¹ On which see, generally, Jennings, R., ‘The Caroline and McLeod Cases’, (1938) 32 AJIL 82 CrossRefGoogle Scholar.

² T. Ruys, ‘Armed Attack’ and Article 51 of the UN Charter (2010), at 488.

³ The discussion assumes that non-state actors operate in a cross-border fashion, from the territory of a different state. States are not prohibited by international law to use force within their own borders to tackle these threats, within limits imposed by human rights law and, if and to the extent applicable, humanitarian law.

⁴ See, e.g., Legal Consequences of the Construction of a Wall in the Occupied Palestinian Territory, Advisory Opinion of 9 July 2004, [2004] ICJ Rep. 136 (Palestinian Wall), at 194, para. 139.

⁵ As asserted by the International Court of Justice in Military and Paramilitary Activities in and Against Nicaragua (Nicaragua v. United States of America), Merits, Judgment of 27 June 1986, [1986] ICJ Rep. 14 (Nicaragua), at 103, para. 195.

⁶ For a review of practice, see Ruys, supra note 2, at 447–72; R. Van Steenberghe, La légitime défense en droit international public (2012), at 292–325; O. Corten, Le droit contre la guerre (2014), at 743–53.

⁷ UN Security Council, Resolution 1368 (2001), UN Doc. S/RES/1368 (2001). The literature on the US's exercise of self-defence in Afghanistan is vast. For a review of the facts and the main scholarly approaches, see T. Becker, Terrorism and the state: Rethinking the Rules of State Responsibility (2006), Ch. 6.

⁸ See summary record of the SC meeting of 20 November 2015, UN Security Council, 7565^th Meeting, UN Doc. S/PV.7565, at 2 (France), 4 (US), 9 (UK). See also the following letters to the Security Council: Letter dated 23 September 2014 from the Permanent Representative of the United States of America to the United Nations addressed to the Secretary-General, UN Doc. S/2014/695 (US); Identical letters dated 25 November 2014 from the Permanent Representative of the United Kingdom of Great Britain and Northern Ireland to the United Nations addressed to the Secretary-General and the President of the Security Council, UN Doc. S/2014/851 (UK); Identical letters dated 8 September 2015 from the Permanent Representative of France to the United Nations addressed to the Secretary-General and the President of the Security Council, UN Doc. S/2015/745 (France). For scholarly analyses of the situation in Syria, see: Arimatsu, L. and Schmitt, M., ‘Attacking “Islamic state” and the Khorasan Group: Surveying the International Law Landscape’, (2014) 53 Columbia Journal of Transnational Law Bulletin 1 Google Scholar; Hakimi, M., ‘Defensive Force against Non-State Actors: The state of Play’, (2015) 91 International Legal Studies 1 Google Scholar (Defensive Force). The matter has also been addressed in numerous blog posts, e.g., J.D. Ohlin, ‘The Unwilling or Unable Doctrine Comes to Life’, Opinio Juris, 23 September 2014, available at www.opiniojuris.org/2014/09/23/unwilling-unable-doctrine-comes-life/; M. Hakimi, ‘Assessing (Again) the Defensive Operations in Syria’, Just Security, 22 January 2015, available at www.justsecurity.org/19313/assessing-again-defensive-operations-syria/; C. Kreß, ‘The Fine Line Between Collective Self-Defense and Intervention by Invitation: Reflections on the Use of Force against “IS” in Syria’, Just Security, 17 February 2015, available at www.justsecurity.org/20118/claus-kreb-force-isil-syria; R. Van Steenberghe, ‘From Passive Consent to Self-Defence after the Syrian Protest against the US-led Coalition’, EJIL: Talk!, 23 October 2015, available at www.ejiltalk.org/13758-2/.

⁹ There are some methodological debates as to whether the practice should prove a modification of the rule or the existence of a new rule, on which see Corten, supra note 6, at 35–63; Van Steenberghe, R., ‘State Practice and the Evolution of the Law of Self-Defence: Clarifying the Methodological Debate’, (2015) 2 JUFIL 81 Google Scholar.

¹⁰ Ruys, supra note 2, at 487–9, 531.

¹¹ Ibid., at 531. The expression is taken from a paper of the UK Foreign and Commonwealth Office, titled ‘Is intervention ever justified’, Foreign Policy Document no 148, 1986, reprinted in ‘UK Materials in International Law’, (1986) 57 BYIL 614. For a similar position, see Antonopoulos, C., ‘Force by Armed Groups as Armed Attack and the Broadening of Self-Defence’, (2008) 55 NILR 159 CrossRefGoogle Scholar.

