1 See, e.g., for such classification, Waxman, M.C., ‘Regulating Resort to Force: Form and Substance of the UN Charter Regime’, (2013) EJIL Vol. 24 No. 1, 151CrossRefGoogle Scholar; Corten, O., ‘The Controversies Over the Customary Prohibition on the Use of Force: A Methodological Debate’, (2006) EJIL Vol. 16 No. 5, 803Google Scholar; Corten, O., ‘Regulating Resort to Force: A Response to Matthew Waxman from a “Bright-Liner”’, (2013) EJIL Vol. 24 No. 1, 191CrossRefGoogle Scholar.

2 See, e.g., on the use of the notion of orthodoxy in relation to the law of self-defence and the shift of that orthodoxy evidenced in recent legal literature on the subject, Kammerhofer, J., ‘The Resilience of the Restrictive Rules on Self-Defence’, in Weller, M. (ed.), The Oxford Handbook on the Use of Force in International Law (2015) 627Google Scholar; Kammerhofer, J., ‘Orthodox Generalists and Political Activists in International Legal Scholarship’, in Happold, M. (ed.), International Law in a Multipolar World (2011) 138Google Scholar.

3 See, for a special focus on US Scholarship, Banks, W.C. and Criddle, E.J., ‘Customary Constraints on the Use of Force: Article 51 with an American Accent’, (2015) 29 LJIL at 67–93.CrossRefGoogle Scholar

4 Differences between the restrictivist and expansionist sides have been largely addressed in legal literature (see, supra notes 1 and 2) but not the changing nature of the arguments used by expansionists following the recent shift in legal scholarship on the use of force.

5 Some of the views expressed in this section have been detailed in van Steenberghe, R., ‘State practice and the evolution of the law of self-defence: clarifying the methodological debate’, (2015) Journal on the Use of Force and International Law, 2:1, 81Google Scholar.

6 Changes are also noticeable in relation to other issues, such as the right to respond in self-defence to minor uses of force. Yet, they are far less apparent and, in any case, they seem to be in line with those characterizing the issues dealt with in this article.

7 At least since the adoption of UNGA Res. 3314 (XXIX) defining the notion of aggression (see, Art. 3(g)) and the ICJ judgment in the Nicaragua case (see, Case Concerning Military and Paramilitary Activities in and against Nicaragua (Nicaragua v. United States of America), Judgment of 27 June 1986, [1986] ICJ Rep. 14, at 105, para. 199).

8 Such qualification is adopted by many legal scholars: see, e.g., T. Ruys, ‘Armed Attack’ and Article 51 of the UN Charter (2010) 368; Ferencz, B.B., ‘Defining Aggression: The Last Mile’, (1973) 12 Col. JTL 430, at 431Google Scholar; Verhoeven, J., ‘Les “étirements” de la légitime défense’, (2002) 48 Annuaire français de droit international 48, at 56CrossRefGoogle Scholar; R. Kolb, Ius contra bellum: Le droit international relatif au maintien de la paix (2009), 274; see also, for a similar qualification, the preparatory works of the resolution adopted in 2007 by the International Law Institute, (2007) 72 Yearbook of the International Law Institute 75, especially at 180, 191 and 206. This qualification also seems in accordance with state practice: see, e.g., the invocation by Pakistan of an indirect aggression committed by India in 1971 with respect to East Pakistan (UN Doc. S/PV.1106, at 10). See nonetheless, contra O. Corten, The Law against War: The Prohibition on the Use of Force in Contemporary International Law (2010), 444.

9 See, e.g., for a similar observation on the long controversial debate on this issue, Henriksen, A., ‘Jus ad bellum and American Targeted Use of Force to Fight Terrorism Around the World’, (2014) 19 Journal of Conflict & Security Law 1, at 15CrossRefGoogle Scholar.

10 See, e.g., in this sense, the opinions of Judge Kooijmans in the Case concerning the Oil Platforms (Islamic Republic of Iran v. United States of America), Judgment of 6 November 2003, [2003] ICJ Rep. 161, at 230, para. 35, and in the case concerning the Armed Activities on the Territory of the Congo (Democratic Republic of the Congo v. Uganda), Judgment of 19 December 2005, [2005] ICJ Rep. 168, at 314, para. 28.

