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Customary Constraints on the Use of Force: Article 51 with an American Accent



This article, prepared for the symposium on ‘The Future of Restrictivist Scholarship on the Use of Force’, examines the current trajectory of restrictivist scholarship in the United States. In contrast to their counterparts in continental Europe, American restrictivists tend to devote less energy to defending narrow constructions of the UN Charter. Instead, they generally focus on legal constraints outside the Charter's text, including customary norms and general principles of law such as necessity, proportionality, deliberative rationality, and robust evidentiary burdens. The article considers how these features of the American restrictivist tradition reflect distinctive characteristics of American legal culture, and it explores the tradition's influence on debates over anticipatory self-defense and the use of force against non-state actors abroad. The article concludes by examining how the American restrictivist tradition is beginning to shape the United States’ approach to the use of force in response to cyber attacks.

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1 Art. 51, 1945 Charter of the United Nations, 1 UNTS XVI (1945) (hereinafter UN Charter).

2 Arts. 1(1) and 2(4), UN Charter.

3 See, e.g., Kreß, C., ‘Some Reflections on the International Legal Framework Governing Transnational Armed Conflicts’, (2010) 15 JCSL 245, at 248; Tams, C.J., ‘The Use of Force Against Terrorists,’ (2009) 20 EJIL 359; Ruys, T. and Verhoeven, S., ‘Attacks by Private Actors and the Right of Self-Defence,’ (2005) 10 JCSL 289.

4 See, e.g., Kammerhofer, J., ‘The Resilience of the Restrictivist Rules of Self-Defence’, in Weller, M., Rylatt, J. W. and Solomou, A. (eds.), The Oxford Handbook on the Use of Force in International Law (2014), 627 at 633; Mrázek, J., ‘The Use of Force and Expanded Conceptions of Self-defence’, (2011) 29 Chinese (Taiwan) Yearbook of International Law and Affairs 135; Gazzini, T., ‘A Response to Amos Guiora: Pre-Emptive Self-Defence Against Non-State Actors?’, (2008) 13 JCSL 25, at 26 (‘Dealing with self-defence . . . essentially means interpreting and applying Article 51 . . . ’).

5 See Kammerhofer, supra note 4, at 633 (reporting the results of an informal survey in which only one of 15 authors ‘identifiable as “US scholars” . . . comes even close to [conventional restrictivism]’).

6 See, e.g., M.E. O’Connell, ‘Lawful Self-Defense to Terrorism’, (2002) 63 University of Pittsburgh Law Review 889, at 894–5.

7 See Corten, O., ‘The Controversies Over the Customary Prohibition on the Use of Force: A Methodological Debate’, (2005) 16 EJIL 803 (discussing the relative flexibility of the customary prohibition against the use of force under the ‘expansive’ and ‘restrictive’ approaches); Byers, M. and Chesterman, S., ‘Changing the Rules about Rules? Unilateral Intervention and the Future of International Law’, in Holzgrefe, R. and Keohane, J.L. (eds.), Humanitarian Intervention: Ethical, Legal, and Political Dilemmas (2003), 177 at 180 (observing that ‘[t]he relationship between [treaties and custom] is similar to the relationship between domestic statutes and the common law’).

8 See J. Kammerhofer, ‘The Resilience of the Restrictivist Rules of Self-Defence’, in Weller, Ryllatt and Solomou (eds.) supra note 4, at 629 (characterizing these constraints as the Nicaragua consensus).

9 See ibid., at 629; Bothe, M., ‘Terrorism and the Legality of Pre-emptive Force’, (2003) 14 EJIL 227, at 229–30; I. Brownlie, International Law and the Use of Force by States (1963) 366 (‘It is considered that the terms “attack”, “use”, and “resort to” imply an act or the beginning of a series of acts.’); Neuhold, H., ‘Legal Crisis Management: Lawfulness and Legitimacy of the Use of Force’, in Fastenrath, al. (eds.), From Bilateralism to Community Interest: Essays in Honour of Judge Bruno Simma (2011) 284, at 2–85 n. 21 (arguing that the ordinary meaning and object and purpose of Art. 51 dictate that ‘the adversary must actually have started offensive military action’ before self-defence is permissible).