¹² Ruys, T. and Verhoeven, S., ‘Attacks by Private Actors and the Right of Self-Defence’, (2005) 10 Journal of Conflict & Security Law 289 CrossRefGoogle Scholar, at 310.

¹³ Trapp, K., ‘The Use of Force against Terrorists: A Reply to Christian J Tams’, (2009) 20 EJIL 1049 CrossRefGoogle Scholar, at 1050.

¹⁴ United States v. von Weizsaecker et al. (Ministries trial) (1949) 14 NMT 314, at 329. For an earlier statement of this principle, see The Maria (1799) 1 C Rob 340, at 361 (‘it is a wild conceit, that wherever force is used, it may be lawfully resisted. A lawful force cannot be lawfully resisted’). See also N. Lubell, Extraterritorial Use of Force Against Non-State Actors (2010), at 41; Y. Dinstein, War, Aggression and Self-Defence (2012), at 190.

¹⁵ For this reason, the encroachment upon the host state's rights must be justified (namely, rendered lawful) and not just excused. Excuse defences exclude the consequences of unlawfulness (e.g., cessation and reparation), without having an effect on the illegality of the relevant conduct. If the infringement of host state rights were merely ‘excused’, this would mean that the victim state may not owe reparations to the host state, but it would not be enough to prevent the host state's right of self-defence from being triggered; the use of force in its territory would remain unlawful and, if it reaches the required gravity, it could constitute an armed attack. On the distinction between justification and excuse in international law, see: Lowe, V., ‘Precluding Wrongfulness or Responsibility: A Plea for Excuses?’, (1999) 10 EJIL 405 CrossRefGoogle Scholar; Christakis, T., ‘Les “circonstances excluant l'illicéité”: une illusion optique?’, in Corten, O. et al. (eds.), Droit du pouvoir, pouvoir du droit: Mélanges offerts à Jean Salmon (2007), 223 Google Scholar; G. Scalese, La rilevanza delle scusanti nella teoria dell'illecito internazionale (2008); F. Paddeu, General Defences in International Law: Justification and Excuse in the Law of State Responsibility (2013), Ch. 3, Unpublished Doctoral Dissertation, University of Cambridge.

¹⁶ See Ruys, T. and Verlinden, N., ‘Digest of State Practice: 1 July-31 December 2014’, (2015) 2 JUFIL 119 Google Scholar, at 135. According to the Syrian Minister of National Reconciliation Affairs, ‘[a]ny action of any type without the approval of the Syrian government is aggression against Syria’, ibid.

¹⁷ See, e.g., the letters sent to the UN Security Council by: Canada: Letter dated 24 October 2001 from the Chargé d'affaires a.i. of the Permanent Mission of Canada to the United Nations addressed to the President of the Security Council, UN Doc. S/2001/1005; the European Union: Letter dated 8 October 2001 from the Permanent Representative of Belgium to the United Nations addressed to the Secretary-General, UN Doc. S/2001/967; Germany: Letter dated 29 November 2001 from the Permanent Representative of Germany to the United Nations addressed to the President of the Security Council, UN Doc. S/2001/1127; and New Zealand: Letter dated 17 December 2001 from the Permanent Representative of New Zealand to the United Nations addressed to the President of the Security Council, UN Doc. S/2001/1193.

¹⁸ See the references in supra note 7.

¹⁹ For an example of this differential treatment, see Becker Lorca, A., ‘Rules for the “Global War on Terror”: Implying Consent and Presuming Conditions for Intervention’, (2012) 45 NYU Journal of International Law and Politics 1 Google Scholar.

²⁰ Even in this case, the interference with the target state's rights of territorial sovereignty and non-intervention, among others, require legal justification, as will be explained in Section 2.

²¹ On which see, generally, Deeks, A., ‘“Unwilling or Unable”: Toward a Normative Framework for Extraterritorial Self-Defense’, (2012) 52 Virginia Journal of International Law 483 Google Scholar. See also Reinold, T., ‘State Weakness, Irregular Warfare, and the Right of Self-Defense Post-9/11’, (2011) 105 AJIL 244 CrossRefGoogle Scholar.