11 See, e.g., in this sense, Corten, supra note 8, at 161 and ff. See, e.g., for some criticisms regarding this argument, which emphasize the more complex nature of the relationships between the prohibition and the exception of self-defence, Kammerhofer, J., ‘Uncertainties of the law on self-defence in the United Nations Charter’, (2004) 35 Netherlands Yearbook of International Law 143, at 183 and ff.CrossRefGoogle Scholar; R. van Steenberghe, La légitime défense en droit international public (2012), 283 and ff.

12 See, e.g., C. Greenwood, ‘International Law and the Pre-emptive Use of Force: Afghanistan, Al-Qaida, and Iraq’, (2003) San Diego Int’l L.J. 7, at 16–18 and 21–3; J. Gardam, Necessity, Proportionality and the Use of Force by States (2004), 150; A. Cassese, ‘Article 51’, in J.-P. Cot, A. Pellet, M. Forteau (dir.), La Charte des Nations Unies: commentaire article par article (2005), 1332–3; C.J. Tams, ‘Light Treatment of a Complex Problem: The Law of Self-Defence in the Wall Case’, (2005) EJIL 963, at 972–3; S. Clavier, ‘Contrasting Perspectives on Preemptive Strike : The United States, France, and the War on Terror’, (2006) Maine L. Rev. 565, at 571–2; N. Ronzitti, ‘The Expanding Law of Self-Defence’, (2006) J. Conflict & Sec. L. 343, at 344 and 348; E. Wilmshurst et al., ‘The Chatham House Principles of International Law on the Use of Force in Self-Defence’, (2006) International Comparative Law Quarterly 963, at 965–71; K.N. Trapp, ‘Back to Basics: Necessity, Proportionality, and the Right of Self-Defence Against Non-State Terrorist Actors’, (2008) International Comparative Law Quarterly 141, at 156; J. Combacau, and S. Sur, Droit international public (2008), 633; J.J. Paust, ‘Self-Defense Targetings of Non-State Actors and Permissibility of U.S. Use of Drones in Pakistan’, (2009–2010) J. Transnat’l L. & Pol’y 237, at 239–41; see in particular, note 3 of the latter article, listing a significant number of scholars adopting such a view. See also, especially about US scholarship, Banks and Criddle, supra note 3, at 82–3. See, e.g., nonetheless contra Corten, supra note 8, at 220; Randelzhofer, A. and Nolte, G., ‘Article 51’, in Simma, B. (ed.), The Charter of the United Nations. A Commentary (2012), 1417Google Scholar.

13 See, e.g., Y. Dinstein, War, Aggression and Self-Defence (2001), 214; Roberts, G.B., ‘Self-Help in Combating State-Sponsored Terrorism: Self-Defense and Peacetime Reprisals’, (1987) 19 Case W. Res. J. Int’l L. 242, at 268Google Scholar; Reisman, W.M., ‘International Legal Responses to Terrorism’, (1999–2000) 22 Hous. J. Int’l L. 3, at 42Google Scholar.

14 See, e.g., Dinstein, supra note 13.

15 See, e.g., ibid., at 215.

16 See, e.g., O. Schachter, ‘The Extraterritorial Use of Force against Terrorist Bases’, (1988–1989) Hous. J. Intl L. 309, at 311.

17 See, e.g., A.C. Arend and R.J. Beck, International Law and the Use of Force: Beyond the UN Charter Paradigm, (1993), 152–5.

18 Turkish incursions into northern Iraq throughout the 1990s or Israeli interventions in Lebanon in the 1980s are examples of precedents mentioned and interpreted as confirming the existence of the right to respond in self-defence to armed attacks by non-state actors (see, e.g., for these two cases, Dinstein, supra note 13, at 218).

19 See, e.g., Paust, supra note 12, at 239–41, especially note 3; Henriksen, supra note 9, at 225.

20 See, e.g., Waxman, supra note 1, at 164, note 54; Kretzmer, D., ‘The Inherent Right to Self-Defence and Proportionality in Jus Ad Bellum’, (2013) 24 EJIL 235, at 246–7CrossRefGoogle Scholar, note 61.

21 See, e.g., Henriksen, supra note 9, at 225; Tsagourias, N., ‘Cyber-attack, self-defence and the problem of attribution’, (2012) 17 Journal of Conflict & Security Law 1, at 14–15CrossRefGoogle Scholar; Bethlehem, D., ‘Self-Defense Against an Imminent or Actual Armed Attack by Nonstate Actors’, (2012) 106 American J. Int’l L. 770, at 774Google Scholar.