10 Kammerhofer, supra note 8, at 629.

11 Military and Paramilitary Activities in and against Nicaragua (Nicaragua v. United States), Merits, Judgment of 27 June 1986, [1986] ICJ Rep. 14, at 103, para. 195 (hereinafter Nicaragua); see also O. Corten, The Law Against War: The Prohibition on the Use of Force in Contemporary International Law (2010) 403; Kretzmer, D., ‘The Inherent Right to Self-Defense and Proportionality in Jus ad Bellum’, (2013) 24 EJIL 235, at 242–4; G. Nolte and A. Randelzhofer, ‘Article 51’, in B. Simma et al. (eds.), The Charter of the United Nations: A Commentary (2012) 1397, at 1409.

12 Ibid.; see also Legal Consequences cf the Construction of a Wall in the Occupied Palestinian Territory, Advisory Opinion of 9 July 2004, [2004] ICJ Rep. 136, at 194, para. 139 (suggesting that ‘Article 51 has no relevance’ if a state ‘does not claim that the attacks against it are imputable to a foreign State’); Armed Activities on the Territory of the Congo (Democratic Republic of Congo v. Uganda), Merits, Judgment of 19 December 2005, [2005] ICJ Rep. 168, at 222, paras. 146–7 (hereinafter Armed Activites) (‘find[ing] that the legal and factual circumstances for the exercise of a right of self-defence by Uganda against the DRC were not present’ because there was ‘no satisfactory proof of the involvement of [the government of the DRC]’ in attacks by non-state actors) [hereinafter Armed Activities]; Corten, supra note 11, at 160–97; Bothe, supra note 9, at 233; Nolte and Randelzhofer, supra note 11, at 1417.

13 See, e.g., Kammerhofer, J., ‘Uncertainties of the Law on Self-Defence in the United Nations Charter’, (2004) 35 Netherlands Yearbook of International Law 143, at 201–2.

14 Ibid.

15 Art. 51, UN Charter.

16 See, e.g., D. Bowett, Self-defense in International Law (1958) 184–8.

17 Art. 51, UN Charter; see also McCormack, T.L.H., ‘Anticipatory Self-Defense in the Legislative History of the United Nations Charter’, (1991) 25 Israel Law Review 1, at 2 (outlining and critiquing this argument); I. Brownlie, International Law and the Use of Force by States (1963) 273; H. Kelsen, The Law of the United Nations (1950) 156, 159, 269, 792, 914.

18 Nicaragua, supra note 11, paras. 51, 64, 191.

19 Ibid., para. 176.

20 Some scholars in the United States have argued, for example, that a use of force in self-defense is not actually ‘against’ the ‘political independence’ or ‘territorial integrity’ of a state. See, e.g., Bradford, W.C., ‘“The Duty To Defend Them”: A Natural Law Justification for the Bush Doctrine of Preventive War’, (2004) 79 Notre Dame Law Review 1365, at 1376–8; cf. Reisman, W.M. and McDougal, M.S., ‘Humanitarian Intervention to Protect the Ibos’, in Lillich, R.B. (ed.), Humanitarian Intervention and the United Nations (1973), 167 at 177. But see Armed Activities, supra note 12, para. 148 (‘Article 51 of the Charter may justify a use of force in self-defence only within the confines there laid down’).

21 See Gazzini, supra note 4, at 27; Hofmeister, H.H., ‘When Is It Right To Attack So-Called “Host States”?’, (2007) 11 Singapore Yearbook of Interntional Law 1, at 4.

22 See Nolte and Randelzhofer, supra note 11, at 1417 (‘[T]he preferable view still seems to be that attacks by organized armed groups need to be attributed to a State in order to enable the affected State to exercise its right of self-defence, albeit under special primary rules of attribution’).

23 See, e.g., US National Security Strategy 22 (2010), available at; Memorandum of Assistant Attorney General Jay S. Bybee, Office of Legal Counsel, Authority of the President Under Domestic and International Law To Use Military Force Against Iraq, Sept. 23, 2002, at 23–33 [hereinafter Bybee Memorandum].

