1 G. Santayana, ‘The Life of Reason: or the Phases of Human Progress, Introduction and Reason in Common Sense’, in M. Wokeck and M. Coleman (eds.), The Works of George Santayana (2011), 172.

2 E.g., T. Ruys, ‘Armed Attack’ and Article 51 of the UN Charter, Evolutions in Customary Law and Practice (2010), 11.

3 B. Baker, ‘The Hague Peace Conferences (1899 and 1907)’, 2009 (November) MEPIL, available at <www.opil.ouplaw.com/view/10.1093/law:epil/9780199231690/law-9780199231690-e305>.

4 See the 1907 (Hague) Convention (II) Respecting the Limitation of Employment of Force for the Recovery of Contract Debts, (1908) 2 AJIL, Supplement, 81–5; the 1919 Covenant on the League of Nations, (1919) 13 AJIL, Supplement, 128–40; the 1928 General Treaty for Renunciation of War as an Instrument of National Policy, XCIV LNTS 57–64 (1929).

5 The 1945 Charter of the United Nations, 24 October 1945, XV United Nations Conference on International Organization, 335–54.

6 See Tams, C., ‘The Use of Force against Terrorists’, (2009) 20 EJIL 359, at 362–73CrossRefGoogle Scholar.

7 Franck, T., ‘Terrorism and the Right of Self-Defense’, (2001) 95 AJIL 839, at 840CrossRefGoogle Scholar; Murphy, S., ‘Terrorism and the Concept of “Armed Attack” in Article 51 of the UN Charter’, (2002) 43 HILJ 41, at 50Google Scholar; Stahn, C., ‘Terrorist Acts as “Armed Attack”: The Right to Self-Defense, Article 51 (½) of the UN Charter, and International Terrorism’, (2003) 27 WFWA 35, at 36Google Scholar; Printer, N., ‘The Use of Force against Non-State Actors under International Law: An Analysis of the U.S. Predator Strike in Yemen’, (2003) 8 UCLA JIL&FA 331, at 351Google Scholar; Feinstein, B., ‘A Paradigm for the Analysis of the Legality of the Use of Armed Force Against Terrorists and States that Aid and Abet Them’, (2004) 17 CJTL 51, at 67Google Scholar; Ronzitti, N., ‘The Expanding Law of Self-Defence’, (2006) 11 JC&SL 343, at 348Google Scholar; McKeever, D., ‘The Contribution of the International Court of Justice to the Law on the Use of Force: Missed Opportunities or Unrealistic Expectations?’, (2009) 78 NJIL 361, at 383Google Scholar; K. Zemanek, Armed Attack, MPEPIL, 2009 (April) para. 16, available at <www.opil.ouplaw.com/view/10.1093/law:epil/9780199231690/law-9780199231690-e241>.

8 Legal Consequences of the Construction of a Wall in the Occupied Palestinian Territory, Advisory Opinion of 9 July 2004, [2004] ICJ Rep. 136, at 215 (Judge Higgins, Separate Opinion) where Judge Higgins claims that the restrictive view is due to the ICJ's holding in Nicaragua that action by irregulars may constitute an armed attack if sent by or on behalf of a state and if reaching a certain scale and effects; Military and Paramilitary Activities in and against Nicaragua (Nicaragua v. United States of America), Merits, Judgment of 27 June 1986, [1986] ICJ Rep. 14, at 103–4, para. 195. Contrary, McKeever, supra note 7, at 383, who argues that the ICJ was broadening the possibilities for actions of non-state actors to fit within the rules on self-defence; see Murphy, supra note 7, at 50–1, as to the silence of Art. 51.

9 Ruys, supra note 2, at 490; R. van Steenberghe, La légitime défense en droit international public (2012) 270–1, who also makes this argument, at 270–81, in relation to the preparatory works and subsequent practice.

10 Murphy, supra note 7, at 50, points out that while Art. 2(4) prohibits uses of force against a state, Art. 51 remains silent as to the origin of the attack; similarly Orr, A., ‘Unmanned, Unprecedented, and Unresolved: The Status of American Drone Strikes in Pakistan Under International Law’, (2011) 44 CILJ 729, at 739Google Scholar.

11 Krajewski, M., ‘Selbstverteidigung gegen bewaffnete Angriffe nicht-staatlicher Organisationen – Der 11. September und seine Folgen’ (2002), 40 AVR 2002, 183, at 195–6Google Scholar, mentions that international law only provides rights and obligations for private persons on an exceptional basis; see also Printer, supra note 7, at 346–7.

12 McKeever, supra note 7, at 382; Tams, supra note 6, at 385. On de facto regimes and the right of peoples to self-determination, see O. Corten, The Law against War, The Prohibition on the Use of Force in Contemporary International Law (2010), 126–97.

13 J. Kammerhofer, Uncertainty in International Law, A Kelsenian perspective (2011), 39–40. This does not turn self-defence into a sanctioning mechanism per se, since its purpose is to protect the state against unlawful armed attacks and not to implement the responsibility of the attacker; see Krajewski, supra note 11, at 186; J. Verhoeven, Les «étirements» de la légitime défense, XLVIII AFDI 2002, 49, at 58–9.

14 Kammerhofer, supra note 13, at 39–40; see also R. Kolb, Ius contra bellum, Le droit international relatif au maintien de la paix (2009), 293.