22 Y. Dinstein, War, Aggression and Self-Defence (2012), 227–30.

23 Ibid., at 227 (emphasis added). See also, Henriksen, supra note 9, at 225: ‘Indeed, despite a few persistent objectors in the academic literature, to most scholars, the international response to 9/11 shows that under certain conditions there is indeed a “right to self-defence against non-state actors for terrorist attacks”’.

24 Legal Consequences of the Construction of a Wall in the Occupied Palestinian Territory, Advisory Opinion of 9 July 2004, [2004] ICJ Rep. 136, at 194, para. 139 (emphasis added).

25 See, e.g., Corten, supra note 8, at 193; Kammerhofer, J., The Armed Activities Case and Non-State Actors in Self-Defence Law’ (2007) 20 LJIL 89, at 96CrossRefGoogle Scholar. See, e.g., for another interpretation, infra notes 32 and 33.

26 Armed Activities on the Territory of the Congo case, supra note 10, at 223, para. 147.

27 See, e.g., Henriksen, supra note 9, at 226.

28 See, e.g., regarding the ICJ assertion in the Wall case, comments of some participants of the Principles of international Law on the Use of Force by States in Self-Defence (Chatham House Principles), 2005, available at www.chathamhouse.org/publications/papers/view/108106; in particular, those of C. Greenwood, ibid., at 21; P. Sands, ibid., at 26; M. Wood, ibid., at 30; See also, Dinstein, supra note 22, at 229.

29 Legal Consequences of the Construction of a Wall in the Occupied Palestinian Territory case, supra note 24, at 230; Armed Activities on the Territory of the Congo case, supra note 10, at 313–14, paras 26–8.

30 Legal Consequences of the Construction of a Wall in the Occupied Palestinian Territory case, supra note 24, at 242; and Armed Activities on the Territory of the Congo case, supra note 10, at 335–7, paras 5–11.

31 Legal Consequences of the Construction of a Wall in the Occupied Palestinian Territory case, supra note 24, at 215.

32 Statement of Wood, M. in Wilmshurst, E. (ed.), Principles of international Law on the Use of Force by States in Self-Defence (Chatham House Principles), supra note 28, at 30Google Scholar. See also, for such interpretation, van Steenberghe, R., ‘Self-Defence Against Non-State Actors in Light of Recent State Practice: A Step Forward?’, (2010) 23 LJIL 183, at 190CrossRefGoogle Scholar and, for further developments, supra note 11, at 285 and ff.

33 Trapp, supra note 12, at 145. See, e.g., in the same way, van Steenberghe, supra note 11, at 282 and ff.

34 See, e.g., Dinstein, supra note 13, at 220.

35 Henriksen, supra note 9, at 226.

36 Bethlehem, supra note 21, at 774.

37 Ibid., at 775.

38 See, e.g., Travalio, G. and Altenburg, J., ‘Terrorism, State Responsibility, and the Use of Military Force’, (2003) 4 Chi. J. Int’l L. 97, at 112Google Scholar; Stahn, C., ‘Terrorist Acts as “Armed Attack”: The Right to Self-Defense, Article 51 (1/2) of the UN Charter, and International Terrorism’, (2002) 23 Fletcher F. World Aff. 35, at 42 and 47–8Google Scholar; van Steenberghe, supra note 11, at 340; Kretzmer, supra note 20, at 247; Trapp, supra note 12, at 141. See, for an express criticism of this principle, Wilmshurst, E. and Wood, M., ‘Self-Defense Against Nonstate Actors: Reflections on the “Bethlehem Principles”’, (2013) 107 American J. Int’l L. 390, at 394CrossRefGoogle Scholar.

39 Bethlehem, supra note 21, at 776.

40 See, e.g., van Steenberghe, supra note 11, at 202; Trapp, supra note 12, at 147; Kress, C., ‘Some Reflections on the International Legal Framework Governing Transitional Armed Conflicts’, (2010) 15 Journal of Conflict and Security Law 245, at 250CrossRefGoogle Scholar; Antonopoulos, C., ‘Force by Armed Groups as Armed Attack and the Broadening of Self-Defence’, (2008) 55 Netherlands International Law Review 159, at 167CrossRefGoogle Scholar.

41 See, e.g., Paust, J.J., ‘Use of Military Force in Syria by Turkey, NATO, and the United States’, (2012–2013) 34 U. Pa. J. Int’l L. 431, at 432Google Scholar.