24 See, e.g., Murphy, S.D., ‘Terrorism and the Concept of “Armed Attack” in Article 51 of the UN Charter’, (2002) 43 HILJ 41, at 50–1; Reisman, W.M., ‘International Legal Responses to Terrorism’, (1999) 22 Houston Journal of International Law 3.

25 See, e.g., H.J. Morgenthau, Politics Among Nations: The Struggle for Power and Peace (1978); E.H. Carr, The Twenty-Years Crisis: 1919–1939 (1939).

26 Franck, T.M., ‘Who Killed Article 2(4)? Or: Changing Norms Governing the Use of Force by States’, 64 (1970) AJIL 809, at 811, 835; see also Glennon, M.J., ‘The Fog of Law: Self-defense, Inherence, and Incoherence in Article 51 of the United Nations Charter’, (2002) 25 Harvard Journal of Law and Public Policy 539, at 540 (suggesting that ‘international “rules” concerning use of force are no longer regarded as obligatory by states’); O. Schachter, ‘The Right of States to Use Armed Force’, (1984) 82 Michigan Law Review 1620, at 1620, 1635 (observing ‘that the obligations of the Charter are widely seen as mere rhetoric, at best idealistic aspirations, or worse as providing a pretext or “cover” for aggression’).

27 Franck, supra note 26, at 809–10; Glennon, supra note 26, at 540–1; cf. Rheingold, T., ‘State Weakness, Irregular Warfare, and the Right to Self-Defense Post 9/11’, (2011) 105 AJIL 244.

28 See Franck, supra note 26, at 816 (‘How is the fact of an armed attack to be established? The Charter provides no answer . . . ’); McDougal, M.S., ‘The Soviet-Cuban Quarantine and Self-Defense’, (1963) 57 AJIL 597, at 600 (asserting that ‘nothing in the “plain and natural meaning” of the words of the Charter requires an interpretation that Article 51 restricts the customary right of self-defense’); cf. N. Lubell, Extraterritorial Use of Force Against Non-State Actors (2010), 50 (observing that it is unsettled whether there is a minimum threshold of severity for an ‘armed attack’ and what that threshold would be).

29 See, e.g., Chevron U.S.A., Inc. v. Natural Resources Defense Council, Inc., 467 U.S. 837, 842–4 (1984) (holding that courts must treat statutory gaps and ambiguities as delegations to the administrative agency that is entrusted to administer the statute).

30 See, e.g., Bressman, L. Schultz, ‘Reclaiming the Fiction of Congressional Delegation’, (2011) 97 Virginia Law Review 2009; Lemos, M.H., ‘The Other Delegate: Judicially Administered Statutes and the Nondelegation Doctrine’, (2008) 81 Southern California Law Review 405.

31 See generally G. Calabresi, A Common Law for the Age of Statutes (1999).

32 Cf. Kant, I., ‘Perpetual Peace: A Philosophical Sketch’ (1795), in Reiss, H. (ed.) and Nisbet, H.B. (trans.), Kant: Political Writings (1991), 93.

33 See Glennon, M.J., ‘The New Interventionism: The Search for a Just International Law’, (1999) 73 Foreign Affairs 2; cf. J. Rawls, The Law of Peoples (1999) 91.

34 Schachter, supra note 26, at 1628 (internal quotation marks omitted).

35 See, e.g., G.P. Fletcher and J.D. Ohlin, Defending Humanity: When Force Is Justified and Why (2008); Bradford, supra note 20; Ohlin, J.D., ‘The Doctrine of Legitimate Defense’, (2015) 91 International Legal Studies 119.

36 See, e.g., Halberstam, M., ‘The Right to Self-Defense Once the Security Council Takes Action’, (1996) 17 Michigan Journal of International Law 229, at 238 (characterizing self-defence as ‘one of the most, if not the most, fundamental rights both of individuals and of states’); Rostow, N., ‘Nicaragua: A Surreply to a Rejoinder’, (1985) 11 Yale Journal of International Law 474, at 478 (‘The inherent right of a state to engage in individual or collective self-defense is the fundamental attribute of sovereignty, and the UN Charter leaves this right unimpaired.’).

