Skip to main content Accessibility help

Self-Defence in Response to Attacks by Non-state Actors in the Light of Recent State Practice: A Step Forward?



This article analyses the recent state practice in which the right of self-defence has been invoked in order to justify the use of force in response to attacks by non-state actors. The main purpose of this analysis is to determine whether the law of self-defence has evolved through this practice. It is submitted that the latter confirms the tendency, evidenced by the US operation ‘Enduring Freedom’ in Afghanistan in 2001, towards allowing states to respond in self-defence to private armed attacks, that is, attacks which are committed by non-state actors only. The article also aims to shed some light on other fundamental conditions of the law of self-defence which played a significant role in the legal assessment of the recent state practice. It is argued in this respect that this practice confirms that any armed attack must reach some level of gravity – which may be assessed by accumulating minor uses of force – in order to trigger the right of self-defence, and that proportionality of the action taken in self-defence may be assessed in quantitative terms, but only as a means of making a prima facie judgement about the necessity of this action.



Hide All

1 See, e.g., on this subject, Y. Dinstein, War, Aggression and Self-Defense (2001), 206–8; C. Gray, International Law and the Use of Force (2008), 198–207; N. Schrijver, ‘Responding to International Terrorism: Moving the Frontiers of International Law for “Enduring Freedom”’, (2001) Netherlands International Law Review 271; M. Byers, ‘Terrorism, the Use of Force and International Law after 11 September’, (2002) ICLQ 401; J. I. Charney, ‘The Use of Force against Terrorism and International Law’, (2001) AJIL 835; T. M. Franck, ‘Editorial Comments: Terrorism and the Right of Self-Defence’, (2001) AJIL 839; S. D. Murphy, ‘Terrorism and the Concept of “Armed Attack” in Article 51 of the UN Charter’, (2002) Harvard International Law Journal 41; M. E. O'Connell, ‘Lawful Self-Defense to Terrorism’, (2001–2) University of Pittsburgh Law Review 889; S. R. Ratner, ‘Jus ad Bellum and Jus in Bello after September 11’, (2002) AJIL 905; C. Stahn, ‘Terrorist Acts as “Armed Attack”: The Right to Self-Defense, Article 51(1/2) of the UN Charter, and International Terrorism’, (2002) Fletcher Forum of World Affairs 35; K. N. Trapp, ‘Back to Basics: Necessity, Proportionality, and the Right of Self-Defense against Non-state Terrorist Actors’, (2007) ICLQ 141; L. Condorelli, ‘Les attentats du 11 septembre et leurs suites: où va le droit international?’, (2001) Revue générale de droit international public 829; O. Corten and F. Dubuisson, ‘Opération “Liberté immuable”: une extension abusive du concept de légitime défense’, (2002) Revue générale de droit international public 51; P. M. Eisemann, ‘Attaque du 11 septembre et exercice d'un droit naturel de légitime défense’, in K. Bannalier et al. (eds.), Le droit international face au terrorisme. Après le 11 septembre 2001 (2002), 239; J. Verhoeven, ‘Les “étirements” de la légitime défense’, (2002) Annuaire français de droit international 49.

2 See Case Concerning Military and Paramilitary Activities in and against Nicaragua (Nicaragua v. United States of America), Judgment of 27 June 1986, [1986] ICJ Rep. 94, para. 176.

3 That Art. 51 of the UN Charter refers itself to the customary right of self-defence may be clearly inferred from the words with which Art. 51 begins – ‘Nothing in the present Charter shall impair. . .’. These words logically imply that the right of self-defence existed prior to the UN Charter and, as a result, is of a customary nature. It is, however, less certain that such an implicit reference may also be inferred from the adjective ‘inherent’ qualifying the right of self-defence under Art. 51, although the ICJ expressly interpreted this adjective as referring to the customary law of self-defence (Nicaragua, supra note 2). Indeed, the adjective ‘inherent’ could be given other possible meanings, such as referring to the imperative nature of the right of self-defence, as it is sustained by some authors (see infra note 8), or, as seems more plausible, emphasizing that states could never be dispossessed of their right of self-defence against their will. The – controversial – interpretation that the Court gave to the adjective ‘inherent’ may perhaps be explained by the fact that it did as much as possible to find customary rules to judge the case since the US multilateral treaty reservation to its competence constrained it to apply only customary law.