42 See, e.g., Paust, J.J., ‘Permissible Self-Defense Targeting and the Death of Bin Laden’, (2011), 39 Denv. J. Int’l L. & Pol’y 569, at 580–1Google Scholar.

43 See, Bethlehem, supra note 21, at 770.

44 Ibid., at 773.

45 See, e.g., for a similar observation, Ruys, supra note 8, at 263.

46 Available at georgewbush-whitehouse.archives.gov/nsc/nss/2002/(last visited 30 September 2014).

47 Ibid., 15 (emphasis added).

48 Pre-emptive versus preventive terminology is only used for the sake of clarity. Different and even contrary terminologies are used in legal scholarship. See, e.g., on this subject van Steenberghe, supra note 11, at 401, note 1602.

49 UN Doc A/59/565 (2004), para. 188.

50 See, e.g., for a similar observation, T. Ruys, supra note 8, at 324 and ff.

51 See, e.g., ibid, at 322.

52 Garwood-Gowers, A., ‘Israel's Airstrike on Syria's Al-Kibar Facility: A Test Case for the Doctrine of Pre-emptive Self-Defence?’, (2011) 16 Journal of Conflict & Security Law 263, at 276 (emphasis added)CrossRefGoogle Scholar.

53 See, e.g., for the factual and legal circumstances surrounding this incident, Jennings, R.Y., ‘The “Caroline” and McLeod Cases’, (1938) 32 American J. Int’l L. 82CrossRefGoogle Scholar.

54 See, e.g., R. Higgins, Problems and process: international law and how we use it (1994), 242–3; Waldock, C.H.M., ‘The Regulation of the Use of Force by Individual States in International Law’, (1952)(II) 81 Collected Courses of The Hague Academy 455, at 498Google Scholar; Malanczuk, P., ‘Countermeasures and self-defence as circumstances precluding wrongfulness in the International Law Commission's Draft Articles on State Responsibility’, (1983) 43 Zeitschrift für Ausländisches Öffentliches Recht und Völkerrecht, at 764Google Scholar; A. Verdross and B. Simma, Universelles Völkerrecht. Theorie und Praxis (1976), 239–40; Schachter, O., ‘Lawful Resort to Unilateral Use of Force’, (1984–1985) 10 Yale J. Int’l L. 291, at 293Google Scholar; T. Franck, Resort to Force. State Action Against Threats and Armed Attacks (2002), 97–8.

55 See, for this diplomatic exchange, avalon.law.yale.edu/19th_century/br-1842d.asp#web2 (last visited 25 July 2015).

56 See, e.g., for this position, Waldock, supra note 54, at 496–9; D.W. Bowett, Self-Defence in International Law (1958), 188–91; Malanczuk, supra note 54, at 761–2, O. Schachter, ‘International Law: The Right of States to Use Armed Force’, (1984) 82 Mich. L. Rev. 1620, at 1634; Mc Cormack, T.L.H., ‘Anticipatory Self-Defence in the Legislative History of the United Nations Charter’, (1991) 25 Isr. L. Rev. 1, at 8Google Scholar. See, similarly, scholarship mentioned in Banks and Criddle, supra note 3, at 70.

57 See, e.g., S.A. Alexandrov, Self-Defence Against the Use of Force in International Law (1996), 149 and ff.; Franck, supra note 54, at 99–107; Malanczuk, supra note 54, at 762–4.

58 M.S. McDougal and F.P. Feliciano, Law and Minimun World Public Order: The legal Regulation of International Coercion (1961), 222. See also, Bowett, supra note 56, at 192; Malanczuk, supra note 54, at 761; Waldock, supra note 54, at 498; Higgins, supra note 54, at 242. See also, especially about US scholarship, Banks and Criddle, supra note 3, at 79. Some references are also made to the case law of the Nuremberg and Tokyo Tribunals (Judgment of the International Military Tribunal (Nuremberg), 1 October 1946, quoted in (1947) 41 American J. int’l L., at 205; Judgment, Cmd. 6964, quoted in L.B. Sohn, Cases on United Nations Law (1967) 915), which quoted and endorsed the Webster formula when judging war criminals, as well as to the first report to the UN Security Council of the UN Atomic Energy Commission in 1946 (avalon.law.yale.edu/20th_century/decad240.asp (last visited 24 July 2015)), according to which ‘a violation [of a treaty or convention on atomic energy issues] might be of so grave character as to give rise to the inherent right of self-defense recognized in Article 51 of the Charter of the United Nations’.

