily been criticized by A. D’Amato, Trashing Customary International Law, 81 American Journal of International Law (1987), 101ff.

  118. 118.

    ICJ, Nuclear Weapons Advisory Opinion, para 73.

  119. 119.

    ICJ, Jurisdictional Immunities Case, para 55 referring to the North Sea Continental Shelf Cases, and Continental Shelf Case (Libya v. Malta).

  120. 120.

    Kammerhofer (supra note 12), 204. See with regard to the ICJ’s rather flexible approach to customary international law Special Rapporteur M. Wood, First Report on Formation and Evidence of Customary International Law, International Law Commission, 65th Session, 6 May-7 June and 8 July-9 August 2013, Official Records of the General Assembly, UN Doc. A/CN.4/663, para 64 footnote 123 with further references.

  121. 121.

    Verdross (supra note 11), 636: “Because it is probable that each of the different theories [on customary law] contain some correct element, the presumption of one mode of creation for all norms of customary international law is probably not correct.” See also Kammerhofer (supra note 12), 204; recently, d’Aspremont in a contemporary formulation has compared “customary international law with a dance floor where (almost) anything goes”, see http://www.ejiltalk.org/customary-international-law-as-a-dance-floor-part-i/#more-10650 (last visited 16 June 2017).

  122. 122.

    Arguing for a modern approach for customary human rights law: O. Schachter, International Law in Theory and Practice, 178 Recueil des Cours (1982), 334; Baker (supra note 33), 180; Roberts (supra note 36), 759; see also T. Meron, Human Rights and Humanitarian Norms as Customary Law (1989).

  123. 123.

    See the differentiation of “consuetudine-fondamento” (fundamental custom) and “consuetudine-fonte” (custom as a source) in G. Sperduti, La fonte suprema dell ordinamento internazionale (1946), 155ff; C. Tomuschat, Obligations Arising for States without or against Their Will, 241 Recueil des Cours (1993), 73; Kolb (supra note 11), 123ff.

  124. 124.

    Schachter (supra note 122), 334.

  125. 125.

    Meron (supra note 122), 131.

  126. 126.

    See O. Schachter, New Custom: Power, Opinio Juris and Contrary Practice, in Theory of International Law at the Threshold of the 21st Century: Essays in Honour of Krzysztof Skubiszewski (1996), 538; on the factual “practice” of torture see the Report of the Special Rapporteur on torture and other forms of cruel, inhuman or degrading treatment, Study on the phenomena of torture, cruel, inhuman, or degrading treatment or punishment in the world, including an assessment of conditions of detention, submitted on 5 February 2010 to the United Nations Human Rights Council, UN-Doc. A/HRC/13/39/Add.5: “Despite the fact that torture constitutes one of the most brutal attacks on human dignity and one of the most serious human rights violations, and notwithstanding the absolute nature of the prohibition of torture and ill-treatment even in the most exceptional circumstances, such as war, internal disturbances and terrorism, torture and ill-treatment are widespread practices in the majority of the countries on our planet. Almost no society is immune against torture, but in many societies torture is practiced systematically, both in fighting ordinary crime and in combating terrorism, extremism or similar politically motivated offences.”

  127. 127.

    Meron (supra note 115), 372–373 referring to R. Higgins, Problems & Process: International Law and How We Use It (1994), 22.

  128. 128.

    Schachter (supra note 126), 538.

  129. 129.

    T. Meron, The Geneva Conventions as Customary Law, 81 American Journal of International Law (1987), 361.

  130. 130.

    Compare further Kirgis (supra note 11), 149. See also T. Meron, International Law in the Age of Human Rights, 301 Recueil des Cours (2003), 388.

  131. 131.

    Cf. Koskenniemi (supra note 2), 1961: “which rights are customary is less a matter of formal tests of legal validity than a deference to their ethico-political importance; indeed, ‘elementary considerations of humanity’ and ‘basic rights of the human person’ receive legal protection regardless of whether lawyers come up with any number of precedents to support them; indeed, the more shocking the violation, the more open is the law for allowing responsibility to be triggered. Would any other conclusion have been acceptable, or possible?”