15 Antonopoulos, C., ‘Force by Armed Groups as Armed Attack and the Broadening of Self-Defence’, (2008) NILR 159, at 169Google Scholar, observing the State must be involved ‘in the activities of the armed group;’ and that responsibility for failure to prevent such activities does not suffice for purposes of self-defence; Kammerhofer, supra note 13, at 39–40.

16 Contrary C. Kreß, Gewaltverbot und Selbstverteidigungsrecht nach Satzung der Vereinten Nationen bei staatlicher Verwicklung in Gewaltakte Privater (1995), 208–9, and footnote 885, who circumvents this demand of identity by holding that self-defence is available against armed attacks that are not permitted (unerlaubt) by international law. Some authors argue for international legal personality of non-state actors: Krajewski, supra note 11, at 196–9; Printer, supra note 7, at 333–4; Tsagourias, N., ‘Non-state actors in peace and security: Non-state actors and the use of force’, in d’Aspremont, J. (ed.), Participants in the International Legal System, Multiple Perspectives on Non-State Actors in International Law (2011), 326 at 326–8Google Scholar. Critical views were given by Verhoeven, supra note 13, at 61–62; Antonopoulos, supra note 15, at 170. Others have noted that Art. 51 does not prohibit armed attacks: Ruys, supra note 2, at 490; Kammerhofer, supra note 13, at 47. Yet the Court claimed that ‘the prohibition of armed attacks may apply to the sending by a State of armed bands to the territory of another state’. Nicaragua, supra note 8, at 103, para. 195 (emphasis added).

17 Kammerhofer, supra note 13, at 49–51; Trapp, K., ‘Back to Basics: Necessity, Proportionality, and the Right of Self-Defence Against Non-State Terrorist Actors’, (2007) 56 ICLQ 141, at 145–6CrossRefGoogle Scholar; Antonopoulos, supra note 15, at 169; Tams, supra note 6, at 385; Kowalski, M., ‘Armed Attack, Non-State Actors and a Quest for the Attribution Standard’, (2010) XXX PYIL 101, at 122Google Scholar.

18 For interpretation of terms, see Randelzhofer, A. and Dörr, O., ‘Article 2(4)’, in Simma, B.et al. (eds.), The Charter of the United Nations, A Commentary, Volume I (2012) 200, at 215–16, paras. 37–9Google Scholar; Henriksen, A., ‘Jus ad bellum and American Targeted Use of Force to Fight Terrorism Around the World’, (2014) 19 JC&SL 211, at 219–20Google Scholar; see also Ruys, supra note 2, at 377.

19 See Michael, B., ‘Responding to Attacks by Non-State Actors: The Attribution Requirement of Self-Defence’, (2009) 16 Aust. JIL 133, at 140–2Google Scholar; Trapp, supra note 17, at 145–6; Molier, G., ‘The War on Terror and Self-Defence against Non-State Actors, An International Law Perspective’, in Ellian, A., Molier, G. and Suurland, D. (eds.), Terrorism: Ideology, Law and Policy (2011), 305, at 316Google Scholar.

20 Kammerhofer, supra note 13, at 38–39.

21 Wolfrum, R., ‘Preamble’, in Simma, B.et al., The Charter of the United Nations, A Commentary, Volume I (2012) 101 at 103–105Google Scholar, paras. 5 and 10; Wolfrum, R., ‘Article 1’, in Simma, B.et al. (eds.), The Charter of the United Nations, A Commentary, Volume I (2012) 107 at 112–13, paras. 15–19Google Scholar. Certain authors have observed that the US response to the 9/11 attacks (unjustifiably) prioritized unilateral responses over collective action, and that the Security Council had been prepared to authorize military action. For example, see Charney, J., ‘The Use of Force against Terrorism and International Law’, (2001) 95 AJIL 835, at 837CrossRefGoogle Scholar; Klein, P., ‘Le droit international a l’épreuve du terrorisme’, (2006) 321 RCADI 203, at 395Google Scholar.

22 Molier, supra note 19, at 310; Kowalski, supra note 17, at 123, adding that expansion to non-state actors would depreciate the prohibition of the use of force; see also Krajewski, supra note 11, at 187; Randelzhofer, A. and Nolte, G., ‘Article 51’, in Simma, B.et al. (eds.), The Charter of the United Nations, A Commentary, Volume II (2012), 1397, at 1403–04, paraGoogle Scholar. 10, in relation to the gap between use of force and armed attack; Kolb, supra note 14, at 269–71.

23 The 1969 Vienna Convention on the Law of Treaties, 1155 UNTS 331.

24 Exceptionally, Schmidl, M., The Changing Nature of Self-Defence in International Law (2009), 43Google Scholar; Kadelbach, S., ‘Interpretation’, in Simma, B.et al. (eds.), The Charter of the United Nations, A Commentary, Volume I (2012), 71, at 75–76Google Scholar, paras. 7–8; Randelzhofer and Nolte, supra note 22, at 1400–01, para. 4.

25 Cf. Section 4, infra; see Orr, supra note 10, at 739; Antonopoulos, supra note 15, at 161; van Steenberghe, R., ‘Self-Defence in Response to Attacks by Non-state Actors in the Light of Recent State Practice: A Step Forward?’, (2010) 23 LJIL 183, at 186CrossRefGoogle Scholar.