59 See, e.g., Waxman, supra note 1, at 160; Rockefeller, M.L., ‘The “Imminent Threat” Requirement for the Use of Preemptive Military Force: Is it Time for a Non-Temporal Standard?’, (2004–2005) 33 Denv. J. Int’l L. & Pol’y 131, at 133Google Scholar.

60 Cf. infra 2.2.2.

61 See, e.g., Henriksen, supra note 9, at 226.

62 UN Doc A/59/2005, 21 March 2005, paras. 124–5.

63 See, e.g., in this sense, Kretzmer, supra note 20, at 248.

64 See, e.g., for a similar conclusion, comments by D. Bethlehem in Wilmshurst, supra note 28, at 41.

65 See, e.g., for a similar conclusion, Ruys, supra note 8, at 322.

66 Gill, T.D., ‘The Temporal Dimension of Self-Defence: Anticipation, Pre-emption, Prevention and Immediacy’, (2006) 11 Journal of Conflict & Security Law 361, at 369 (emphasis added)CrossRefGoogle Scholar.

67 See, e.g., Wilmshurst, supra note 28, at 8; Akande, D. and Liefländer, T., ‘Clarifying Necessity, Imminence, and Proportionality in the Law of Self-Defense’, (2014) 107 American J. Int’l L. 563, at 565–566Google Scholar; Schrijver, N. and van den Herik, L., ‘Leiden Policy Recommendations on Counter-terrorism and International Law’ (2010) 57 Netherlands International Law Review, at 543Google Scholar; Sofaer, A.D., ‘On the Necessity of Pre-emption’, (2003) 14 EJIL 209, at 220CrossRefGoogle Scholar; Rockefeller, supra note 59, at 144; Pierson, C., ‘Preemptive Self-Defense in an Age of Weapons of Mass Destruction: Operation Iraqi Freedom’, (2004–2005) 33 Denv. J. Int’l L. & Pol’y 150, at 176Google Scholar; Cohan, J.A., ‘The Bush Doctrine and the Emerging Norm of Anticipatory Self-Defense in Customary International Law’, (2003) 15 Pace Int’l L. Rev. 283, at 355Google Scholar; Schmitt, M.N., ‘Premptive Strategies in International Law’, (2002–2003) 24 Mich. J. Int’l L. 513, at 535Google Scholar; Yoo, J., ‘International Law and the War in Iraq’, (2003) 97 American J.Int’l L. 563, at 572CrossRefGoogle Scholar; ibid., ‘Force Rules: UN Reform and Intervention’, (2005) 6 Chi. J. Int’l L. 641, at 650. See also, especially for US scholarship on the notion of imminence and in particular on the requirement of a high level of evidence in that matter, Banks and Criddle, supra note 3, at 75–6.

68 Yoo, ‘International Law and the War in Iraq’, supra note 67, at 574.

69 See, e.g., Wilmshurst, supra note 28, at 9; Rockefeller, supra note 59, at 143–4.

70 Akande and Liefländer, supra note 67, at 566; Sofaer, supra note 67, at 220; Pierson, supra note 67, at 175.

71 See, e.g., D. Bethlehem, comments to the Chatham House Principles, supra note 28, at 41; A. Robert, ibid., at 43–4; P. Sands, ibid., at 44 and 47; M. Shaw, ibid., at 48; M. Wood, ibid., at 51–2; Sofaer, supra note 67, at 214; Rockefeller, supra note 59, at 141, Pierson, supra note 67, at 174; Cohan, supra note 67, at 353; Schmitt, supra note 67, at 534; Yoo, ‘International Law and the War in Iraq’, supra note 67, at 574; supra note 67, at 651.

72 Schmitt, supra note 67, at 534.

73 Cohan, supra note 67, at 354.

74 See, e.g., Corten, supra note 8, at 407 and 411; Ruys, supra note 8, at 59–60.

75 See, for more details on this, van Steenberghe, supra note 11, at 100.

76 See, e.g., Corten, supra note 8, at 411, 416 and ff.; Ruys, supra note 8, at 60.

77 See, e.g., Corten, supra note 8, at 12 and 21.

78 Second report on identification of customary international law, by M. Wood, 22 May 2014, UN Doc. A/CN.4/672, at 18–19, para. 37.