  132. 132.

    O. Schachter, Entangled Treaty and Custom, Dinstein (ed.), International Law at a Time of Perplexity: Essays in Honour of Shabtai Rosenne (1988), 734, referring to a “higher normativity” of certain norms which he considers as resulting from a fact of contemporary life; P. Weil, Towards Relative Normativity in International Law, 77 American Journal of International Law (1983), 413ff is strictly opposed to relative normativity.

  133. 133.

    See the references in supra note 123f.

  134. 134.

    Kolb (supra note 11), 123; see also Tomuschat (supra note 123), 292, who considers that the constitutional foundations of the international community have not evolved in State practice and must necessarily be a core element of the constitutional framework.

  135. 135.

    Preamble of Hague Conventions of 1899 and 1907 respecting the Laws and Customs of War on Land and included in Common Article 3 of the Geneva Conventions 1949.

  136. 136.

    Tomuschat (supra note 123), 300–2.

  137. 137.

    Ibid. The meshing of different sources of international law finds followers in ICL as K. Ambos, Der allgemeine Teil des Völkerstrafrechts (2002), 43f proposed that customary international law in statu nascendi shall be verified by general principles.

  138. 138.

    See for instance the evaluation of Common Article 3 of the Geneva Conventions which the ICJ considers to be the minimum yardstick applicable to international and non-international armed conflicts. See ICJ, Nicaragua Case, paras 218–229; ICJ, Nuclear Weapons Advisory Opinion, para 75 referring to “The Hague Law” and “Geneva Law”. ICJ, Wall Case, Advisory Opinion, para 78, referring to the customary international law as reflected in Art. 42 Hague Regulations of 1907. ICJ, Armed Activities Case (Democratic Republic of the Congo v. Uganda), Judgment, 19 December 2005, ICJ Reports (2005), para 219 referring to customary international law as reflected in Art. 25, 27, 28, 43, 46, 47 Hague Regulations of 1907.

  139. 139.

    ICJ, Corfu Channel Case (United Kingdom v. Albania), Judgment, 9 April 1949, ICJ Reports (1949), 22 referring to the recognized principle of elementary considerations of humanity; ICJ Nicaragua Judgment, para 215, referring to principles of humanitarian law underlying Hague Convention VIII of 1907; furthermore in para 218 it referred to “elementary considerations of humanity” that are reflected in Common Article 3.

  140. 140.

    ICJ, Nicaragua Judgment, paras 218–220.

  141. 141.

    ICJ, Nuclear Weapons Advisory Opinion, para 78, referring to the principle of protection of civilians and the prohibition on causing unnecessary suffering as the fabric of humanitarian law.

  142. 142.

    First referred to in the ICJ, Corfu Channel Case, 22; ICJ, Advisory Opinion Nuclear Weapons, para 79; ICJ, Wall Case, Advisory Opinion, para 157, the Court stating that these obligations have an erga omnes character.

  143. 143.

    In its modern formulation in Additional Protocol I it states that states are bound by “the principles of international law derived from established custom, from the principles of humanity and from the dictates of public conscience.” The Martens Clause was referred to and considered customary law in ICJ, Nuclear Weapons Advisory Opinion, paras 84, 87.

  144. 144.

    That confusion is also present in academic writing: G. Zyberi, The Humanitarian Face of the International Court of Justice. Its Contribution to Interpreting and Developing International Human Rights and Humanitarian Law Rules and Principles (2008), 285, alleges that the ICJ applied in Nicaragua fundamental principles of customary international humanitarian law; T. Meron, Editorial Comment, Revival of Customary Humanitarian Law, 99 American Journal of International Law (2005), 819: “In that case, the Court held that common Articles 1 and 3 of the Geneva Conventions constitute general principles of humanitarian law that are binding on the United States – in other words, that they are customary law” (emphasis added); J. Wouters, C. Ryngaert, The Impact on the Process of the Formation of customary International Law, in Kamminga, Scheinin (eds.), The Impact of Human Rights Law on General International Law (2009), 123f holding that the ICJ in relation to international humanitarian law was “relying on general principles” (emphasis added).