²² Ruys, T., ‘Crossing the Thin Blue Line: An Inquiry into Israel's Recourse to Self-Defense Against Hezbollah’, (2007) 43 Stanford Journal of International Law 265 Google Scholar, 283 (noting that state involvement with non-state actors ‘is not easy to assess [and] ultimately boils down to a factual and contextual assessment’).

²³ See infra Section 3.2.2.

²⁴ Pursuant to Art. 51: ‘Nothing in the present Charter shall impair the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security. Measures taken by Members in the exercise of this right of self-defence shall be immediately reported to the Security Council and shall not in any way affect the authority and responsibility of the Security Council under the present Charter to take at any time such action as it deems necessary in order to maintain or restore international peace and security’, 1945 Charter of the United Nations, 1 UNTS 16.

²⁵ See, e.g., Murphy, S., ‘Terrorism and the Concept of “Armed Attack” in Article 51 of the UN Charter’, (2002) 43 Harvard Interational Law Journal 42 Google Scholar; Van Steenberghe, R., ‘Self-Defence in Response to Attacks by Non-State Actors in the Light of Recent Practice: A Step Forward?’, (2010) 23 LJIL 183 CrossRefGoogle Scholar; Ratner, S., ‘Self-Defence Against Terrorists: The Meaning of Armed Attack’, in van den Herik, L. and Schrijver, N. (eds.), Counter-Terrorism Strategies in a Fragmented International Legal Order - Meeeting the Challenges (2013), 334 CrossRefGoogle Scholar.

²⁶ Van Steenberghe usefully reviews the arguments made under several of the ARS rules on attribution in Van Steenberghe, supra note 6, at 311–23.

²⁷ Articles on the Responsibility of States for Internationally Wrongful Acts (hereinafter ARS and Commentary) Annexed to UN Doc. A/RES/56/83 (2002).

²⁸ On this provision see, generally, Thouvenin, J.M., ‘Self-Defence’, in Crawford, J. et al. (eds.), The Law of International Responsibility (2010), 455 Google Scholar; J. Crawford, State Responsibility: The General Part (2013); Paddeu, F., ‘Self-Defence as a Circumstance Precluding Wrongfulness: Understanding Article 21 of the Articles on State Responsibility’, (2015) 86 BYIL 90 Google Scholar.

²⁹ As an example, the recent Oxford Handbook on the Use of Force in International Law, edited by Marc Weller, and containing over 50 chapters, does not contain a chapter on Article 21.

³⁰ See, e.g., Christakis, T. and Bannelier, K., ‘La légitime défense en tant que “circonstance excluant l'illicéité”’, in Kherad, R. (ed.), Légitimes défenses (2007) 233 Google Scholar; Christakis, T. and Bannelier, K., ‘La légitime défense a-t-elle sa place dans un code sur la responsabilité internationale?’, in Constantinides, A. and Zaikos, N. (eds.), The Diversity of International Law: Essays in Honour of Professor Kalliopi K Koufa (2009), 519 CrossRefGoogle Scholar. These authors consider that Article 21 is ‘useless’ (they use the term ‘inutile’ in French) insofar as the relations between the victim and aggressor states are concerned.

³¹ For this, see Deeks, supra note 21.

³² The state of war consisted of the situation, condition or status during which the extraordinary law of war substituted the law of peace in the regulation of the relations between the parties to the conflict, see Wright, Q., ‘When does War Exist?’, (1932) 26 AJIL 362 CrossRefGoogle Scholar, at 363.

³³ S. Neff, War and the Law of Nations (2005), at 177.

³⁴ See, e.g., North Atlantic Coast Fisheries Case (Great Britain/US) (1910) 11 RIAA 167, at 181 (‘International law in its modern development recognizes that a great number of Treaty obligations are not annulled by war, but at most suspended by it’). The doubts surrounding this question in the early twentieth century were described by Hurst, C., ‘The Effect of War on Treaties’, (1921) 2 BYIL 37 Google Scholar.

³⁵ For a summary of the scholarly debate see Neff, supra note 33, at 335–40.

³⁶ See the exhaustive review of practice in M. Mancini, Stato di guerra e conflitto armato nel diritto internazionale (2009), Ch. 4. See also, for practice related to commercial and economic relations, S. Silingardi, Gli effetti giuridici della guerra sui rapporti economici e commerciali (2012).

³⁷ ARS, supra note 27, Art. 21 Commentary, para. 2. See also J. Crawford, Second Report on State Responsibility, UN Doc. A/CN.4/498 and Add.1–4 (1999), at 75, para. 299.