79 This is supported not only by the work of the International Law Commission on the identification of customary international law in legal literature (see, supra note 78) but also by most legal scholars (see, e.g., the numerous authors mentioned in the Second report on identification of customary international law, supra note 78, at 19, footnote 84) and the ICJ (see, for instance, Legality of the Threat or Use of Nuclear Weapons, Advisory Opinion of 8 July 1996, [1996] ICJ Reports 226, at 254–5, paras. 70–3).

80 See, for a similar observation, Akehurst, M., ‘Notes and Comments: Letter to the Editor in Chief’, (1986) 80 American J. of Int’l L. 141, at 147Google Scholar.

81 See e.g., the qualification given by Franck to the military operation launched by Israel in Egypt on 5 June 1967 as a relevant precedent of anticipatory self-defence; such a qualification is made on the basis of both the ‘acts and words’ of the Israeli authorities (supra note 54, at 103). See, for a similar qualification, A. M. Weisburd, Use of Force: The Practice of States since World War II (1997), 137. See, concerning the qualification of some other precedents on the basis of not only state material conduct but also state political and moral declarations, F.R Teson, Humanitarian Intervention: An Inquiry into Law and Morality (1988), 169 and 192.

82 See, e.g., Weisburd, A. M., ‘Customary International Law: The Problem of Treaties’, (1988) 21 Vanderbilt Journal of International Law 1, at 45Google Scholar; Donaghue, S., ‘Normative Habits, Genuine Beliefs and Evolving Law: Nicaragua and the Theory of Customary International Law’, (1995) 16 Australian Yearbook of International Law 327, at 342Google Scholar.

83 See, for further developments on that issue, van Steenberghe, R., ‘The Law against War or Jus contra Bellum: A New Terminology for a Conservative View on the Use of Force?’, (2011) 24 LJIL 747, at 750–1CrossRefGoogle Scholar.

84 Kammerhofer, ‘The Resilience of the Restrictive Rules on Self-Defence’, supra note 2, at 644.

85 See, e.g., supra notes 43–4 and 73.

86 See, e.g., on such notion in relation to the law on use of force, C. Gray, International Law and the Use of Force (2008), 25 and ff.

87 See, van Steenberghe, supra note 11, at 18 and ff.

88 See, for more details on this, van Steenberghe, supra note 11, at 99–100.

89 See, e.g., Ruys, supra note 8, at 19 and ff.; Corten, supra note 8, at 29 and ff.; Dinstein, supra note 22, at 227–8; van Steenberghe, supra note 11, at 185–6; see, at least implicitly, Gray, supra note 86, at 201 and ff.; Trapp, supra note 12, at 149 and ff.; Tams, supra note 12, at 378 and ff. See nonetheless contra Kammerhofer, ‘The Resilience of the Restrictive Rules on Self-Defence’, supra note 2, at 638 and ff.

90 See nonetheless, for a contrary (minority) position, Verhoeven, J., ‘Considérations sur ce qui est commun. Cours général de droit international public (2002)’, (2008) 334 Recueil des Cours 9, at 274–5Google Scholar. See also, the dissenting opinion of Judge Jennings in Nicaragua, supra note 7, at 530–1. Judge Jennings took the view that Art. 51 – contrary to Art. 2(4) – of the UN Charter is not declaratory of customary international law. However, he did not exclude that part of the Article (which deals with individual self-defence) expresses prior customary international law.

91 See, e.g., Nicaragua case, supra note 7, para. 175. See also, point 1 of the resolution adopted on 27 October 2007 by the Institute of International Law on the right of self-defence, (2007) Yearbook of Institute of International Law, at 234. It may be reasonably argued that the conditions of necessity and proportionality are only of a customary nature as they are not provided for by Art. 51 of the UN Charter (see, e.g., Nicaragua case, supra note 7, para. 176; Nuclear Weapons case, supra note 79, para. 41). The same may be said with respect to the obligation for a state victim of an armed attack to request the assistance of a third state in order for this state to be allowed to repel the armed attack on the basis of collective self-defence (see, e.g., Nicaragua case, supra note 7, para. 199). By contrast, the aspects of the right of self-defence related to the UN collective security mechanism may arguably be considered as being only of conventional nature (see, e.g., Nicaragua case, supra note 7, para. 200; Nuclear Weapons case, supra note 79, para. 44). Regarding the ‘armed attack’ condition, proponents of a restrictive conception of the law governing self-defence acknowledge that this condition is of both a customary and conventional nature but argue that these two sources have a similar content (see, e.g., I. Brownlie, International Law and the Use of Force by States (1963), 238–239; Corten, supra note 8, at 410), while proponents of an extensive approach consider that customary international law differs in that regard from Art. 51 to the extent that it allows a broad interpretation of the notion of armed attack set out in that Art. or provides for other possible situations in which states may act in self-defence, not excluded by Art. 51 (see, e.g., C. Humphrey Waldock, ‘The Regulation of the Use of Force by Individual States in International Law’, (1952) Recueil des Cours, 496–9).