4 See more particularly, for instance, statements from Israel (UN Doc. S/PV.2280, at 8 (Israeli intervention in Iraq, 1981)); Tajikistan, Kazakhstan, Kirghizstan, and Russia (UN Doc. S/26290, at 2 (conflict between Tajikistan and Afghanistan, 1993)); the United States (UN Doc. S/1998/780, at 1 (US intervention in Afghanistan and Sudan, 1998); UN Doc. A/C.6/35/SR.51, 1980, para. 4 (declaration at the Sixth Committee of the UN General Assembly concerning the ILC Works on State Responsibility).

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

6 See, e.g., statements from Iran (UN Doc. S/23786, at 1 (Iranian intervention in Iraq, 1992); UN Doc. S/1994/1273, at 1 (Iranian intervention in Iraq, 1994); UN Doc. S/1997/768, at 1 (Iranian intervention in Iraq, 1997)); Tajikistan, Kazakhstan, Kirghizstan, and Russia (UN Doc. S/26290, at 2 (conflict between Tajikistan and Afghanistan, 1993)); see also the dissenting opinion of Judge Koroma, annexed to the Advisory Opinion rendered by the ICJ in Legality of the Threat or Use of Nuclear Weapons, Advisory Opinion of 8 July 1996, [1996] ICJ Rep. 562.

7 See on this subject G. Distefano, ‘La pratique subséquente des Etats parties à un traité’, (1994) Annuaire français de droit international 55; J. M. Sorel, ‘Article 31 de la Convention de Vienne de 1969’, in O. Corten and P. Klein (eds.), Les Conventions de Vienne sur le droit des traités. Commentaires article par article (2006), 1320; M. K. Yassen, ‘L'interprétation des traités d'après la Convention de Vienne sur le droit des traités’, (1976-III) RCADI 51; J.-P. Cot, ‘La conduite subséquente des parties à un traité’, (1966) Revue générale de droit international public 664.

8 The latter requirement must still be specified in the light of the nature of the law of self-defence. Indeed, it is generally upheld that the prohibition on the use of force – or, at least, the prohibition on aggression – has a peremptory nature. It has been expressly considered as such by most states (see, more particularly, the declarations pronounced by the states at the Sixth Committee of the UNGA in the frame of the adoption of GA Resolution 42/22 (1987)). It is not the case with respect to the law of self-defence. Only few states expressly recognized it as a peremptory norm (see, e.g., statements from Iraq, UN Doc. A/C.6/35/SR.51, at 17, para. 62, and Jamaica, UN Doc. A/C.6/35/SR.53, at 15, para. 51). Moreover, it is not possible to give to the right of self-defence a peremptory nature on the ground that no derogation is permitted from it or on the ground that it modifies a peremptory norm (two grounds on which a norm may possibly be given a peremptory nature according to Art. 53 of the Vienna Convention on the Law of Treaties). Indeed, the right of self-defence is a right, and not an obligation, from which it is therefore conceptually impossible to derogate and it is contained in the scope of a peremptory norm, the prohibition on the use of force, but does not properly modify this norm. However, as contained in the scope of the prohibition on the use of force, any evolution of the law of self-defence actually implies an evolution of this prohibition and, therefore, must fulfil the conditions under which a peremptory norm may evolve – that is, ‘to be accepted and recognized by the international community of States as a whole’ (Vienna Convention on the Law of Treaties, Art. 53). In other words, the law of self-defence has an indirect peremptory nature. This does not mean that all the states must approve the evolution of this law. Unanimity is not required. It is enough if it is approved by all the different state groupings of the world, provided that it is not opposed by the other states. In this respect, opposition from one or a few does not prevent the evolution from happening.

9 Letter from the Permanent Representative of Israel, UN Doc. S/2006/515, at 1–2.

10 See note 33, infra.

11 See, e.g., ‘Iraq Moves to Dissuade Turkey from Raids’, New York Times, 17 October 2007: ‘“We have reached the point of self-defense, and we are ready to do whatever is necessary in light of common sense”, said Mr. Erdogan’; Reuters, ‘Turkey to Approve Troops to Iraq in Defiance of US’, available at (16 October 2007): ‘“[Adoption of this plan] does not mean an immediate incursion will follow, but we will act at the right time and under the right conditions”, Erdogan told his ruling AK Party on Tuesday [16 October 2007]. “This is about self-defense”, he said in televised remarks’;, ‘Turkey's Erdogan Says Parliament to Approve Iraq Raid (Update4)’, (16 October 2007): ‘[Erdogan] said a military strike would come under international laws governing self-defense’; Reuters, ‘Erdogan hopes “developments” will avert incursion’, (22 October 2007): ‘“The fact that [Iraq does not take steps to end rebel activities in Northern Iraq] renders the possibility of Turkey using her right of self-defense inevitable”, [Erdogan] said in Oxford, speaking through an interpreter’; ‘Erdogan threatens to raid Iraq’, The News (Pakistan), available at (8 October 2008): ‘“Turkey is in a position of self-defence when it comes to terrorism”, [Erdogan] added’.