26 For example, the doctrine of implied powers as evidenced by ICJ, Reparation for injuries suffered in the service of the United Nations, Advisory Opinion of 11 April 1949, [1949] ICJ Rep. 174; Effect of awards of compensation made by the U.N. Administrative Tribunal, Advisory Opinion of 13 July 1954, [1954] ICJ Rep. 47, at 56–7; Legality of the Threat or Use of Nuclear Weapons by a State, Advisory Opinion of 8 July 1996, [1996] ICJ Rep. 66, at 78–9; and on the interpretation of silence, I. van Damme, Treaty Interpretation by the WTO Appellate Body (2009), 110.

27 Cf. Section 3, infra.

28 ILC, Commentary Interpretation of Treaties, Introduction, (1966) Vol II YBILC 218, at 219, para. 6.

29 Kreß, supra note 16, at 214–215; Kowalski, supra note 17, at 119.

30 But with Kowalski, supra note 17, at 123, setting the purpose of Art. 51 against the fundamental purpose of the Charter.

31 See Ruys, T. and Verhoeven, S., ‘Attacks by Private Actors and the Right of Self-Defence’, (2005) 10 JC&SL 289, at 291Google Scholar; Orr, supra note 10, at 739; Kowalski, supra note 17, at 97; Ruys, supra note 2, at 369–70; van Steenberghe, La légitime défense, supra note 9, at 270–1.

32 Orr, supra note 10, at 739, mentions that Art. 51 is ambiguous and then moves on to the preparatory works.

33 Exceptionally, e.g., Ruys, supra note 2, at 369.

34 Nicaragua, supra note 8 at 103–104, para. 195, drawing upon the Definition of Aggression, G.A. 3314 (XXIX), UN Doc. A/RES/3314(XXIX) (14 December 1974), Art. 3(g) available at <www.un.org/documents/ga/res/29/ares29.htm>.

35 van Steenberghe, supra note 25, at 199–202; van Steenberghe, La légitime défense, supra note 9, at 352–353; see also Kolb, supra note 14, at 274.

36 van Steenberghe, supra note 25, at 200–201; van Steenberghe, La légitime défense, supra note 9, at 352; G. Wettberg, The International Legality of Self-Defense Against Non-State Actors, State Practice from the U.N. Charter to the Present (2007), at 208; C. Greenwood, ‘Self-Defence’, MPEPIL, para. 18, at <opil.ouplaw.com/view/10.1093/law:epil/9780199231690/law-9780199231690-e401?prd=EPIL>, arguing the necessity of self-defence in relation to unwilling or unable States.

37 van Steenberghe, supra note 25, at 200–1; van Steenberghe, La légitime défense, supra note 9, at 352.

38 van Steenberghe, supra note 25, 201–2; van Steenberghe, La légitime défense, supra note 9, at 352–354; Trapp, supra note 17, at 145–7, who does make reference (147) to breach of obligations for this scenario; and more generally, Deeks, A., ‘“Unwilling or Unable”: Toward a Normative Framework for Extraterritorial Self-Defense’, (2012) 52 VJIL 483, at 494–5Google Scholar.

39 Trapp, supra note 17, at 142–5, 150, and 155; Krajewski, supra note 11, at 202–5, argues that in case of self-defence against non-state actors the territorial State has a duty of tolerance by analogy to the law of neutrality, and considers, 205–7, the US actions against the Taliban unjustified; see also Stahn, supra note 7, at 42–3; Kolb, supra note 14, at 275, claims self-defence is allowed against a non-state actor with a territorial basis.

40 Kowalski, supra note 17, at 125–8, adding that necessity and proportionality would require targeting only the non-state actor, unless the territorial State were to come in on its side; in a similar vein, Feinstein, supra note 7, at 57–67, but continuing, 67–73, with an aiding and abetting construction; Trapp, supra note 17, at 147.

41 Trapp, supra note 17, at 147 and footnote 34; Kowalski, supra note 17, at 125–8; Declaration on Principles of International Law concerning Friendly Relations and Co-operation among States in accordance with the Charter of the United Nations, GA Resolution 2525 (XXVI), Annex, (emphasis added), UN Doc. A/RES/2625(XXV) (24 October 1970) available at <www.un.org/documents/ga/res/25/ares25.htm>.

42 Randelzhofer and Nolte, supra note 22, at 1417–18, paras. 37–8, pointing to encouragement, direct support, planning, awareness, and shelter, and (419, para. 41) to inability to prevent such acts; Ruys and Verhoeven, supra note 31, at 305–6.

43 Trapp, supra note 17, at 147; Kowalski, supra note 17, at 125, indicating that breach of the positive obligation alone would not suffice and linking it to inability or unwillingness of the territorial State to prevent such acts.

44 Ruys and Verhoeven, supra note 31, at 312.

45 Ruys and Verhoeven, supra note 31, at 305–8; also, Antonopoulos, supra note 15, at 169, holding that responsibility for failing to prevent armed activities of non-state actors is not in itself sufficient for a plea of self-defence; Michael, supra note 19, at 154–5, claiming toleration does not suffice to attribute armed actions of non-state actors to the territorial State.

46 Ruys, supra note 2, at 375–7; and Ruys and Verhoeven, supra note 31, at 318.

47 ILC, Articles on Responsibility of States for Internationally Wrongful Acts (2001) Vol II YBILC 26–30.

48 ILC, Commentary on Articles on Responsibility of States for Internationally Wrongful Acts (2001) YBILC, at 140–1, Art. 55.