³⁸ Including treaties, at least as a matter of principle. See Art. 3 of the ILC's Articles on the Effects of Armed Conflict on Treaties, with Commentaries, Report of the ILC on the work of its sixty-third session, UN Doc. A/66/10 (2011).

³⁹ Nicaragua, Memorial of Nicaragua, 30 April 1985, ICJ Pleadings, vol. IV, at 115; Border and Transborder Armed Actions (Nicaragua v. Honduras), ICJ, Application instituting proceedings, 28 July 1986, at 5–7; Border and Transborder Armed Actions (Nicaragua v. Costa Rica), ICJ, Memorial of Nicaragua, 19 August 1987, at 109; Armed Activities on the Territory of the Congo (DRC v. Burundi), ICJ, Application instituting proceedings, 23 June 1999, at 15–19; Armed Activities on the Territory of the Congo (DRC v. Rwanda), ICJ, Application instituting proceedings, 23 June 1999, at 16–19; Aerial Incident of 10 August 1999 (India v. Pakistan), ICJ, Application instituting proceedings, 21 September 1999, at sec. II; Land and Maritime Boundary between Cameroon and Nigeria (Cameroon v. Nigeria: Equatorial Guinea intervening), ICJ, Cameroon statement, CR 2002/7, at 36, para. 7; Legality of the Use of Force (Federal Republic of Yugoslavia v. Belgium), Preliminary Objections, Judgment of 15 December 2004, [2004] ICJ Rep. 279, at 283, para. 1; Certain Activities Carried out by Nicaragua in the Border Area (Costa Rica v. Nicaragua), ICJ, Application instituting proceedings, 18 November 2010, at 4.

⁴⁰ Nicaragua, Memorial of Nicaragua, supra note 39, at 120; Nicaragua v. Honduras, Application instituting proceedings, supra note 39, 5–7; Cameroon v. Nigeria, CR 2002/7, supra note 39, at 36, para. 7; DRC v. Burundi, Application instituting proceedings, supra note 39, at 15–19; DRC v. Rwanda, Application instituting proceedings, supra note 39, at 16–19; Certain Activities (Nicaragua v. Costa Rica), Memorial of Nicaragua, supra note 39, at 55ff.

⁴¹ Nevertheless, ambiguities and differences of opinion remain about the meaning and scope of the various conditions and requirements of the exercise of the right.

⁴² See, e.g., Murphy, supra note 25, at 44.

⁴³ The expression is taken from: Baxter, R., ‘International Law in “Her Infinite Variety”’, (1980) 29 ICLQ 549 CrossRefGoogle Scholar.

⁴⁴ ARS, supra note 27, Art. 21 Commentary, para. 4. The expression is the ICJ's: Legality of the Threat or Use of Nuclear Weapons, Advisory Opinion of 8 July 1996, [1996] ICJ Rep. 226 (Nuclear Weapons), at 242, para. 30.

⁴⁵ It may be worthwhile to recall that, as clarified by the ICJ in Nicaragua, Art. 51 is not a complete statement of the law of self-defence as this exists in customary law. Customary law (to which a renvoi is made by the reference in Art. 51 to the ‘inherent right’) complements the Charter provision. See Nicaragua, supra note 5, at 94, para. 176.

⁴⁶ For a thorough review of the relevant practice, see Corten, supra note 6, 341–59. A significant number of judges and scholars support the peremptory status of the prohibition of force: Nicaragua, supra note 5, sep. op. President Singh, at 153; sep. op. Judge Sette-Camara, at 199; Oil Platforms (Islamic Republic of Iran v. United States of America), Merits, Judgment of 6 November 2003, [2003] ICJ Rep. 161, sep. op. Judge Kooijmans, at 260, para. 46; diss. Op. Judge Elaraby, at 291; sep. op. Judge Simma, at 327, para. 6; Palestinian Wall, supra note 4, sep. op. Judge Elaraby, at 254; Ruys, supra note 2, at 27; O. Dörr and A. Randelzhofer, ‘Article 2(4)’, in B. Simma et al. (eds.), The Charter of the United Nations: A Commentary (2012) vol 1, 200, at 231; Van Steenberghe, supra note 6, at 137–40.

⁴⁷ There are other exceptions too, see Helmersen, S., ‘The Prohibition of the Use of Force as Jus Cogens: Explaining Apparent Derogations’, (2014) 61 NILR 167 CrossRefGoogle Scholar.