37 See E.J. Criddle and E. Fox-Decent, International Law's Fiduciary Constitution (forthcoming 2016); Marks, S.P. and Cooper, N., ‘The Responsibility To Protect: Watershed or Old Wine in New Bottles?’, (2010) 2 Jindal Global Law Review 86, at 94 (‘It is by fulfilling the social contract of protecting the rights of its members that the state acquires legitimacy.’); cf., E. de Vattel, The Law of Nations (Fenwick, transl. 1916), 256 (‘Self-defense against an unjust attack is not only a right which every Nation has, but it is a duty, and one of its most sacred duties.’).

38 Fenwick, C.J., Editorial, ‘The Quarantine Against Cuba: Legal or Illegal?’, (1963) 57 AJIL 588, at 592.

39 For a discussion of these aspects of proportionality in the context of European public law, see Sweet, A.S. and Mathews, J., ‘Proportionality, Judicial Review, and Global Constitutionalism’, in Bongiovanni, G. et al. (eds.), Reasonableness and Law (2009), 173 at 174.

40 See, e.g., Rostow, N., ‘Nicaragua and the Law of Self-Defense Revisited’, (1987) 11 Yale Journal of International Law 437, at 453 (‘To be lawful, a responsive use of force under article 51 must aim to cure the breach that gave rise to the exercise of the right of self-defense. It must be proportional, involving no more than the force reasonably required to cure the breach’).

41 See, e.g., Bybee Memorandum, supra note 23.

42 See, e.g., Erakat, N.S., ‘New Imminence in the Time of Obama: The Impact of Targeted Killing on the Law of Self-Defense’, (2014) 56 Arizona Law Review 195, at 202 (‘Imminence indicates that an attack has not yet taken place but is already in motion or is otherwise inevitable.’).

43 See, e.g., Daskal, J.C., ‘The Geography of the Battlefield: A Framework for Detention and Targeting Outside the “Hot” Conflict Zone’, (2013) 161 University of Pennsylvania Law Review 1165; Waxman, M., ‘The Use of Force Against States that Might Have Weapons of Mass Destruction’, (2009) 31 Michigan Journal of International Law 1.

44 See, e.g., Schmitt, M.N., ‘U.S. Security Strategies: A Legal Assessment’, (2004) 27 Harvard Journal of Law and Public Policy 737, at 756–7 (discussing and endorsing the U.S. government's assertion of this standard when responding in Afghanistan to the 9/11 terrorist attacks).

45 See, e.g., Alston, P., ‘The CIA and Targeted Killing Beyond Borders’, (2011) 2 Harvard National Security Law Journal 283.

46 See, e.g., Corten, supra note 7, at 813.

47 Viewed from this perspective, the principles that govern contemporary jus ad bellum have affinities with global administrative law, which plausibly constitutes ‘a revived version of jus gentium’ based on ‘norms emerging among a wide variety of diverse actors and in very diverse settings, rather than depending on a ius inter gentes built upon agreements between states.’ Kingsbury, B., Krisch, N. and Stewart, R.B., ‘The Emergence of Global Administrative Law, The Emergence of Global Administrative Law’, (2005) 68 Law & Contemporary Problems 15, at 29.

48 See Dyzenhaus, D., ‘The Rule of (Administrative) Law in International Law’, (2005) 68 Law & Contemporary Problems 127, at 131 (‘[T]he history of the common law of judicial review is a history of judges imposing controls on public officials that are not prescribed by any statute.’).

49 Ibid., at 129.

50 Whether these norms meet the legal requirements for customary international law is, of course, a separate question. See, R. Scoville, ‘Finding Custom’ (forthcoming 2016) Iowa Law Review 101 (finding that US courts tend to consult an artificially narrow spectrum of state practice and opinio juris when determining the content of customary).