49 Application of the Convention on the Prevention and Punishment of the Crime of Genocide (Bosnia and Herzegovina v. Serbia and Montenegro), Judgment of 26 February 2007, [2007] ICJ Rep. 43, at 208–9, para. 401.

50 ILC Commentary, supra note 48, Art. 8.

51 Nicaragua, supra note 8 at 64–5, paras. 115–16.

52 Prosecutor v. Dusko Tadić, Judgment, Appeals Chamber, IT-94–1-A, 15 July 1999, available at <www.icty.org/x/cases/tadic/acjug/en/tad-aj990715e.pdf>, 39–62, paras. 98–145 and especially 117, 120–2, 130–1, 137 and 145.

53 Genocide, supra note 49, at 207–11, paras. 398–407.

54 Molier, supra note 19, at 414–15.

55 Tadić, supra note 52, at 58–9, paras. 131, 137, 145; see Paust, J., ‘Armed Attacks and Imputation: Would a Nuclear Weaponized Iran Trigger Permissible Israeli and U.S. Measures of Self-Defense?’, (2014) 45 GJIL 411, at 432–4Google Scholar.

56 ILC, Commentary supra note 48, Art. 8 at 47–9, paras. 3 and 8, indicating that conduct is not attributable if it related incidentally or peripherally to an operation and escaped the control of the State. Contrary Tadić, supra note 52, at 58–9, para. 137.

57 Corten, O. and Dubuisson, F., ‘Opération «liberté immuable»: Une extension abusive du concept de légitime défense’, (2002) RGDIP 51, at 55–70Google Scholar; Hofmeister, H., ‘When is it right to Attack So-called ‘Host States’? An Analysis of the Necessary Nexus between Terrorists and their Host States’, (2007) 11 SYBIL 75–84, 78Google Scholar, but proposing, 80–3, a refined harbouring theory.

58 Corten and Dubuisson, supra note 57, at 55–65 and 65–70.

59 Murphy, supra note 7, at 50–1.

60 Krajewski, supra note 11, at 189–191; Ruys and Verhoeven, supra note 31, at 313–14; Kowalski, supra note 17, at 115–18. The ‘sending by or on behalf of a State’ in Art. 3(g) of the Definition of Aggression has been interpreted to call for application of Art. 8 of ASR: M. Byers, ‘Terrorism, the Use of Force and International Law after 11 September’, (2002) 51 ICLQ 401, 407–8 and footnote 38; Corten and Dubuisson, supra note 57, at 65–70); Randelzhofer and Nolte, supra note 22, at 1415, referring to the theory of de facto organs; Ruys and Verhoeven, supra note 31, at 300–1; Klein, supra note 21, at 387–8; Tams, supra note 6, at 368–9; Zemanek, supra note 7, at para. 6. Others have noted that the Court was not discussing attribution but the concept of armed attack: Michael, supra note 19, at 137–9; and van Steenberghe, supra note 25, at 196. On attribution, see earlier de Hoogh, A., ‘Articles 4 and 8 of the 2001 ILC Draft Articles on State Responsibility, The Tadić Case and Attribution of Acts of Bosnian Serb Authorities to the Federal Republic of Yugoslavia’, (2001) 72 BYBIL 255Google Scholar.

61 Verhoeven, supra note 13, at 56–9; Ruys and Verhoeven supra note 31, at 314.

62 Definition of Aggression, supra note 34, at Art. 3(g); Nicaragua, supra note 8, at para. 195.

63 Stahn, supra note 7, at 37 and 47, adding the trend may be to provide for the responsibility of host states for merely tolerating or harbouring terrorists; contrary Kowalski supra note 17, at 113–15, who – while supporting the overall control standard – denies that this would allow attribution of the 9/11 attacks, and noting that this would not provide a remedy if a state were not sufficiently involved or unable or unwilling to repress armed activities of non-state actors; Kolb, supra note 14, at 276–7, noting that logistical support and financing are sufficient to turn a State into a complicit. O’Connell, M.E., ‘Lawful Self-Defense to Terrorism’, (2002) 63 University of Pittsburgh Law Review 889, at 899–902Google Scholar, finds Afghanistan responsible for the acts of al-Qaeda while noting (at 902, footnote 85) that the links between the two were less substantial than required for purposes of attribution.

64 Michael, supra note 19, at 145–55, requiring sanctuary in combination with State cooperation or logistical or other support.

65 Hofmeister, supra note 57, at 80–3, asserting state support as a conditio sine qua non for the armed activities, clear evidence of harbouring, and awareness moderated by a due diligence requirement; see also Ruys, supra note 2, at 503. In a similar vein Antonopoulos observes that practice appears to accept self-defence in response to armed activities of an armed group of great intensity, when in progress, and the armed group is permanently hosted in another State: Antonopoulos, supra note 15, at 171.

66 Schmidl, supra note 24, at 238, when the government oppresses the population and is considered illegitimate.

67 Tams, supra note 6, at 384–7, drawing analogies to inter-State complicity and aiding and abetting in criminal law. ILC Commentary, supra note 48, at 65–7, Art. 16; applied by analogy in Genocide, supra note 49, at 216–19, paras. 418–24.

68 Kowalski, supra note 17, at 126–128, adding that necessity and proportionality would require targeting only the non-state actor, unless the State were to come in on its side; similarly Feinstein, supra note 7, at 57, with an aiding and abetting construction. See also Beard, J., ‘America's New War on Terror: The Case for Self-Defense under International Law’, (2002) 25 HJL&PP 559, at 578–82Google Scholar.