⁴⁸ ARS, supra note 27, Art. 21 Commentary, para. 1. This reading was endorsed by Judge Tomka in Case Concerning Armed Activities on the Territory of the Congo (DRC v. Uganda), Merits, Judgment of 19 December 2005, [2005] ICJ Rep. 168, Declaration Tomka, at 353–4.

⁴⁹ See e.g., Ethiopia: UN Doc. A/C.6/35/SR.51, at 12, para. 46; Hungary: UN Doc. A/C.6/35/SR.55, at 12, para. 45; Mongolia: UN Doc. A/C.6/35/SR.53, at 8, paras. 29–30; Spain: UN Doc. A/C.6/35/SR.55, at 4, para. 11; Trinidad and Tobago: UN Doc. A/C.6/35/SR.56, at 6, para. 26; (then) USSR: UN Doc. A/C.6/35/SR.52, at 14, para. 63. On this point, see Crawford, supra note 37, at 74–5.

⁵⁰ Kammerhofer describes this approach as a ‘gap’ in the prohibition of force: J. Kammerhofer, Uncertainty in International Law: A Kelsenian Perspective (2011), at 9.

⁵¹ E.g., Fragmentation of International Law: Difficulties Arising from the Diversification and Expansion of International Law, Report of the Study Group of the International Law Commission, UN Doc. A/CN.4/L.682, 13 April 2006, at 52–3, para. 95; J. Green, ‘Questioning the Peremptory Status of the Prohibition of the Use of Force’, (2011) 32 Michigan Journal of International Law 215; de Hoogh, A., ‘ Jus Cogens and the Use of Armed Force’, in Weller, M. (ed.), Oxford Handbook of the Use of Force in International Law (2015), 1161 Google Scholar, at 1172–5.

⁵² As confirmed by ARS, supra note 27, Art. 26: ‘The wrongfulness of any act of a State which is not in conformity with an obligation arising under a peremptory norm of general international law.’

⁵³ Assuming that conflicts between peremptory rules are resolved by reference to the principles of priority.

⁵⁴ Van Steenberghe, supra note 6, at 118.

⁵⁵ See references at supra note 47. It may be added that distinguishing ‘aggression’ from ‘force’ is not a simple task either in theory or in practice, on which see C. Díaz Barrado, El consentimiento, causa de exclusión de la ilicitud del uso de la fuerza en derecho internacional (1989), 74–5.

⁵⁶ See further, Paddeu, supra note 28, at Section II.

⁵⁷ From the standpoint of Charter law, it could be argued that the language of Art. 51 does indeed authorize the impairment of other Charter-rights of the aggressor state. Thus, Art. 51 states that ‘nothing in the present Charter shall impair. . .’. Nevertheless, Art. 51 cannot authorize the impairment of customary rights. Art. 103 of the Charter is not helpful in this regard. To begin with, it is limited to other conventional rights – though it seems logical that it may extend to customary rights as well. At any rate, Art. 103 gives priority to obligations arising under the Charter and not to rights arising thereunder.

⁵⁸ See, e.g., Nicaragua's claims at: Memorial of Nicaragua, Nicaragua, ICJ Pleadings, vol IV, 110–11, and oral statement, ICJ Pleadings, vol V, 210–12. For Iran's claims, see Oil Platforms, supra note 46, at 166, para. 1; also at 176, para. 26.

⁵⁹ See, e.g., Memorial of Iran, Aerial Incident of 3 July 1988 (Iran v. USA), at 146 (breach of the Chicago Convention), 182 (breach of the Treaty of Amity), 238 (both).

⁶⁰ See, e.g., Guyana v. Suriname, Award, 17 September 2007, UNCLOS Annex VII Tribunal, at 147, para. 445, and dispositif at 165, para. 2, on the obligation to solve disputes peacefully contained in Art. 279 of the 1982 United Nations Convention on the Law of the Sea (UNCLOS), 1833 UNTS 397.

⁶¹ Oil Platforms (Islamic Republic of Iran v. United States of America), Preliminary Objection Judgment of 12 December 1996, [1996] ICJ Rep. 803, at 811–12, para. 21: ‘The Treaty of 1955 imposes on each of the Parties various obligations on a variety of matters. Any action by one of the Parties that is incompatible with those obligations is unlawful, regardless of the means by which it is brought about. A violation of the rights of one party under the Treaty by means of the use of force is as unlawful as would be a violation by administrative decision or by any other means.’