51 For a comparison of expansivist and restrictivist approaches to customary jus ad bellum, see Corten, supra note 7.

52 See generally Calabresi, supra note 31, at 1–5, 163–6 (discussing these features of the American tradition and encouraging judges to use ‘common law’-style adjudication to update obsolete statutes).

53 Letter from Webster to Fox, Apr. 24, 1841, 29 British Foreign State Papers 1129, at 1137–8.

54 Ibid.

55 See, e.g., Kammerhofer, J., ‘The Armed Activities Case and Non-State Actors in Self-Defence Law’, (2007) 20 LJIL 89, at 99 (expressing skepticism that the Caroline doctrine, ‘a statement on the law on the use of force made in 1842[,] is still correct despite the developments over the last 165 years’).

56 See, e.g., Schachter, supra note 26, at 1635; McDougal, supra note 28, at 599.

57 See Schachter, supra note 26, at 1635 (observing that during ‘debates in the Security Council on [the legality of Israel's 1981 strike against the Iraqi Osirak nuclear reactor], several delegates referred to the Caroline Case formulation of the right of anticipatory defense as an accepted statement of customary law’). In response to the ICJ's Nicaragua Judgment, US State Department Legal Adviser Abraham Sofaer stated unequivocally: ‘The United States rejects the notion that the UN Charter supersedes customary international law on the right of self defense.’ Sofaer, A., ‘Terrorism, the Law, and the National Defense’, (1989) 126 Military Law Review 89, at 95.

58 See, e.g., Paust, J.J., ‘Use of Force Against Terrorists in Afghanistan, Iraq, and Beyond’, (2002) 35 Cornell International Law Journal 533, at 535–6.

59 See Security Strategy of the United States of America, September 2002, available at

60 Sapiroa, M., ‘Iraq: The Shifting Sands of Preemptive Self-Defense’, (2003) 98 AJIL 599, at 602.

61 Ibid.

62 See Secretary of State George Shultz, ‘Low-Intensity Warfare: The Challenge of Ambiguity’, 86 Department of State Bulletin Mar. 1986, at 17 (‘The UN Charter is not a suicide pact.’).

63 W.H. Taft IV, Legal Adviser, Department of State, The Legal Basis for Preemption, Nov. 18, 2002, available at

64 Ibid.; see also Sofaer, A., ‘On the Necessity of Preemption’, (2003) 14 EJIL 209, at 220 (distilling a series of factors that would determine whether the use of force is legitimate under Art. 51 of the Charter, including ‘(1) the nature and magnitude of the threat involved; (2) the likelihood that the threat will be realized unless pre-emptive action is taken; (3) the availability and exhaustion of alternatives to using force; and (4) whether using pre-emptive force is consistent with the terms and purposes of the UN Charter and other applicable international agreements’).

65 Bybee Memorandum, supra note 23, at 26–7.

66 See Senate Select Committee on Intelligence, Report on the U.S. Intelligence Community's Prewar Intelligence Assessments on Iraq (7 July 2004).

67 See Schmitt, M.N., ‘U.S. Security Strategies: A Legal Assessment’, (2004) 27 Harvard Journal of Law and Public Policy 737, at 756–7 (suggesting that absence of objections to this standard in the U.N. Security Council and among NATO members testifies to the international legal authority of the ‘clear and compelling’ evidence standard).

68 See, e.g., Erakat, supra note 42, at 203–4, 243–7; Franck, T.M., ‘What Happens Now? The United Nations After Iraq’, (2003) 97 AJIL 607, at 610, 619.

69 For a summary, see Bethlehem, D., ‘Self-Defense Against an Imminent or Actual Armed Attack by Non-State Actors’, (2012) 106 AJIL 769.

70 Schmitt, M.N., ‘Responding to Transnational Terrorism Under the Jus Ad Bellum: A Normative Framework’, (2008) 56 Naval Law Review 1, at 7.

71 President Ronald Reagan, Address to the Nation (Apr. 14, 1986), in Department of State Bulletin June 1986, at 1–2.

72 Letter to Congressional Leaders Reporting on Military Action Against Terrorist Sites in Afghanistan and Sudan, 34 Weekly Comp. Pres. Doc. 1650 (Aug. 21, 1998).