69 Randelzhofer and Nolte, supra note 22, at 1417–19, paras. 37 and 41, stressing such factors as encouragement, direct support, planning, awareness, reasonable steps, and shelter.

70 Ruys, supra note 2, at 491. van Steenberghe, supra note 25, at 194–5, asserts that practice does not support the requirement of attribution since states invoking self-defence do not claim that the territorial state itself committed an armed attack.

71 Ruys, supra note 2, at 490–1.

72 Ruys, supra note 2, at 491–2, by reference to the ICJ's statement in the Genocide case, supra note 49 at 210, para. 406.

73 Ruys, supra note 2, at 493.

74 Dubuisson and Corten, supra note 57, at 55–65; Paust, supra note 5, at 433–4.

75 Verhoeven, supra note 13, at 56–8; and also Klein, supra note 21, at 387.

76 Verhoeven, supra note 13, at 57–8.

77 van Steenberghe, supra note 25, at 195.

78 Krajewski, supra note 11, at 191–3, envisaging the armed attack in terms of substantial involvement of the State, and continuing, 193–5, to reject safe haven as a ground for attribution, since the Court had denied (Nicaragua, supra note 8, at para. 195) that the provision of weapons, or logistical or other support, could be considered an armed attack.

79 Kowalski, supra note 17, at 106–7 and 109 (more broadly, 105–10); Ruys, supra note 2 at 490, observed that a complicating factor is that Art. 51 of the UN Charter obtains characteristics of both primary and secondary law.

80 Verhoeven, supra note 13, at 56–8; Byers, supra note 60, at 409–10, arguing that due to the contested character of the rule and the legal strategy of the US, the right to self-defence could be exercised against states which actively support or willingly harbour terrorists that already committed attacks; Feinstein, supra note 7, at 72–3, suggesting a harbour or harbour and support standard.

81 Ruys and Verhoeven, supra note 31, at 314–17, otherwise denying that moral support could qualify as aiding and abetting.

82 Nicaragua, supra note 8, at 103–4, para. 195.

83 Krajewski, supra note 11, at 191–2; Ruys and Verhoeven, supra note 31, at 303–4; Ruys, supra note 2, at 415–16.

84 Nicaragua, supra note 8, at 543 (Dissenting Opinion, Judge Jennings); Nicaragua, supra note 8, at 341–7, paras. 162–171 (more broadly, 331–47, paras. 154–71) (Dissenting Opinion, Judge Schwebel), discussing the preparatory works of the Definition of Aggression.

85 Dubuisson and Corten, supra note 57, at 55–65; Klein, supra note 21, at 370–4; Ruys, supra note 2, at 389 and 418; see also Corten, supra note 12, at 447–50.

86 Dubuisson and Corten, supra note 57, at 56; Klein, supra note 21, at 372, notes that support of terrorist groups, taken by itself, was rejected in the drafting process.

87 Corten, supra note 12, at 446–7. Supported by Ruys, supra note 2, at 387–90, indicating that relatively minor forms of assistance and certainly toleration would be insufficient to justify the invocation of self-defence, and suggesting that overall control of a state could transform the armed activities of a non-state actor into an armed attack by a state; Paust, supra note 55, at 433–4, making a distinction between general support for a non-state actor and support for specific armed attacks; Sicilianos, L.-A., ‘L’invocation de la légitime défense face aux activités d’entités non-étatiques’, (1989) 2 HYIL 147, at 153–4Google Scholar, had earlier raised the question whether the ICJ in Nicaragua had emptied ‘substantial involvement’ of its substance.

88 Genocide, supra note 49, at 202, para. 385, at 207–8, para. 398.

89 van Steenberghe, supra note 25, at 194–5.

90 Klein, supra note 21, at 389–90; Corten, supra note 12, at 450–4, invoking the Genocide case to deny the relevance of the overall control standard.

91 van Steenberghe, La légitime défense, supra note 9, at 311–12.

92 Affirming attribution: Murphy, supra note 7, at 50–1; seemingly, O’Connell, supra note 63, at 899–902; Stahn, supra note 7, at 47, on the basis of the overall control standard. Denying attribution: Krajewski, supra note 11, at 189–91 and 195, in relation to Art. 8 and 11 of the ASR; Stahn, supra note 7, at 37, on the basis of the effective control standard; Ruys and Verhoeven, supra note 31, at 300–1 and 313–14, in relation to Art. 8 and 11 of the ASR; Kowalski, supra note 17, at 113–18, in relation to Art. 8–11 of the ASR.

93 Genocide, supra note 49 at 204–6, paras. 390–5.

94 Ruys, supra note 2, at 490–3.

95 Krajewski, supra note 11, at 191–3, denying substantial involvement of Afghanistan in the 9/11 attacks; Dubuisson and Corten, supra note 57, at 54–64; Klein, supra note 21, at 370–4; Ruys, supra note 2, at 382–90.