⁶² Application of the Convention on the Prevention and Punishment of the Crime of Genocide (Croatia v. Serbia), Merits, Judgment of 3 February 2015, para. 474 (not yet reported).

⁶³ For additional examples in practice, see Paddeu, supra note 28.

⁶⁴ Nicaragua, supra note 5, at 126, paras. 247–9, on prohibition of intervention; at 128, para. 251, on territorial sovereignty.

⁶⁵ DRC v. Uganda, supra note 48, at 227, para. 165, and at 280, para. 345.1. Note that the Court only refers to the principle of non-use of force and the principle of non-intervention in the dispositif, even though the DRC had claimed also a breach of its territorial sovereignty and the Court had addressed it in its reasoning. For the DRC's prayer for relief, see Memorial of the DRC, at 273.

⁶⁶ 1957 Iran-US Treaty of Amity, Economic Relations and Consular Rights, 284 UNTS 93.

⁶⁷ Oil Platforms (Islamic Republic of Iran v. United States of America), Rejoinder Submitted by the United States of America, 23 March 2001, at 141, para. 5.02. The US's argument on this point changed throughout the pleadings, for a brief summary (in what concerns the point of Art. 21), see Paddeu, supra note 28.

⁶⁸ Oil Platforms (Islamic Republic of Iran v. United States of America), Iran statement, CR 2003/5, at 41, para. 29 (references omitted).

⁶⁹ Oil Platforms, Iran statement, CR 2003/7, at 51, para. 3.

⁷⁰ Nuclear Weapons, supra note 44, at 242, para. 30.

⁷¹ ARS, supra note 27, Art. 21 Commentary, para. 4.

⁷² The term is, once again, taken from the ICJ's judgment in Nuclear Weapons, supra note 44, at 242, para. 30.

⁷³ ARS, supra note 27, Art. 21 Commentary, para. 4.

⁷⁴ Oil Platforms, Iran statement, CR 2003/5, at 41, para. 29. (references omitted).

⁷⁵ DRC v. Uganda, Uganda Memorial, at 232–3, paras. 5.93–5.94.

⁷⁶ ARS, supra note 27, Art. 21 Commentary, para. 5.

⁷⁷ Commentary to Art. 34, ILC, Report of the Commission on the work of its thirty-second session, UN Doc. A/35/10 (1980), at 61, para. 28.

⁷⁸ Ibid.

⁷⁹ Crawford, supra note 37, at 76, para. 302.

⁸⁰ ILC, Report of the Commission on the work of its fifty-first session, UN Doc. A/54/10 (1999), at 77, para. 321.

⁸¹ ARS, supra note 27, Art. 21 Commentary, para. 5.

⁸² Art. 21 requires compatibility both with the jus ad bellum and with the jus in bello, see Commentary, para. 6. The discussion in this article will only consider the issue from the standpoint of the jus ad bellum.

⁸³ ARS, supra note 27, Art. 21 Commentary, para. 5.

⁸⁴ Through whichever rule of attribution recognized in the ARS or, even, a lower standard of attribution recognized in the primary law, on which see Crawford, General Part, supra note 28, at 158. See also Tams, C., ‘The Use of Force against Terrorists’, (2009) 20 EJIL 359, at 385–8Google Scholar; G. Nolte and A. Randelzhofer, ‘Article 51’, in B. Simma et al. (eds.), The Charter of the United Nations: A Commentary (2012) vol. 2, 1397, at 1416; Corten, supra note 6, at 717–58. See also Van Steenberghe, supra note 6, 311–23, for a review of relevant arguments.

⁸⁵ On the distinction between self-defence against a state or within the state, see Trapp, K., ‘Back to Basics: Necessity, Proportionality, and the Right of Self-Defence against non-State Terrorist Actors’, (2007) 56 ICLQ 141 CrossRefGoogle Scholar, at 142; Lubell, supra note 14, at 36. For a contrary view, see O'Connell, M.E., ‘Dangerous Departures’, (2013) 107 AJIL 380, at 383 Google Scholar.

⁸⁶ This interpretation of Art. 51 and the right of self-defence is accepted here for the sake of argument. As noted in the introduction, however, it is important to recall that whether this is accepted as a matter of customary law is debatable.