12 See, e.g., ‘L'opération turque en Irak est un acte légitime d'autodéfense (Erdogan)’, Le Monde, 26 February 2008; ‘La Turquie entend poursuivre son offensive dans le nord de l'Irak’, Le Monde, 28 February 2008: ‘Ankara justifie ses opérations militaires contre les rebelles séparatistes kurdes par son “droit légitime à l'autodéfense”.’

13 See, e.g., statements from Belgium: Chambre des représentants de Belgique, Questions et réponses écrites (21 February 2008), in QRVA 52 010, at 1357; and the Netherlands: Ministerial Statement, 3 March 2008, available at,2008/03/Beantwoording-vragen-van-het-lid-Van-Bommel-over-e.html), quoted in T. Ruys, ‘Quo Vadit Jus Ad Bellum?: A Legal Analysis of Turkey's Military Operation against the PKK in Northern Iraq’, (2008) Melbourne Journal of International Law 23.

14 See, e.g., UN Doc. S/1996/479, at 2. While the notions of self-preservation and necessity are used in the English text, the French translation refers to ‘un souci justifié de légitime défense’.

15 See, e.g., Le Monde, 21 July 2005, 4.

16 See, e.g., statements from the United States UN Doc. S/1995/566, at 1 (Turkish intervention in Iraq, 1996)), the United Kingdom: ‘We recognize Turkey's right to self-defence from the terrorist activities of the Kurdish Workers Party’ ((1997) BYIL 631); Iraq (UN Doc. S/1996/561, at 2 and UN Doc. S/1997/129, at 2 (Turkish intervention in Iraq, 1996)).

17 An agreement was concluded in 1984 between Iraq and Turkey in order to authorize the latter to pursue the Kurdish rebels into Northern Iraq. This agreement was no longer in effect at the time of the 2008 Turkish incursion into Iraq.

18 Some authors argue that the concept of necessity may preclude the wrongfulness resulting from the violation of the prohibition on the use of force, provided that this violation does not amount to the gravity of an act of aggression (see, e.g., T. Christakis, ‘Unilatéralisme et multilatéralisme dans la lutte contre la terreur: l'exemple du terrorisme biologique et chimique’, in Bannelier et al., supra note 1, at 172). This is founded on the assumption that the prohibition on use of force, in contrast to the prohibition on aggression, has not a jus cogens nature; the violation thereof could therefore be ‘excused’ by a circumstance precluding the wrongfulness (according to Art. 26 of the Articles on State Responsibility). Authors supporting such a view generally refer to the considerations held on this subject by the former ILC Rapporteur, R. Ago, (1980)Yearbook of the International Law Commission, vol. II, Part 1, UN Doc. A/CN.4/318/Add.5 to 8, at 38–9, paras. 55–56). This position has not, however, been confirmed by the following works of the ILC itself. It is moreover based on a distinction between the prohibition on the use of force and the prohibition on aggression, which is highly difficult to establish in practice or is, at least, very debatable. At any rate, Turkey did not elaborate any justification based on the concept of necessity to justify its invasion of Northern Iraq in 2008.

19 UN Doc. S/2008/814, at 1; UN Doc. S/2008/816, at 1; UN Doc. S/2009/6, at 1; UN Doc. S/PV.6060, at 6.

20 See, e.g., statements from South Africa (UN Doc. S/PV.6060, at 8); Italy (UN Doc. S/PV.6060, at 13); Vietnam (UN Doc. S/PV.6060, at 13); Costa Rica (UN Doc. S/PV.6060, at 16); Belgium (UN Doc. S/PV.6060, at 17); Croatia (UN Doc. S/PV.6060, at 17); Denmark (; Hungary (ibid.); the United States (ibid., and ‘In Interview, Obama Talks of “New Approach” to Iran’, New York Times, 12 January 2009); Germany (; see also the statement from the UN Secretary-General (

21 See the Gaza Disengagement Plan, Section II.A.3, available at; see also the decision ruled by the Israeli Supreme Court on 30 January 2008 (Jaber Al-Bassiouni Ahmed and others v. 1. Prime Minister 2. Minister of Defence): ‘We should point out in this context that since September 2005 Israel no longer has effective control over what happens in the Gaza Strip. The military government that was in force in this territory in the past was ended by a decision of the government, and Israeli soldiers are no longer stationed in the territory on a permanent basis, nor are they in charge of what happens there’ (available at, para.12).