73 Wedgwood, R., ‘Responding to Terrorism: The Strikes Against bin Laden’, (1999) 24 Yale Journal of International Law 559, at 563–4.

74 Sofaer, A.D., ‘Sixth Annual Waldemar A. Solf Lecture in International Terrorism: The Strikes Against bin Laden’, (1989) 126 Military Law Review 89, at 92. Unsurprisingly, before 9/11 some leading U.S. scholars were not persuaded that non-state terrorists could engage in an ‘armed attack’. See, e.g., L. Henkin, International Law: Politics and Values (1995), 126 (‘It is difficult to make an “armed attack” out of a limited, isolated terrorist attack or even a few sporadic ones.’).

75 Wedgwood, supra note 72, at 565.

76 Ibid., at 576.

77 See, e.g., Lobel, J., ‘The Use of Force to Respond to Terrorist Attacks: The Bombing of Sudan and Afghanistan’, (1999) 24 Yale Journal of International Law 537, at 547 (‘unilateral attacks based on secret information gained largely be inference’ are unlawful).

78 Letter from the Permanent Representative of the United States of America to the President of the Security Council (Oct. 7, 2001), 40 ILM 1281 (2001).

79 S.C. Res. 1386, U.N. Doc. S/RES/1386 (Dec. 20, 2001); see also Roberts, A., ‘Afghanistan and International Security’, in Schmitt, M.N. (ed.), The War in Afghanistan: A Legal Analysis (2009), 4 at 15 (concluding that the Security Council had earlier ‘accepted that a right of self-defense could apply to a State when it was attacked by a non-state entity’).

80 See, e.g., L. Henkin, International Law: Politics and Values (1995) 126 (‘It is difficult to make an “armed attack” out of a limited, isolated terrorist attack or even a few sporadic ones.’).

81 S.D. Murphy, ‘The International Legality of US Cross-Border Operations from Afghanistan to Pakistan’, in Schmitt, The War in Afghanistan: A Legal Analysis, supra note 78, at 109.

82 Ibid.

83 See, e.g., O’Connell, supra note 6, at 890–3. But see O’Connell, M.E., ‘Unlawful Killing with Combat Drones: A Case Study of Pakistan’, in Bronitt, S. et al. (eds.), Shooting To Kill: The Law Governing Lethal Force in Context (2012) (‘An armed response to a terrorist attack will almost never meet [the] parameters for the lawful exercise of self-defense.’)

84 Letter from Webster to Fox, supra note 52.

85 Murphy, supra note 80, at 109; see also Sofaer, supra note 63, at 220.

86 O’Connell, M.E., ‘Combatants and the Combat Zone’, (2009) 43 University of Richmond Law Review 845, at 858.

87 Paust, J.J., ‘Post 9/11 Overreaction and Fallacies Regarding War and Defense, Guantanamo, The Status of Persons, Treatment, Judicial Review of Detention, and Due Process in Military Commissions’, (2007) 79 Notre Dame Law Review 1335, at 1342; see also Rona, G., ‘Interesting Times for International Law: Challenges from the “War on Terror”’, (2003) 27 Fletcher Forum of World Affairs 55, at 62.

88 U.N. ECOSOC, Civil and Political Rights, Including the Questions of Disappearances and Summary Executions, Submitted to Commission on Human Rights, U.N. Doc. E/CN.4/2003/3 (Jan. 13, 2003).

89 H.H. Koh, Legal Advisor, U.S. Department of State, Speech to the Annual Meeting of the American Society of International Law, The Obama Administration and International Law (25 March 2010).

90 Ibid.

91 Ibid.

92 Ibid.

93 Ibid.

94 Letter from A.D. Romero, Executive Director of the American Civil Liberties Union, to Barack Obama, President of the United States, 28 April 2010, at 1, available at

95 Declaration of Prof. M.E. O’Connell, para. 14; Al-Aulaqi v. Obama, 727 F. Supp. 2d 1 (D.D.C. 2010).

96 P. Alston, Report of the Special Rapporteur on Extrajudicial, Summary or Arbitrary Executions, A/HRC?14/24/Add.6 (28 May 2010).