⁸⁷ Each of these approaches will be presented in its essential form, in a way which it is believed will be more or less acceptable to all scholars maintaining it. Of course, there may be variations in the details across the writings of various scholars, though these are immaterial to the point being made in this article.

⁸⁸ Hakimi, Defensive Force, supra note 8, at 3–4.

⁸⁹ Trapp, supra note 85, at 146–7; Van Steenberghe, supra note 25, at 199–202; Deeks, supra note 21, at 494–5; Tams, C. and Devaney, J., ‘Applying Necessity and Proportionality to Anti-Terrorist Self-Defence’, (2012) 45 Israel Law Review 91 CrossRefGoogle Scholar, at 98–101; Akande, D. and Liefländer, T., ‘Clarifying Necessity, Imminence, and Proportionality in the Law of Self-Defence’, (2013) 107 AJIL 563 Google Scholar, at 564–6.

⁹⁰ Gray, C., ‘The Limits of Force’ (2016) 376 RCADI 93 Google Scholar, at 111.

⁹¹ E.g., Wolfrum, R. and Philipp, C., ‘The Status of the Taliban: Their Obligations and Rights under International Law’, (2002) 6 Max Planck Yearbook of United Nations Law 559 CrossRefGoogle Scholar, at 594–5; Wolfrum, R., ‘The Attacks of September 11, 2001, the Wars Against the Taliban and Iraq: Is There a need to Reconsider International Law on the Recourse to Force and the Rules in Armed Conflict?’, (2003) 7 Max Planck Yearbook of United Nations Law 1 CrossRefGoogle Scholar, at 37–8; Ruys and Verhoeven, supra note 12, at 315–17. For a summary and critique of this approach see K. Trapp, State Responsibility for International Terrorism (2011), 58–61.

⁹² ARS, supra note 27, Art. 16 Commentary, para. 10. In the literature, see Graefrath, B., ‘Complicity in the Law of International Responsibility’, (1996) 29 RBDI 370, at 371 Google Scholar (complicity ‘constitutes itself an internationally wrongful act of the State . . . [i]t does not create a kind of co-responsibility in another State's responsibility . . . [i]t has its own identity as a separate violation of international law’).

⁹³ See, generally, H. Aust, Complicity and the Law of State Responsibility (2011), at 269–86.

⁹⁴ Becker, supra note 7, at 224–5.

⁹⁵ Corfu Channel (UK v. Albania), Compensation Assessment, Judgment of 15 December 1949, [1949] ICJ Rep. 4, at 22. See also, in the context of terrorism, General Assembly, Declaration on Principles of International Law concerning Friendly Relations and Co-operation among States in accordance with the Charter of the United Nations, UN Doc. A/RES/25/2625 (XXV) (24 October 1970). On this obligation see, generally, Trapp, supra note 91, at 64–82.

⁹⁶ On due diligence obligations see Pisillo Mazzeschi, R., ‘The Due Diligence Rule and the Nature of the International Responsibility of States’ (1992) 35 German Yearbook of International Law 5 Google Scholar.

⁹⁷ See, e.g., Brown, D., ‘Use of Force against Terrorism after September 11th: State Responsibility, Self-Defense and Other Responses’, (2003) 11 Cardozo Journal of International and Comparative Law 1 Google Scholar, at 15, 30–2; T. Gazzini, The Changing Rules on the Use of Force in International Law (2005), 189; Reinold, supra note 21, at 284; Trapp, K., ‘Can Non-State Actors Mount an Armed Attack?’, in Weller, M. (ed.), Oxford Handbook of the Use of Force in International Law (2015), 679 Google Scholar, at 695; Moir, L., ‘Action Against Host States of Terrorist Groups’, in Weller, M. (ed.), Oxford Handbook of the Use of Force in International Law (2015), 720 Google Scholar, at 730. For a similar argument, see: Wedgwood, R., ‘Responding to Terrorism: The Strikes against Bin Laden’, (1999) 24 Yale Journal of International Law 559 Google Scholar, at 565; 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 Transnational Lawyer 51 Google Scholar, at 77.

⁹⁸ It is moreover problematic in that, by definition, a state's inability to deal with the non-state actor will not infringe its due diligence obligation. Due diligence obligations are obligations of means, not of result, and are premised upon the capacity of the state, in the circumstances, to engage in that conduct. See F. Lozano Contreras, La noción de debida diligencia en derecho internacional público (2007), at 220–8.

⁹⁹ Similarly, Ruys and Verhoeven, supra note 7, at 317–18; Ruys, supra note 22, at 285; Antonopoulos, supra note 11, at 169–70.