22 See, e.g., UN Doc. S/RES/1860 (adopted on 8 January 2009), in which the Security Council stresses ‘that the Gaza Strip constitutes an integral part of the territory occupied in 1967 and will be part of the Palestinian state’.

23 See, e.g., the statement of the Special Rapporteur for the Palestinian Territories occupied since 1967 for Presentation to the Special Session of the Human Rights Council on the Situation in the Gaza Strip, 9 January 2009, available at

24 According to this article, ‘[a] territory is considered occupied when it is actually placed under the authority of the hostile army’. The article adds that ‘the occupation extends only to the territory where such authority has been established and can be exercised’.

25 Art. 43 of the Hague Regulations.

26 See, e.g., UNGA resolutions such as UN Doc. A/RES/63/165 (2008).

27 Legal Consequences of the Construction of a Wall in the Occupied Palestinian Territory, Advisory Opinion of 9 July 2004, [2004] ICJ Rep. 194, para. 139.

30 One may distinguish between two main aspects of the condition of necessity. The first aspect regulates the triggering of the right of self-defence. Its effect is to impose on states the right to resort to force only after all the available alternatives to protect themselves have been exhausted. This aspect will be addressed in relation to the issue discussed in subsection, infra. There is another aspect which concerns the exercise of the right of self-defence. It implies that all the measures taken in self-defence must not exceed what is necessary to achieve the final result, that is, to repel the armed attack. This aspect will be addressed in relation to the condition of proportionality, which is analysed in subsection 3.2.2, infra.

31 See, e.g., F. Dubuisson, ‘La guerre du Liban de l'été 2006 et le droit de la légitime défense’, (2006) Revue belge de droit international 562.

32 Supra note 13.

33 See, e.g., regarding the Israeli intervention in Lebanon, statements from Russia (UN Doc. S/PV.5489, at 7); Argentina (UN Doc. S/PV.5489, at 9); Qatar (UN Doc. S/PV.5489, at 10); China (UN Doc. S/PV.5489, at 11); Japan (UN Doc. S/PV.5489, at 12); Congo (UN Doc. S/PV.5489, at 13); Tanzania (UN Doc. S/PV.5489, at 13); Denmark (UN Doc. S/PV.5489, at 15); Greece (UN Doc. S/PV.5489, at 17); France (UN Doc. S/PV.5489, at 17); Ghana (UN Doc. S/PV.5493 (Resumption 1), at 8); Brazil (UN Doc. S/PV.5493 (Resumption 1), at 19); New Zealand (UN Doc. S/PV.5493 (Resumption 1), at 33). See, e.g., regarding the Israeli intervention in Gaza, statements from Brazil (; Chile (ibid.); Denmark (ibid.); EU (ibid.); India (ibid.); Latvia (ibid.); Spain (ibid.); France (ibid., and UN Doc. S/PV.6060, at 9); South Africa (UN Doc. S/PV.6060, at 9); Indonesia (UN Doc. S/PV.6060, at 10); Vietnam (UN Doc. S/PV.6060, at 13); Burkina Faso (UN Doc. S/PV.6060, at 15); Costa Rica (UN Doc. S/PV.6060, at 16); Belgium (UN Doc. S/PV.6060, at 17); Egypt (UN Doc. S/PV.6060, at 18); Turkey (UN Doc. S/PV.6061, at 10); Austria (UN Doc. S/PV.6061, at 14); Mexico (UN Doc. S/PV.6061, at 19); Argentina (UN Doc. S/PV.6061 (Resumption 1), at 8); Pakistan (UN Doc. S/PV.6061 (Resumption 1), at 10); Iceland (UN Doc. S/PV.6061 (Resumption 1), at 15); Ecuador (UN Doc. S/PV.6061 (Resumption 1), at 16); Bolivia (UN Doc. S/PV.6061 (Resumption 1), at 17); Paraguay (UN Doc. S/PV.6061 (Resumption 1), at 17).