97 K. Anderson, Hoover Inst., Targeted Killing and Drone Warfare (2011), 2–3 (noting that drones disturb the long-accepted ‘implied geography of war’ based on ‘where hostilities took place.’)

98 J.O. Brennan, Assistant to the President for Homeland Security and Counterterrorism, Remarks at the Woodrow Wilson International Center for Scholars: The Ethics and Efficacy of the President's Counterterrorism Strategy (30 April 2012) [hereinafter Brennan Remarks]; E.H. Holder, Jr., Attorney Gen., U.S. Dept. of Justice, Remarks at Northwestern University School of Law (5 March 2012); J.C. Johnson, Gen. Counsel, U.S. Dep't of Defense, Speech at the Oxford Union: The Conflict Against al Qaeda and its Affiliates: How Will it End? (30 November 2012) [hereinafter Johnson Speech]; Bradley, C.A. and Goldsmith, J.L., ‘Congressional Authorization and the War on Terrorism’, (2005) 118 Harvard Law Review 2047, at 2117–23 (international law does not limit the geographic scope of the battlefield); Lewis, M.W., ‘Drones and the Boundaries of the Battlefield’, (2012) 47 Texas International Law Journal 293, at 312–13 (limiting strikes to armed conflict zones would create sanctuaries for terrorists); Paust, J.J., ‘Self-Defense Targetings of Non-State Actors and Permissibility of U.S. Use of Drones in Pakistan’, (2010) 19 Journal of Transnational Law and Policy 237, at 280 (suggesting that in lawful self-defence, targeted killings of non-state actors engaged in armed attacks can be permissible no matter where such attacks occur); Ohlin, J.D., ‘The Duty to Capture’, (2013) 97 Minnesota Law Review 1268 (no strict geographic limits in applying IHL).

99 See Brennan Remarks, supra note 97; Holder Remarks, supra note 97; Johnson Speech, supra note 97; Letter from Samantha J. Power, US Representative to the United Nations to Ban Ki Moon, Secretary-General of the United Nations, 23 September 2014 [hereinafter Power Letter].

100 Daskal, supra note 43, at 1209.

101 Ibid., at 1209, 1230–1.

102 See Brennan Remarks, supra note 97; Holder Remarks, supra note 97; Johnson Speech, supra note 97.

103 Stephen W. Preston, Gen. Counsel, CIA, Remarks at Harvard Law School (Apr. 10, 2012) (arguing that to justify the use of force ‘we need look no further than the inherent right of national self-defense, which is recognized by customary international law’).

104 A. Deeks, ‘Narrowing Down the U.S. International Legal Theory for ISIS Strikes in Syria’, Lawfare blog, 12 September 2014, available at For background, see Deeks, A., ‘“Unwilling or Unable”: Toward a Normative Framework for Extraterritorial Self-Defense’, (2012) 52 Virginia Journal of International Law 483. Indeed, in her 23 September 2014 letter to the Secretary General, Ambassador Samantha Power noted that Art. 51 self-defence is lawful where ‘the government of the State where the threat is located is unwilling or unable to prevent the use of its territory for such attacks. The Syrian regime has shown that [it is unwilling or unable]. Accordingly, the United States has initiated necessary and proportionate military actions in Syria.’ Power Letter, supra note 98.

105 Schmitt, M., ‘Preemptive Strategies in International Law’, (2003) 24 Michigan Journal of International Law 513, at 546.

106 Waxman, M.C., ‘Cyber-Attacks and the Use of Force: Back to the Future of Article 2(4)’, (2011) 36 Yale Journal of International Law 421, at 428–30.

107 Ibid., at 431, at 436–7; The White House, International Strategy for Cyberspace: Prosperity, Security, and Openness in a Networked World (2011), 14; Hathaway, al., ‘The Law of Cyber-Attack’, (2012) 100 California Law Review 817, at 848; Banks, W., ‘The Role of Counterterrorism Law in Shaping ad Bellum Norms for Cyber War’, (2012) 89 International Legal Studies 157, at 169–70.