¹⁰⁰ ARS, supra note 27, Art. 50(1)(a).

¹⁰¹ Recall that Art. 51 of the Charter and Art. 21 of the ARS reflect different effects of the customary right of self-defence: Art. 51 concerns the effect of self-defence on the prohibition of force, Art. 21 its effect on the second set of legal relations.

¹⁰² Bothe, M., ‘The Law of Neutrality’, in Fleck, D. (ed.), Handbook of International Humanitarian Law (2013), 549 Google Scholar.

¹⁰³ ARS, supra note 27, Art. 50(1)(a).

¹⁰⁴ Bothe, supra note 102, at 558.

¹⁰⁵ Ibid.

¹⁰⁶ Ibid., fn. 39.

¹⁰⁷ Note that the applicability of the law of neutrality to conflicts between states and non-state actors is contested. See, e.g., the exchange between Chang, K., ‘Enemy Status and Military Detention in the War Against Al-Qaeda’, (2011) 47 Texas International Law Journal 1 Google Scholar; Ingber, R., ‘Untangling Belligerency from Neutrality in the Conflict with Al-Qaeda’, (2011) 47 Texas International Law Journal 76 Google Scholar; Heller, K., ‘The Law of Neutrality does not Apply in the Conflict with Al-Qaeda, and it is a Good Thing too: A Response to Chang’, (2011) 47 Texas International Law Journal 115 Google Scholar.

¹⁰⁸ Michaelsen, C., ‘Maritime Exclusion Zones in Times of Armed Conflict at Sea: Legal Controversies Still Unresolved’, (2003) 8 Journal of Conflict and Security Law 363 CrossRefGoogle Scholar, at 388.

¹⁰⁹ N. Klein, Maritime Security and the Law of the Sea (2011), 275.

¹¹⁰ Humphrey, D., ‘Belligerent Interdiction of Neutral Shipping in International Armed Conflict’, (1997) 2 Journal of Conflict and Security Law 23 CrossRefGoogle Scholar, at 29.

¹¹¹ See, e.g., Debate on the subject of the Falkland Islands, in (1982) 53 BYIL 540 (statement by Margaret Thatcher); and the various letters to the President of the UN Security Council in (1982) 53 BYIL 539–49.

¹¹² British letter, 9 April 1982, addressed to the President of the UN Security Council in (1982) 53 BYIL 539.

¹¹³ For the UK's position, see Gray, C., ‘The British Position in Regard to the Gulf Conflict’, (1988) 47 ICLQ 420 CrossRefGoogle Scholar.

¹¹⁴ The US subsequently asserted a different position, and it may be wondered if this was in response to the indiscriminate policy followed by Iran and Iraq in the enforcement of these rights. On the evolution of the US position, see Humphrey, supra note 110, at 33–4.

¹¹⁵ Ibid., at 32–3.

¹¹⁶ Quoted by Gray, supra note 113, at 423.

¹¹⁷ On which see Klein, supra note 109, at 274.

¹¹⁸ P. Wendel, State Responsibility for Interferences with the Freedom of Navigation in Public International Law (2007), 233–6.

¹¹⁹ Klein, supra note 109, at 275.

¹²⁰ For a similar view, see O'Keefe, R., Book review: Stephen C. Neff, War and the Law of Nations: A General History , (2007) 66 CLJ 461, at 462 Google Scholar.

¹²¹ For an overview, see Corten, supra note 6, Ch. 4.

¹²² On which see, e.g., Ruys, supra note 2, at 490 (arguing that Art. 51 neither prohibits armed attacks nor points to their illegality).

¹²³ Ministries trial, supra note 14, at 329; The Maria, supra note 14, at 361. See also, Lubell, supra note 14, at 41; Dinstein, supra note 14, at 190.

¹²⁴ Kammerhofer, supra note 50, at 40; de Hoogh, A., ‘Restrictivist Reasoning on the Ratione Personae Dimension of Armed Attacks in the Post 9/11 World’, (2016) 29 LJIL 19, at 22 CrossRefGoogle Scholar. But see Van Steenberghe, supra note 6, at 289–90. For a proposal in this regard, see Slaughter, A. and Burke-White, W., ‘An International Constitutional Moment’, (2002) 43 HILJ 1 Google Scholar, at 2.

¹²⁵ Gray, supra note 90, at 93–197.