34 See, e.g., regarding the Israeli intervention in Lebanon, statements from Argentina (UN Doc. S/PV.5489, at 9); Japan (UN Doc. S/PV.5489, at 12); United Kingdom (UN Doc. S/PV.5489, at 12); Peru (UN Doc. S/PV.5489, at 14 and UN Doc. S/PV.5493 (Resumption 1), at 4); Denmark (UN Doc. S/PV.5489, at 15); Slovakia (UN Doc. S/PV.5489, at 16, and UN Doc. S/PV.5493, at 19); Greece (UN Doc. S/PV.5489, at 17, and UN Doc. S/PV.5493 (Resumption 1), at 3); the United States (UN Doc. S/PV.5493, at 17); Russia (UN Doc. S/PV.5493 (Resumption 1), at 2; Ghana (UN Doc. S/PV.5493 (Resumption 1), at 8; France (UN Doc. S/PV.5493 (Resumption 1), at 12); Finland speaking on behalf of the European Union (UN Doc. S/PV.5493 (Resumption 1), at 16); Switzerland (UN Doc. S/PV.5493 (Resumption 1), at 18); Brazil (UN Doc. S/PV.5493 (Resumption 1), at 19); Norway (UN Doc. S/PV.5493 (Resumption 1), at 23); Australia (UN Doc. S/PV.5493 (Resumption 1), at 27); Turkey (UN Doc. S/PV.5493 (Resumption 1), at 28); Djibouti (UN Doc. S/PV.5493 (Resumption 1), at 32); Canada (UN Doc. S/PV.5493 (Resumption 1), at 39); and Guatemala (UN Doc. S/PV.5493 (Resumption 1), at 41).

See, e.g., regarding the Israeli intervention in Gaza, supra note 20.

35 Supra note 13.

36 Only Iraq expressly condemned the Turkish invasion of its territory. This criticism came only some days after the launching of the Turkish operation (‘Bagdad condamne l'intervention militaire turque, Ankara justifie son action contre le PKK’, Le Monde, 26 February 2008).

37 See, e.g., statements from the EU (Presidency of the EU, Statement on the Terrorist Attacks of the PKK in Turkey over the Weekend, available at Declaracoes_PESC/20071022PESCPKK.html; Presidency of the EU, Statement on the Military Action Undertaken by Turkey in Iraqi Territory’, available at; the United States (BBC News, ‘Turkey Must End Iraq Raid – Bush’, 28 February 2008, available at

38 It could therefore hardly be argued that states were not aware of the Turkish operation.

39 See, e.g., Y. Arai-Takahashi, ‘Shifting Boundaries of the Right of Self-Defence – Appraising the Impact of the September 11 Attacks on Jus ad Bellum’, (2002) International Lawyer 1096; Ratner, supra note 1, at 908; Murphy, supra note 1, at 50 ff.; Stahn, supra note 1, at 50–1; Schrijver, supra note 1, at 285; O'Connell, supra note 1, at 899.

40 See, regarding the Israeli intervention in Lebanon, UN Doc. S/PV.5489, at 6. See, regarding the Turkish invasion of Northern Iraq, ‘Erdogan Hopes “Developments” Will Avert Incursion’, ( (22 October 2007)).

41 See, regarding the Israeli intervention in Lebanon, statements from Peru (UN Doc. S/PV.5489, at 14), France (UN Doc. S/PV.5489, at 17), Denmark (UN Doc. S/PV.5493 (Resumption 1), at 7), Ghana (UN Doc. S/PV.5493 (Resumption 1), at 8); see, regarding the Turkish operation in Iraq, statements from Belgium and the Netherlands (supra note 13).

42 Even interpreted in the most flexible way, the criteria provided in Art. 8 of the Articles on State Responsibility and, more particularly, the control criterion do not allow the attribution of attacks by non-state actors to a state if the latter is merely unable or unwilling to prevent or stop these attacks. Indeed, the control criterion requires at least an ‘effective’ control by the state over the activities conducted by non-state actors.

43 Some authors argue that Art. 9 of the Articles on State Responsibility can be interpreted in such a way that the Hezbollah activities could be assimilated into activities of the Lebanese government (T. Ruys, ‘Crossing the Thin Blue Line: An Inquiry into Israel's Recourse to Self-Defense against Hezbollah’, 2007 Stanford Journal of International Law 285). Art. 9 provides that ‘the conduct of a person or group of persons shall be considered an act of a State under international law if the person or group is in fact exercising elements of the governmental authority in the absence or default of the official authorities and in circumstances such as to call for the exercise of those elements of authority’. In the view of the above-mentioned authors, Art. 9 would be applicable to the relationships between Hezbollah and the Lebanese government. As a result, the Hezbollah attacks against Israel in 2006 could be attributed to Lebanon and qualified as an armed attack by a state, attack against which the right of self-defence could be exercised. The classical conception of the law of self-defence would therefore be preserved. This interpretation is certainly plausible. But the ILC comments on Art. 9 of the Articles on State Responsibility does not seem to envisage such an interpretation. At any rate, as will be demonstrated further, relying on state attribution rules and attempting to adapt them in order to address particular use of force issues does not seem relevant (infra).

44 See, for the same position, Verhoeven, supra note 1, at 59.

45 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, para. 405.