96 The 1945 Statute of the International Court of Justice (1945), 39 AJIL, Supplement, 215–29.

97 For account of events, see Murphy, S. (ed.), ‘Terrorist Attacks on World Trade Center and Pentagon’, (2002) 96 AJIL 273Google Scholar; Schmidl, supra note 24, at 108–16; Wettberg, supra note 36, at 152–63; Ruys, supra note 2, at 394–406, 419–33, and 443–72; Corten, supra note 12, at 455–66; K. Szabó, ‘Anticipatory Action in Self-Defence, The Law of Self-Defence – Past, Present and Future’, (2010) Ph.D. thesis, University of Amsterdam,195–215 and 226–30; UK, Responsibility for the Terrorist Atrocities in the United States: BBC, The UK's bin Laden dossier in full, 4 October 2014, paras. 1, 4, 11–13, at <news.bbc.co.uk/2/hi/uk_news/politics/1579043.stm>; US, National Commission on Terrorist Attacks Upon the United States, The 9/11 Commission Report, especially 65–7 and 251–2, at <govinfo.library.unt.edu/911/report/911Report.pdf>.

98 SC Resolution 1368, preamble, para. 3; SC Resolution 1373, S/RES/1373 (2001), preamble para. 4, both available at <www.un.org/en/sc/documents/resolutions/2001.shtml>.

99 E.g., O’Connell, supra note 63, at 892; Zemanek, supra note 7, para. 15.

100 US, letter 7 October 2001, S/2001/746; UK, letter 7 October 2001, S/2001/747.

101 Murphy, supra note 7, at 46–50, arguing broad acceptance of the US claims compared to earlier incidents; Byers, supra note 60, at 405–10, arguing a change of customary international law to allow for self-defence against States that willingly harbour or actively support a terrorist group; Beard, supra note 68, at 561–78, contrasting the wide support or acceptance of the US response to earlier examples; Krajewski, supra note 11, at 195–9, using SC practice and NATO and OAS responses to argue on subjectivity of non-state actors; Stahn, supra note 7, at 36, arguing that criticism was based on other grounds than armed attack by non-state actors and that many supported or did not criticize; Printer, supra note 7, at 353–5, arguing the UN failed to renounce the US self-defence claim in relation to the 9/11 attacks; Langille, B., ‘It's “Instant Custom”: How the Bush Doctrine became Law after the Terrorists Attacks of September 11, 2001’, (2003) 26 BCI&CLR 145, at 151–6Google Scholar, arguing that the resolutions and their acceptance constitute instant custom.

102 Kreß, supra note 16, at 42–102, and 346–54 for his conclusions (English summary).

103 Corten, supra note 12, at 184; van Steenberghe, supra note 25, at 307; Ruys, supra note 2, at 464–6.

104 Corten, supra note 12, at 183; Wettberg, supra note 6, at 192–5; Ruys, supra note 2, at 447–9.

105 van Steenberghe, supra note 25, at 307–8; Ruys, supra note 2, at 466–8.

106 Ruys, supra note 2, at 468–9.

107 Ruys, supra note 2, at 469–71.

108 Corten, supra note 12, at 183–4 and 463–4; van Steenberghe, supra note 25, at 296–8; Wettberg, supra note 36, at 114–23; Schmidl, supra note 24, at 143–98; Ruys, supra note 2, at 449–57.

109 Wettberg, supra note 36, at 195–203; van Steenberghe, supra note 25, at 298–9.

110 Corten, supra note 12, at 184–5; van Steenberghe, supra note 25, at 303–6; Ruys, supra note 2, at 457–62.

111 Corten, supra note 12, at 185 and 464–5; van Steenberghe, 308–9; Ruys, supra note 2, at 462–4.

112 Ruys, supra note 2, at 471–2.

113 Verhoeven, supra note 13, at 62–4.

114 Ruys and Verhoeven, supra note 31, at 311–12; McKeever, supra note 7, at 384–5, adding that the 9/11 attacks involved some degree of state complicity; Kowalski, supra note 17, at 124, noting the resolutions do not refer to armed attack and cannot be read to conclusively affirm that non-state actors can be an autonomous source of armed attacks; Ruys, supra note 2, at 441, observing that no in depth discussion took place of the Taleban's responsibility for al-Qaeda's acts.

115 Corten, supra note 12, at 181–2; similarly, McKeever, supra note 7, at 384–5, invoking the World Summit Outcome, which stipulates that the provisions of the Charter are sufficient to address the full range of threats to the peace. GA Resolution 60/1, World Summit Outcome, A/RES/60/1, 12 September 2005, para. 79.

116 Ruys and Verhoeven, supra note 31, at 311–12; McKeever, supra note 7, at 384–5, adding that self-defence and non-state actors are not combined in one paragraph; Kowalski, supra note 17, at 124.

117 Corten, supra note 12, at 182.

118 Corten, supra note 12, at 45–6; similarly, Ruys, supra note 2, at 39 and 49.

119 See Legal Consequences for States of the Continued Presence of South Africa in Namibia (South West Africa) notwithstanding Security Council Resolution 276 (1970), Advisory Opinion of 21 June 1971, [1971] ICJ Rep. 16, at 53, para. 114; Accordance with International Law of the Unilateral Declaration of Independence in Respect of Kosovo, Advisory Opinion of 22 July 2010, [2010] ICJ Rep. 403, at 442, para 94; see generally Wood, M., ‘The Interpretation of Security Council Resolutions’, (1998) 2 MPYUNL 73Google Scholar; and K. Boon, Are Security Council acts relevant to the formation of customary international law?, 26 June 2014, at <opiniojuris.org/2014/06/26/security-council-acts-relevant-formation-customary-international-law/>.

120 Murphy, supra note 7, at 46, noted that the GA after 9/11 condemned the terrorist acts but did not characterize them as an armed attack and did not recognize the right of self-defence. By contrast, O’Connell, supra note 63, at 892, observed that the GA did not condemn the use of force against Afghanistan.