  145. 145.

    See, ICJ, Nuclear Weapons Advisory Opinion, paras 79–82, referring to “intransgressible principles of international customary law”, the “affirmation” of which as customary law is established by the Nuremberg Tribunal and Report of the Secretary-General Pursuant to Paragraph 2 of Security Council Resolution 808 (1993), 3 May 1993, UN Doc S/25704 (SG Report on ICTY); See also the Dissenting Opinion of Judge Koroma at p. 580: “By reference to the humanitarian principles of international law, the Court recognized that the Conventions themselves are reflective of customary law and as such universally binding”.

  146. 146.

    B. Schlütter, Developments in Customary International law (2010), 154 (in relation to the ICJ Nicaragua Case); V. Chetail, The Contribution of the International Court of Justice to International Humanitarian Law, 85 ICRC Review No. 850 (2003), 245.

  147. 147.

    Chetail, id, 244.

  148. 148.

    ICJ, Nuclear Weapons Advisory Opinion, paras 79–81.

  149. 149.

    ICJ, Nuclear Weapons Advisory Opinion, para 82.

  150. 150.

    The perception of general principles is not restricted to those derived from the foro domestico but also include general principles of international law that have been generated by acceptance on the international plane via, e.g. declarations of the UN General Assembly.

  151. 151.

    B. Simma, International Human Rights and General International Law, in collected Courses of the Academy of European Law, Volume IV (1995), 225; compare also M.C. Bassiouni, A Functional Approach to General Principles of International Law, 11 Michigan Journal of International Law (1990), 768ff.

  152. 152.

    Simma, Alston (supra note 1), 102–6; see also Schachter (supra note 126), 539; compare also A. Verdross, B. Simma, Universelles Völkerrecht (1984), 386, § 606.

  153. 153.

    Simma, Alston (supra note 1), 104.

  154. 154.

    A. Pellet, Shaping the Future of International Law: The Role of the World Court in Law-Making, in Arsanjani et al. (eds.), Looking to the Future: Essays on International Law in Honor of W. M. Reisman (2011), 1076.

  155. 155.

    ICJ, Jurisdictional Immunities Case, paras 52 and 81–91.

  156. 156.

    P. Tomka, Custom and the International Court of Justice, 12 The Law and Practice of International Courts and Tribunals (2013), 197.

  157. 157.

    Official Records of the General Assembly, UN Doc. A/CN.4/L.796, International Law Commission, 63rd Session, 26 April – 3 June and 4 July – 12 August 2011, Report of the Planning Group; See also Official Records of the General Assembly, Sixty-sixth Session, Supplement No. 10 (A/6610), paras 365ff.

  158. 158.

    First Report of Special Rapporteur Wood (supra note 120).

  159. 159.

    Special Rapporteur M. Wood, Second Report on Identification of Customary International Law, International Law Commission, 66th Session, 5 May-6 June and 7 July-8 August 2014, Official Records of the General Assembly, UN Doc. A/CN.4/672.

  160. 160.

    Special Rapporteur M. Wood, Third Report on Identification of Customary International Law, International Law Commission, 67th Session, 4 May-5 June and 6 July-7 August 2015, Official Records of the General Assembly, UN Doc. A/CN.4/682. Special Rapporteur M. Wood, Third Report on Identification of Customary International Law, International Law Commission, 67 Session, 4 May – 5 June and 6 July – 7 August 2015, Official Records of the General Assembly, UN Doc. A/CN.4/682. Special Rapporteur M. Wood, Fourth Report on Identification of Customary International Law, International Law Commission, 68th Session, 2 May – 10 June and 4 July – 12 August 2016, Official Records of the General Assembly, UN Doc. A/CN.4/695.

  161. 161.