46 Supra note 2, at 103, para. 195.

47 Armed Activities on the Territory of the Congo (Democratic Republic of the Congo v. Uganda), Judgment of 19 December 2005, [2005] ICJ Rep., at 222–3, para. 146.

48 See, for the same position, T. Becker, Terrorism and the State: Rethinking the Rules of State Responsibility (2006), 182–3.

49 During the final stages of the preparatory works of UNGA Resolution 3314 (XXIX), the draft committee submitted a text in which the notion of substantial involvement included very flexible forms of support, such as tolerating armed activities on one's territory. This text has been clearly refuted by all the other states: see GA, Report of the Special Committee on the Definition of Aggression, UN Doc. A/8719 (1972), annex II, appendixes A and B.

50 See, for a similar interpretation, the separate opinion of Judge Kooijmans, annexed to the judgment rendered by the ICJ in Armed Activities, supra note 47, at 314, para. 28.

51 Discussions during the preparatory works of Art. 51 of the UN Charter were mainly devoted to finding a solution accommodating the regional arrangements with the UN Charter provisions concerning the use of force. There was no discussion on the (state or non-state) nature of the author of the armed attack triggering the right of self-defence. In fact, one of the first US projects on Art. 51 provided that the right of self-defence could be exercised in case of an armed attack by a state (Foreign Relations of the United States. 1945 (1967), I, 685–6). The words ‘by a state’ were deleted in the final version of Art. 51 without this suppression giving rise to any substantial discussion.

52 The (state or non-state) nature of the author of the attack, which was the casus foederis of the mutual defensive obligation, was not discussed in many of these treaties. Some treaties make this defensive obligation dependent upon an armed attack by a state while others (sometimes concluded in the same region) merely require the occurrence of an armed attack.

53 Supra note 2.

54 Supra note 47.

55 See, e.g., the international reactions to past Israeli interventions in Egypt (UN Doc. S/PV.685, at 6; UN Doc. S/PV.694, at 24; UN Doc. S/PV.749, at 22; UN Doc. S/PV.748, at 6–7); Syria (UN Doc. S/PV.710, at 12; UN Doc. S/PV.710, at 7; UN Doc. S/PV.710, at 12; UN Doc. S/PV.844, at 11; UN Doc. S/PV.1000, at 11; UN Doc. S/PV.1004, at 2–3; UN Doc. S/PV.1165, at 15; UN Doc. S/PV.1166, at 6; UN Doc. S/PV.1292, at 13); Jordan (UN Doc. S/PV.1320, at 19; UN Doc. S/PV.1320, at 20–21; UN Doc. S/PV.1323, at 3; UN Doc. S/PV.1327, at 4; UN Doc. S/PV.1402, at 1; UN Doc. S/PV.1402, at 2; UN Doc. S/PV.1402, at 5; UN Doc. S/PV.1403, at 6; UN Doc. S/PV.1437, at 3); Lebanon (UN Doc. S/PV.1460, at 6–7; UN Doc. S/PV.1461, at 4; UN Doc. S/PV.1643, at 12; UN Doc. S/PV.1643, at 13; UN Doc. S/PV.1643, at 14; UN Doc. S/PV.1643, at 15; UN Doc. S/PV.1644, at 3; UN Doc. S/PV.1644, at 16; UN Doc. S/PV.1644, at 19; UN Doc. S/PV.1648, at 11; UN Doc. S/PV.1649, at 17; UN Doc. S/PV.1649, at 19; UN Doc. S/PV.1650, at 2; UN Doc. S/PV.1650, at 10; UN Doc. S/PV.1661, at 6; UN Doc. S/PV.1767, at 6; UN Doc. S/PV.2374, at 3; UN Doc. S/PV.2375, at 10; UN Doc. S/PV.2377, at 1–2; UN Doc. S/PV.2377, at 3–4); and Tunisia (UN Doc. S/PV.2611, at 4). See also the international reactions to the past incursions of South Africa into neighbouring countries (UN Doc. S/PV.2616, at 11), the United States into Libya (UN Doc. S/PV.2671, at 16) or the United Kingdom into Yemen (UN Doc. S/PV.1106, at 14–15; UN Doc. S/PV.1108, at 7–8; UN Doc. S/PV.1108, at 10; UN Doc. S/PV.1110, at 4–5; UN Doc. S/PV.1109, at 9).

56 See, e.g., statements from Sudan (UN Doc. S/1998/786, at 2) concerning US attacks on a Sudanese factory in 1998; and the United States and the United Kingdom (UN Doc. S/PV.2655, at 113 and 118) concerning the Israeli hijacking of a Libyan aircraft in 1986.