121 Nicaragua, supra note 8, at 99–100, para. 188, holding that opinio juris may be derived, with due caution, from the attitude of states towards certain General Assembly resolutions; Legality of the Threat or Use of Nuclear Weapons, supra note 26, at 226, para. 70, affirming the need to look at the content of GA resolutions and the conditions of their adoption.

122 Reisman, W.M., ‘International Legal Responses to Terrorism’, (1999) 22 HJIL 3, at 42–7Google Scholar; Murphy, supra note 7, at 50; Orr, supra note 10, at 740. See thorough discussion, in relation to necessity and proportionality of self-defence, in J. Green, The International Court of Justice and Self-Defence in International Law (2009), 63–76.

123 Antonopoulos, supra note 15, at 161, in relation to imminent armed attacks. Also critical Kolb, supra note 14, at 265–7; and T. Ruys, ‘Guest Post: Self-Defence and Non-State Actors in the Cold War Era – A Response to Marty Lederman’, 12 March 2015, available at <www.opiniojuris.org/2015/03/12/guest-post-self-defence-and-non-state-actors-in-the-cold-war-era-a-response-to-marty-lederman/>.

124 Antonopoulos, supra note 15, at 161.

125 Langille, supra note 101, at 154–6.

126 Krajewski, supra note 11, at 206–7; Verhoeven, supra note 13, at 63–4; Corten, supra note 12, at 40–1.

127 Michael, supra note 19, 150. ICJ, North Sea Continental Shelf case (Germany v. Denmark), Judgment of 20 February 1969, [1969] ICJ Rep. 3, at 43, para. 74.

128 Corten, supra note 12, at 40–1.

129 Ruys and Verhoeven, supra note 31, at 294–96; Tams, supra note 6, at 378–81, both taking the post-Cold War period as frame of reference.

130 Asylum case (Colombia v. Peru) Judgment of 20 November 1950, [1950] ICJ Rep. p. 266, at 276–7; Nicaragua, supra note 8, at 98, para. 186.

131 van Steenberghe, supra note 25, at 187–91. Ruys supra note 2, 469–71. Ruys notes that Ethiopia's use of force in Somalia should rather be seen as intervention by invitation.

132 Michael, supra note 19, at 146–7; and Ruys, supra note 2, at 424–6, both mentioning the bombing of the West Berlin disco in 1986 and the failed assassination attempt on former President Bush in 1993.

133 Nicaragua, supra note 8, at 108–9, para. 207.

134 Nicaragua, supra note 8, at 108–9, paras. 207–8. See Corten supra note 12, 31–2; Ruys, supra note 2, at 36, 38–9, and 40; and van Steenberghe, La légitime défense, supra note 9, at 135.

135 Corten, supra note 12, at 461–2, noting that NATO's declaration invoking Art. 5 of its constituent treaty was premised on the attacks being ‘directed from abroad’, and suggesting, 463–4, attribution of Hezbollah actions to Lebanon because of its close ties to and members within the Lebanese government; Ruys, supra note 2, at 454–5, denying attribution of Hezbollah actions, and noting, 459, that the connection between the PKK and Iraqi authorities remained below the Nicaragua and Tadić thresholds and that no active support was given.

136 Corten, supra note 12, at 462–3.

137 Ruys, supra note 2, at 439–42, but denying attribution and claiming Taleban support remained below the threshold of substantial involvement. Rather, so he claims, 440, the US position was premised on a harbouring doctrine.

138 Corten, supra note 12, at 463–4.

139 Ruys, supra note 2, at 455–7.

140 Corten, supra note 12, at 462.

141 Ruys, supra note 2, at 449.

142 Corten, supra note 12, at 462, further asserting that some states were pressured into support. Ruys and Verhoeven, supra note 31, at 292–4, note that in the decolonisation period condemnations were politically motivated rather than based on legal grounds; Trapp, supra note 17, at 153, observing that condemnations of Israel were due to concerns for the Middle East peace process and the implementation of the Roadmap.

143 Corten, supra note 12, at 462; Ruys, supra note 2, at 441–2, pointing to Ratner's qualification of the Eiffel Tower factor.

144 Stahn, supra note 7, at 36; Trapp, supra note 17, at 153–5, and noting, 152–3, that states did not comment on the legal issue of action in self-defence targeting terrorist bases in states complicit with or acquiescing in terrorist operations.

145 Wettberg, supra note 36, at 71–2, makes this argument in general; Ruys, supra note 2, at 451–5, but does observe, 455, that States did not explain why the right to self-defence was applicable in the circumstances, no opinions were expressed on the legality of self-defence against attacks by non-state actors, and no reference was made to Resolutions 1368 and 1373; see generally van Steenberghe, La légitime défense, supra note 9, at 167–8.

146 Corten, supra note 12, at 463–4.

147 Verhoeven, supra note 13, at 62–3; Stahn, supra note 7, at 36; Printer, supra note 7, 353–5, discussing possible limitation of self-defence as a result of measures by the Council; Ruys and Verhoeven, supra note 31, at 296 and 314, in relation to cross-border military action by Burundi and the US.

148 Security Council Resolution 1267, S/RES/1267 (1999), Preamble paras. 5–7 and paras. 1–2, available at <www.un.org/en/ga/search/view_doc.asp?symbol=S/RES/1267(1999)>.