108 White House Strategy, supra note 106, at 14.

109 M.N. Schmitt (gen. ed.), Tallinn Manual on the International Law Applicable to Cyber Warfare (2013).

110 Ibid., Rule 13(3), (4).

111 Ibid., Rule 13(8).

112 Ibid., Rule 13(7).

113 Ibid., Rule 13(21).

114 Ibid., Rule 13(9).

115 Ibid., Rule 13(17). A majority accepted self-defense in the cyber realm against non-state actors.

116 See, e.g., W.A. Owens et al. (eds.), National Research Council, Technology, Policy, Law and Ethics Regarding U.S. Acquisition and Use of Cyberattack Capabilities (2009), 33–4 [hereinafter NRC report]; Jensen, E.T., ‘Computer Attacks on Critical National Infrastructure: A Use of Force Invoking the Right of Self-Defense’, (2002) 38 Stanford Journal of International Law 207, at 223–9; Schmitt, M.N., ‘Computer Network Attack and the Use of Force in International Law: Thoughts on a Normative Framework’, (1999) 37 Columbia Journal of Transnational Law 885, at 930–4; Watts, S., ‘Low-Intensity Computer Network Attacks and Self-Defense’, in Pedrozo, R.A. and Wollschlaeger, D.P. (eds.), International Law and the Changing Character of War (2011).

117 W.G. Sharp, Cyberspace and the Use of Force (1999), 129–30.

118 NRC Report, supra note 115, at 253–4; Silver, D.B., ‘Computer Network Attack as a Use of Force Under Article 2(4) of the United Nations Charter’, in Schmitt, M.N. and O’Donnell, B.T. (eds.), Computer Network Attack and International Law (2002), 89.

119 Silver, supra note 117, at 90–1.

120 See Waxman, , ‘Self-Defensive Force against Cyber Attacks: Legal, Strategic and Political Dimensions’, (2013) 89 International Law Studies 109, at 112; Jensen, supra note 115, at 230; Shulman, M.R., ‘Discrimination in the Laws of Information Warfare’, (1999) 37 Columbia Journal of Transnational Law 939, at 955–6.

121 Advance Questions for Lieutenant General Keith Alexander, USA, Nominee for Commander, United States Cyber Command: Before the S. Armed Services Comm., 111th Cong. 11 (2010).

122 H.H. Koh, Remarks at the U.S. Cyber Command Inter-Agency Legal Conference: International Law in Cyberspace (Sept. 18, 2012). Koh offered several examples of the types of cyber attacks that would trigger self-defence rights: cyber events that trigger a nuclear plant meltdown; intrusions that open a dam above a populated area causing destruction, or operations disabling air traffic control causing airplane crashes.

124 Ibid.

125 Ibid.

126 Wales Summit Declaration, Issued by the Heads of State and Government Participating in the North Atlantic Summit in Wales, Sep. 5, 2014, para. 72, at

127 Ibid.

128 Ibid.

129 Koh, supra note 121, at 5; Schmitt, M.N., ‘Cyber Operations in International Law: The Use of Force, Collective Security, Self-Defense, and Armed Conflicts’, in Lin, H. (ed.), Proceedings of a Workshop on Deterring Cyberattacks (2009), 151 at 167; Waxman, supra note 119, at 112.

130 Art. 38, UN Charter.

131 See C.A. Bradley, A State Preferences Account of Customary International Law Adjudication, available at (casting doubt on the traditional formula and suggesting a new approach expressly modeled on common law adjudication).

132 See, e.g., Kammerhofer, supra note 8, at 641 (arguing that even if customary international law offers a wider birth to self-defense than the Charter, this ‘does not help in justifying a prima facie breach of Article 2(4)’).

* Interim Dean; Board of Advisors Distinguished Professor; Professor of Law; Professor of Public Administration and International Affairs, Maxwell School of Citizenship and Public Affairs; Director, Institute for National Security and Counterterrorism [].

** Professor of Law, William and Mary Law School [].



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