57 Supra note 41; infra note 62.

58 See, for the same conclusion, D. W. Bowett, ‘Reprisals Involving Recourse to Armed Force’, (1972) AJIL 13; Gray, supra note 1, at 136.

59 See, e.g., statements on the Israeli incursion into Lebanon in 1968 by the United States (UN Doc. S/PV.1460, at 6); China (UN Doc. S/PV.1461, at 6); Brazil (UN Doc. S/PV.1462, at 2); France (UN Doc. S/PV.1462, at 3); and the USSR (UN Doc. S/PV.1462, at 5).

60 See, for a similar observation, F. Megret, ‘“War”? Legal Semantics and the Move to Violence’, (2002) EJIL, at 378.

61 See, e.g., statements from Japan (UN Doc. S/PV.5489, at 12); Ghana (UN Doc. S/PV.5489, at 8 and UN Doc. S/PV.5493 (Resumption 1), at 9); Slovakia (UN Doc. S/PV.5493, at 19); Mexico (UN Doc. S/PV.5493 (Resumption 1), at 45). See also Bowett's observations concerning the Lebanese situation in 1972, supra note 58, at 20; these observations remain particularly relevant.

62 See, e.g., UN Doc. S/1996/479, at 2, para. 2.

63 See, e.g., ibid., para. 3. In this letter Turkey insists on the fact that it can ‘neither ask the Government of Iraq to fulfil its obligation [to prevent the use of its territory for the staging of terrorist acts against Turkey] nor find any legitimate authority in the north of Iraq to hold responsible under international law for terrorist acts committed or originated there’.

64 This obligation is provided in UNGA Resolution 2625 (XXV). It is also contained in UNGA Resolutions 49/60 (1994) and 1373 (2001), some UNSC resolutions adopted in particular situations and some treaties regulating specific cases. It is almost uncontested that it has a customary nature.

65 It is an obligation of due diligence which compels states to do as much as they can to prevent hostile acts from being committed against another state from their territory. See, on the nature of the due diligence obligations, R. Pisillo-Mazzeschi, ‘The Due Diligence Rule and the Nature of International Responsibility of States, (1992) German Yearbook of International Law 22. See also, for a jurisprudential application of a due diligence obligation, Application of the Convention on the Prevention and Punishment of the Crime of Genocide, supra note 45, para. 430.

66 See, for a same opinion, S. A. Barbour and Z. A. Salzman, ‘“The Tangled Web”: The Right of Self-Defense against Non-state Actors in the Armed Activities Case’, (2008) New York University Journal of International Law and Politics 84; contra Trapp, supra note 1, at 147. The author contends that a state unable to stop hostile activities on its territory is under an obligation to accept any foreign assistance in countering such activities. According to the author, ‘a state's consistent failure to address its counter-terrorism incapacity, where such assistance is available, could be interpreted as an unwillingness to meet its international terrorism prevention obligations [and this state should be held responsible]’. However, as explained above, a state may be prevented from accepting some foreign help because of national political pressure or other particular factors. Such a state should not therefore be automatically considered unwilling to fulfil its due diligence obligation. One could consider it as merely unable to do so and, as a result, not necessarily responsible.

67 Supra note 30.

68 In this way, it is nonsense to maintain, as some authors do (see, e.g., Barbour and Salzman, supra note 66, at 90), that there is a clash between the law of self-defence and the law of state responsibility on the ground that an action taken in self-defence in response to attacks by non-state actors may eventually take place on the territory of a state which is not responsible for these attacks.

69 See Nicaragua, supra note 2, at 180, para. 231; Oil Platforms (Islamic Republic of Iran v. United States of America), Judgement of 6 November 2003, [2003] ICJ Rep. 192, para. 64; Armed Activities, supra note 47, at 222–3, paras. 146–147.

70 See, for a similar conclusion, Gray, supra note 1, at 155.

71 See the following judgments in which the ICJ asserted that an armed attack under Art. 51 of the UN Charter constituted one of the most grave uses of force: Nicaragua, supra note 2, at 101, para.191; Oil Platforms, supra note 69, at 192, para. 64.