92 This seems to be the main position adopted by scholars. See, e.g., those mentioned in Kammerhofer, ‘The Resilience of the Restrictive Rules on Self-Defence’, supra note 2, at 641, ff. 56.

93 See, Ruys, supra note 8, at 19–22.

94 See, e.g., Corten, supra note 8, at 29. The author nonetheless considers that this way through which the law of self-defence may evolve, i.e., interpretation of Art. 51 on the basis of subsequent state practice, does not have to be distinguished from the customary evolution of that law.

95 Supra note 91.

96 Baxter, R.R, ‘Treaties and Custom’ (1970–I) 129 Recueil des Cours 25, at 64 and 96Google Scholar.

97 See, e.g., Kolb, R., ‘Selected Problems in the Theory of Customary International Law’, (2003) 50 Netherlands International Law Review 119, at 145–6CrossRefGoogle Scholar.

98 Ibid., at 146.

99 Meron, T., ‘The Geneva Convention as Customary Law’, (1987) 81 American J. of Int’l L. 348, at 367CrossRefGoogle Scholar.

100 That Art. 51 of the UN Charter refers to the customary right of self-defence may be inferred from the words that Art. 51 begins with: ‘Nothing in the present Charter shall impair . . .’. These words logically imply that the right of self-defence existed prior to the UN Charter and, as a result, as part of customary international law. It is, however, less certain that this may also be inferred from the adjective ‘inherent’ qualifying the right of self-defence under Art. 51, although the ICJ expressly interpreted this adjective as referring to the customary international law of self-defence (Nicaragua case, supra note 7, para. 176). Indeed, the adjective ‘inherent’ could be given other possible meanings, such as referring to the imperative nature of the right of self-defence, as it is claimed by some authors (see, e.g., R. Ago, ‘Addendum – Eighth Report on State Responsibility by Mr. Roberto Ago, Special Rapporteur – The Internationally Wrongful Act of the State, Source of International Responsibility (Part 1)’ (1980), UN Doc A/CN.4/318/Add.5–7, at 67, footnote 263), or, as seems to be more plausible, emphasising that states could never be dispossessed of their right of self-defence against their will. The (controversial) interpretation that the Court gave to the adjective ‘inherent’ may perhaps be explained by the fact that it did as much as possible to find customary rules to judge the case since the United States multilateral treaty reservation to its competence constrained it to apply only customary international law.

101 See more particularly, for instance, statements from Israel: UN Doc S/PV.2280 (1 September 1988), 8 (regarding Israeli intervention into Iraq – 1981); Tajikistan, Kazakhstan, Kirghizstan and Russia: UN Doc S/26290 (11 August 1993), 2 (concerning conflict between Tajikistan and Afghanistan – 1993); the United States: UN Doc S/1998/780 (2 August 1998), 1 (in relation to US intervention into Afghanistan and Sudan – 1998); and UN Doc A/C.6/35/SR.51 (17 November 1980), 2, para. 4 (declaration at the Sixth Committee of the UN General Assembly concerning the ILC's work on State Responsibility).

102 See, e.g., Nicaragua case, supra note 7, at 101, paras. 191 and 195.

103 See, e.g., J. Verhoeven, Droit international public (2000), 682.

104 See, for a very clear example of an interpreting role played by a reference to the state practice predating the UN Charter (and, more particularly, the Caroline case), the statement pronounced by SEM Mohamed Bennouna, Ambassador, Permanent Representative of Morocco in New York in relation to the High-Level Panel report on the Threats, Challenges and Change, UN Doc A/59/565 (31 January 2005): ‘[le Maroc est] reconnaissant au groupe de personnalités de haut niveau pour la clarté de son analyse de l’article 51 de la charte, en rappelant la seule interprétation de la légitime défense admise en droit international, dans la lignée de l’affaire du Caroline . . . et qui consiste à réagir en cas d’agression armée ou lorsque celle-ci est “imminente”’.