149 Corten, supra note 12, at 182–3.

150 Ruys, supra note 2, at 459–60.

151 Ruys, supra note 2, at 461–2.

152 Ruys, supra note 2, at 404–5, 428 and 471.

153 Corten, supra note 12, at 465; and also Ruys, supra note 2, at 433, regarding Iranian incursions into Iraq.

154 van Steenberghe, supra note 25, at 187; see also Corten, supra note 12, at 38–40.

155 Beard, supra note 68, at 560–6, mentioning both the Charter and customary international law; Ruys and Verhoeven, supra note 31, at 292–8, only on the last page mentioning customary international law; Trapp, 145–155, only referencing the Court's statements on the customary self-defence requirements of necessity and proportionality; Tams, 378–82, coming to the conclusion that ‘the current law’ is ‘in a state of flux’; Michael, 145–55, mentioning both Art. 31(3)(c) VCLT and customary international law.

156 Ibid. Exceptionally: Antonopoulos, supra note 15, at 160; Wettberg, supra note 36, at 67–8, 70–72; Corten, supra note 12, at 4–49; Ruys, supra note 2, at 6–52; van Steenberghe, La légitime défense, supra note 9, at 141–81.

157 But see Wettberg, supra note 36, at 67–8, 70–2; Corten, supra note 12, at 4–49; Ruys, supra note 2, at 6–52; van Steenberghe, La légitime défense, supra note 9 141–81.

158 Michael, supra note 19, at 145, adding that practice may be relevant to a rule of customary international law independently; Ruys, supra note 2, at 19–22, who also contemplates, 22–9, a modification of the Charter through customary international law; van Steenberghe, supra note 25, at 141–7, discusses practice modifying a treaty independently from a rule of customary international law.

159 Verhoeven, supra note 13, at 52; van Steenberghe, supra note 25, at 185–7; Schmidl, supra note 24, at 43 and 108; and van Steenberghe, La légitime défense, supra note 9, at 141–3.

160 Ibid. Exceptionally: van Steenberghe, supra note 25, at 185– 7.

161 Contrary, van Steenberghe, supra note 25, at 185–187, who claims that keeping the analysis separate in order to determine an interpretation or a rule of customary international law is not strictly required, and he concludes, suggesting the similarity of the processes concerned, that ‘the evolution of the law of self-defence through state practice merely requires that this practice be followed in application of this law and be constant and general.’

162 ILC Commentary, supra note 28, at 220.

163 See also Ruys, supra note 2, at 32–3, pointing to the increased weight of physical practice accompanied by legal justification. For early reference to opinio juris, see Asylum case, supra note 130, at 276–7; and North Sea Continental Shelf case, supra note 127, at 43–5.

164 Ruys, supra note 2, at 30–1. Corten, supra note 12, at 29–34, and Ruys, supra note 2, at 34–42, reference Nicaragua, supra note 8, at 108–9, para. 207 (emphasis added): ‘[t]he significance for the Court of cases of State conduct prima facie inconsistent with the principle of non-intervention lies in the nature of the ground offered as justification. Reliance by a State on a novel right or an unprecedented exception to the principle might, if shared in principle by other States, tend towards a modification of customary international law.’

165 Ruys and Verhoeven, supra note 31, at 292–8; Tams, supra note 6, at 362–92.

166 Corten, supra note 12, at 4–49; Ruys, supra note 2, at 6–52; van Steenberghe, supra note 25, at 141–81.

167 van Steenberghe, supra note 25, at 185–202.

168 Corten, supra note 12, at 179–86.

169 Corten, supra note 12, at 450–4 and 460–6.

170 Ruys, supra note 2, at 487.

171 US, letter 23 September 2014, S/2014/695. For recent comment, K. Heller, ‘The Seemingly Inexorable March of “Unwilling or Unable” Through the Academy’, 6 March 2015, <opiniojuris.org/2015/03/06/the-seemingly-inexorable-march-of-unwilling-or-unable-through-the-academy/>.

172 Iraq, letter 22 September 2014, S/2014/691; also Iraq, letter 25 June 2014, S/2014/440. Note that Iraq has not declared itself the victim of an armed attack and does not invoke self-defence. In Nicaragua, supra note 8, at 103–5, paras. 195–200, the ICJ held that in case of collective self-defence one would expect a state to declare itself to be the victim of an armed attack, that a request to a third State is made to assist in collective self-defence, and that the absence of a report to the Security Council may indicate that the victim State may not have been convinced that it was acting in self-defence. See comment: M. Weller, ‘Striking ISIL: Aspects of the Law on the Use of Force, ASIL Insight’, 11 March 2015, <www.asil.org/insights/volume/19/issue/5/striking-isil-aspects-law-use-force>.

173 G. Nolte, First report on subsequent agreements and subsequent practice in relation to treaty interpretation, 19 March 2013, A/CN.4/660; G. Nolte, Second report on subsequent agreements and subsequent practice in relation to the interpretation of treaties, 26 March 2014, A/CN.4/671; M. Wood, First report on identification and evidence of customary international law, 17 May 2013, A/CN.4/663; M. Wood, Second report on identification of customary international law, 22 May 2014, ACN.4/672.

174 J. d’Aspremont, ‘Customary International Law as a Dance Floor: Part II,’ 15 April 2014, <www.ejiltalk.org/customary-international-law-as-a-dance-floor-part-ii/>.