72 See on this subject R. Kolb, Ius contra bellum. Le droit international relatif au maintien de la paix (2003), 201–2.

73 See, e.g., Gray, supra note 1, at 180–1.

74 In the Nicaragua case, the ICJ clearly asserted that (collective) self-defence can never be exercised in response to uses of force which do not amount to an act of aggression. However, in a very ambiguous statement, the Court admitted that less grave uses of force could justify some ‘proportionate countermeasures on the part of the State which had been the victim of [these uses of force]’. It added that ‘[t]hey could not justify counter-measures taken by a third State . . . and particularly could not justify intervention involving the use of force’ (Nicaragua, supra note 2, at 127, para. 249). One may interpret this statement in a way that the Court considered the notion of countermeasure, which was used in the judgment, as synonymous with the notion of intervention involving the use of force. According to this interpretation, such an intervention would thus be admitted in reaction to less grave uses of force, provided that it is resorted to only by the victim state (and not by a third state). At least one may contend that the Court has not excluded such an interpretation. The latter has been supported by many scholars (see, e.g., J. L. Hargrove, ‘The Nicaragua Judgment and the Future of the Law of Force and Self-Defense’, (1987) AJIL 138; T. J. Farer, ‘Drawing the Right Line’, (1987) AJIL 113; L. B. Sohn, ‘The International Court of Justice and the Scope of the Right of Self-defense and the Duty of Non-intervention’, in Y. Dinstein (ed.), International Law at a Time of Perplexity: Essays in Honour of Shabatai Rosenne (1989), 877–8).

75 Separate Opinion of Judge Simma, annexed to the judgment of the ICJ in Oil Platforms, supra note 69, at 332).

76 According to para. 5 of the resolution, ‘An armed attack triggering the right of self-defence must be of a certain degree of gravity . . . [but,] [i]n case of an attack of lesser intensity the target State may . . . take strictly necessary police measures to repel the attack’ (available at The notion of police measures is particularly ambiguous. It normally entails only the use of force limited to the territory of the state which takes these measures. But what is therefore the point of recognizing such a right? Asserting this right is only useful if it implies extraterritorial use of force, since every state is empowered to take police measures on its territory. In fact, para. 5 of the resolution is the result of a compromise between the members of the International Law Institute, some of them (such as Mr Roucounas ((2007) 72 Yearbook of the Institute of International Law, at 170), Mr Koroma (ibid., at 174), Mr Sucharitkul (ibid., at 179), Mr Reisman (ibid., at 184)) rejecting any extraterritorial use of force in reaction to less grave uses of force than an armed attack, while others (such as Mr Gaja (ibid., at 178), Mr Arsanjani (ibid., at 179), Mr Momtaz (ibid., at 182), Mr Hafner (ibid., at 184), Mr Tomuschat (ibid., at 207), Mr Schewebel (ibid., at 208) supported the view that some strictly proportionate force could still be used in this situation, provided that it was resorted to by the victim state only. That is why the resolution remains ambiguous on the subject.

77 See the letter published in the Sunday Times (11 January 2009) and signed by many scholars including I. Brownlie, R. Falk, C. Chinkin, and M. C. Bassiouni (‘Israel's Bombardment of Gaza Is Not Self-Defence – It's a War Crime’, available at

78 See more particularly, for these two last conditions, para. 5 of the resolution adopted by the International Law Institute on the law of self-defence (available at and the records of the meetings of the Institute on this subject (see supra note 76). See also, for the condition of proportionality, the same records of the Institute as well as the opinion of Judge Simma (supra note 75) and the letter published in the Sunday Times (supra note 77).

79 See, for the same position, I. Brownlie, International Law and the Use of Force by States (1963), 366; R. Higgins, Problems and Process (1994), 251.

80 Supra note 33.

81 Supra note 37.

82 Supra note 33.

83 See Nicaragua, supra note 2, at 122, para. 237; Oil Platforms, supra note 69, at 198, paras. 76–77; Armed Activities, supra note 47, at 223, para. 147.

84 Most of the scholars who support such a conception refer to the considerations held on this subject by the former ILC Rapporteur, R. Ago (see (1980) Yearbook of the International Law Commission, II, Part 1, UN Doc. A/CN.4/318/Add.5–8, at 67, para. 121).

85 Supra note 30.

* Postdoctoral researcher, Belgian National Fund for Scientific Research (FNRS); lecturer in humanitarian law at the University of Louvain; member of the International Law Centre at the University of Louvain [].

Recommend this journal

Email your librarian or administrator to recommend adding this journal to your organisation's collection.

Leiden Journal of International Law
  • ISSN: 0922-1565
  • EISSN: 1478-9698
  • URL: /core/journals/leiden-journal-of-international-law
Please enter your name
Please enter a valid email address
Who would you like to send this to? *



Altmetric attention score

Full text views

Total number of HTML views: 0
Total number of PDF views: 0 *
Loading metrics...

Abstract views

Total abstract views: 0 *
Loading metrics...

* Views captured on Cambridge Core between <date>. This data will be updated every 24 hours.

Usage data cannot currently be displayed