105 See, for instance, statements from Iran: UN Doc S/23786 (6 April 1992), 1 (concerning Iranian intervention into Iraq – 1992); UN Doc S/1994/1273 (9 November 1994), 1 (concerning Iranian intervention into Iraq – 1994); UN Doc S/1997/768 (2 October 1997), 1 (concerning Iranian intervention into Iraq – 1997); and Tajikistan, Kazakhstan, Kirghizstan and Russia: UN Doc S/26290 (n 45), 2 (concerning conflict between Tajikistan and Afghanistan – 1993). See also, the dissenting opinion of Judge Koroma, annexed to the advisory opinion of the ICJ in Nuclear Weapons case, supra note 79, at 562.

106 See Baxter, supra note 96, at 64 and 96.

107 Meron, supra note 99, at 367 (emphasis added).

108 The latter requirement must still be specified in the light of the nature of the law of self-defence. Indeed, it is generally upheld that the prohibition on use of force – or, at least, the prohibition on aggression – has a peremptory nature (see nonetheless, for a critical view on this position, Green, J.A., ‘Questioning the Peremptory Status of the Prohibition of the Use of Force’, (2011) 32 Michigan Journal of International Law 215)Google Scholar. It has been expressly considered as such by most states (see in this respect, the numerous declarations mentioned in Corten, supra note 8, at 204–7, and, more particularly, the declarations pronounced by the states at the Sixth Committee of the UN General Assembly in the frame of the adoption of General Assembly Resolution 42/22 [1987]). It is not the case with respect to the law of self-defence (or the entire jus ad bellum, which includes this law). Only few states expressly recognized self-defence as a peremptory norm (see for instance, statements from Iraq: UN Doc A/C.6/35/SR.51 (n 45) 17, para. 62; and Jamaica: UN Doc A/C.6/35/SR.53 (19 November 1980), 15, para. 51). Moreover, it is not possible to give to the right of self-defence a peremptory nature on the ground that no derogation is permitted from it or on the ground that it modifies a peremptory norm (i.e., the two grounds on which a norm may possibly attain peremptory status according to Art. 53 of the VCLT). Indeed, the right of self-defence is a right, and not an obligation, from which it is therefore conceptually impossible to derogate and it is contained in the scope of a peremptory norm, the prohibition on use of force, but does not properly modify this norm. However, as contained in the scope of the prohibition on use of force, any evolution of the law of self-defence actually implies an evolution of this prohibition and, therefore, must fulfil the conditions under which a peremptory norm may evolve, i.e., ‘to be approved by the accepted and recognized by the international community of States as a whole’. In other words, the law of self-defence has an indirect peremptory nature. This does not mean that all the states must approve the evolution of this law: unanimity is not required. It is enough if it is approved by all the different states’ groups of the world, provided that it is not opposed by other states. In this respect, one instance or a few remote instances of opposition would not prevent the evolution from happening.

109 Ruys, supra note 8, at 23.

110 See, for a more detailed analysis, van Steenberghe, supra note 11, at 270–92.

111 See, in this way, Separate Opinion of Judge Kooijmans annexed to the judgment of the ICJ in Armed Activities on the Territory of the Congo case, supra note 8, at 313–14, para. 28.

112 See, for further developments on that issue, van Steenberghe, supra note 11, at 97–106; see also, van Steenberghe, supra note 83, at 776–7.

113 See, e.g., Kammerhofer, ‘The Resilience of the Restrictive Rules on Self-Defence’, supra note 2, at 639–40.

114 See e.g., for such a view, Distefano, G., ‘La pratique subséquente des Etats parties à un traité’, (1994) 40 Annuaire français de droit international 41, at 61–70CrossRefGoogle Scholar; I. Brownlie, Principles of Public International Law (2008), 630; Yassen, M.K., ‘L’interprétation des traités d’après la Convention de Vienne sur le droit des traités’, (1976) Collected Courses of The Hague Academy of International Law, 51Google Scholar; Cot, J-P., ‘La conduite subséquente des parties à un traité’, (1966) 70 Revue générale de droit international public 632, at 664–6Google Scholar.

115 See e.g., statements from Chile, UN Doc. A/CONF.39/C.1/SR.37, §75; see e.g., on this subject, P. Daillier, M. Forteau and A. Pellet, Droit international public (2009), 325.