    Statute of the International Law Commission, adopted by the General Assembly in Resolution 174 (II), 21 November 1947, and amended by resolutions 485 (V), 12 December 1950, 984 (X), 3 December 1955, 985 (X), 3 December 1955 and 36/39, 18 November 1981, Art. 24.

  162. 162.

    See Yearbook of the International Law Commission (1950), Volume II, 24ff Working Paper by M. O. Hudson, Special Rapporteur and addendum thereto as well as 364ff Report of the International law Commission to the General Assembly, Document A/1316.

  163. 163.

    Survey of International Law in Relation to the Work of Codification of the International Law Commission, Memorandum submitted by the Secretary General, 10 February 1949, UN Doc. A/CN.4/1/Rev.1, para 33: “It is doubtful whether any useful purposes would be served by the attempts to make it more specific, as, for instance by defining the conditions of the creation and of the continued validity of international custom”; see also First Report of Special Rapporteur Wood (supra note 120), 4–5.

  164. 164.

    See First Report of Special Rapporteur Wood (supra note 120), 5, referring to the 1998 unofficial survey as quoted in Report of the Study Group on the Future Work of the International Law Commission, in Anderson et al. (eds.), The International Law Commission and the Future of International Law (1998), 42.

  165. 165.

    First Report of Special Rapporteur Wood (supra note 120), 6.

  166. 166.

    ILC Report on the Work of its 65th Session to the General Assembly, 6 May – 7 June and 8 July – 9 August 2013, Supplement No. 10 (A/68/10), paras 69 and 76. SR Wood was criticized that his approach would not take into account the distinction between formal and material sources of law, see id, para 87.

  167. 167.

    Thus the ILC will not shed light on the first layer of sources of law, see Sect. 4.1, Preliminary Remarks.

  168. 168.

    ILC Report on the Work of its 65th Session (supra note 166), para 83: “General support was also expressed for the proposed focus on the practical process of identifying rules of customary international law, rather than the content of such rules.”

  169. 169.

    First Report of Special Rapporteur Wood (supra note 120), 7–8, referring to ICJ, Ahmadou Sadio Diallo Case (Republic of Guinea v. Democratic Republic of the Congo), Judgment – Compensation owed by the Democratic Republic of the Congo to the Republic of Guinea, 19 June 1012, ICJ Reports (2012), 391ff, Declaration of Judge Greenwood, para 8.

  170. 170.

    ILC Report on the Work of its 65th Session (supra note 166), paras 84 and 86. In his Second Report Special Rapporteur Wood (supra note 159), para 28 adheres to the commission’s proposal.

  171. 171.

    ILC Report on the Work of its 68th Session to the General Assembly, 2 May – 10 June and 4 July – 12 August 2016, Supplement No. 10 (A/71/10), Chapter V.

  172. 172.

    Koskenniemi (supra note 2), 1947.

  173. 173.

    Verdross (supra note 11), 636.

  174. 174.

    ILC Report on the Work of its 65th Session (supra note 166), para 102. Second Report of Special Rapporteur Wood (supra note 159), para 28.

  175. 175.

    ILC Report on the Work of its 65th Session (supra note 166), para 102, para 101. Second Report of Special Rapporteur Wood (supra note 159), Draft Conclusion 2.

  176. 176.

    Second Report of Special Rapporteur Wood (supra note 159), para 27, determining that “[w]hile such writings [i.e. on one-element approaches] are always interesting and provocative, and have been (and should be) duly taken into account, it remains the case that they do not seem to have greatly influenced the approach of States or courts. The two-element approach remains dominant.” The Special Rapporteur lists numerous examples in the legal literature (para 26), State practice (para 24) and international courts (para 25) to substantiate this finding.

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Rauter, T. (2017). The Formation of Customary International Law and Its Methodological Challenges. In: Judicial Practice, Customary International Criminal Law and Nullum Crimen Sine Lege. Springer, Cham. https://doi.org/10.1007/978-3-319-64477-6_4

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