Ch.VII Action with Respect to Threats to the Peace, Breaches of the Peace, and Acts of Aggression, Article 51
Georg Nolte, Albrecht Randelzhofer
Edited By: Bruno Simma, Daniel-Erasmus Khan, Georg Nolte, Andreas Paulus, Nikolai Wessendorf (Assistant Editor)
- UN Charter — United Nations (UN) — Armed attack — Self-defence — International peace and security
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 the 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.
See also the select bibliography on Art. 2 (4).
- Bothe M, ‘Die Erklärung der Generalversammlung der Vereinten Nationen über die Definition der Aggression’ (1975) 18 JIR 127.
- ——— ‘Terrorism and the Legality of Pre-emptive Force’ (2003) 13 EJIL 227.
- Cannizzaro E, ‘Contextualizing Proportionality: Jus Ad Bellum and Jus In Bello in the Lebanese War’ (2006) 864 IRRC 779.
- Cassin V, Debevoise W, Kailes H, and Thompson T, ‘The Definition of Aggression’ (1975) 16 Harv Intl LJ 589.
- (p. 1398) Chainoglou K, Reconceptualising the Law of Self-Defence (Ant N Sakkoulas, Bruylant 2008).
- Constantinou A, The Right of Self-Defence under Customary International Law and Article 51 of the UN Charter (Ant N Sakkoulas, Bruylant 2000).
- Corten O, The Law Against War: The Prohibition on the Use of Force in Contemporary International Law (Hart 2010).
- Dinstein Y, War, Aggression and Self-Defence (5th edn, CUP 2012).
- Fassbender B, ‘Selbstverteidigung und Staatengemeinschaftsinteresse in der Zeit des Völkerbundes’ in T Bruha, S Heselhaus, and T Marauhn (eds), Legalität, Legitimität und Moral (Mohr Siebeck 2008) 99.
- Franck TM, ‘On Proportionality of Countermeasures in International Law’ (2008) 102 AJIL 715.
- Gardam J, Necessity, Proportionality and the Use of Force by States (CUP 2004).
- Gazzini T, The Changing Rules on the Use of Force in International Law (Juris Publishing 2005).
- Gill TD, ‘The Law of Armed Attack in the Context of the Nicaragua Case’ (1988) 1 Hague YB Intl L 30.
- Gray C, International Law and the Use of Force (3rd edn, OUP 2008).
- Green J A, The International Court of Justice and Self-Defence in International Law (Hart 2009).
- Greenwood C, ‘Self-Defence’ MPEPIL (online edn).
- Greig DW, ‘Self-Defence and the Security Council: What Does Article 51 Require?’ (1991) 40 ICLQ 366.
- Kreß C, Gewaltverbot und Selbstverteidigungsrecht nach der Satzung der Vereinten Nationen bei staatlicher Verwicklung in Gewaltakte Privater (Duncker & Humblot 1995).
- Krisch N, Selbstverteidigung und kollektive Sicherheit (Springer 2001).
- Lagoni R, ‘Gewaltverbot, Seekriegsrecht und Schiffahrtsfreiheit im Golfkrieg’ in W Fürst, R Herzog, and DC Umbach (eds), Festschrift für Wolfgang Zeidler, vol 2 (de Gruyter 1987) 1833.
- Lubell N, Extraterritorial Use of Force Against Non-State Actors (OUP 2010).
- McCormack TLH, Self-Defense in International Law: The Israeli Raid on the Iraqi Reactor (Palgrave Macmillan 1996).
- Murphy SD, ‘Terrorism and the Concept of “Armed Attack” in Article 51 of the U.N. Charter’ (2002) 43 Harv Intl LJ 41.
- ——— ‘Self-Defense and the Israeli Wall Advisory Opinion: An Ipse Dixit from the ICJ?’ (2005) 99 AJIL 62.
- Reisman WM and Armstrong A, ‘The Past and Future of the Claim of Preemptive Self-Defense’ (2006) 100 AJIL 525.
- Ronzitti N, ‘The Expanding Law of Self-Defence’ (2006) 11(3) J Conflict & Security L 343.
- Roscini M, ‘World Wide Warfare—Jus Ad Bellum and the Use of Cyber Force’ (2010) 14 Max Planck YB UN L 85.
- Ruys T, Armed Attack and Article 51 of the UN Charter (CUP 2010).
- Ruys T and Verhoeven S, ‘Attacks by Private Actors and the Right of Self-Defence’ (2005) 10 J Conflict & Security L 289.
- Schachter O, ‘Self-Defense and the Rule of Law’ (1989) 83 AJIL 259.
- Tams C J, ‘Light Treatment of a Complex Problem: The Law of Self-Defence in the Wall Case’ (2006) 16 EJIL 963.
- ——— ‘The Use of Force Against Terrorists’ (2009) 20 EJIL 359.
- Tomuschat C, ‘Der 11. September 2001 und seine rechtlichen Konsequenzen’ (2001) 33 EuGRZ 535.
- Wilmshurst E, ‘The Chatham House Principles of International Law on the Use of Force in Self-Defence’ (2006) 55 ICLQ 963.
- Zimmermann A, ‘The Second Lebanon War: Jus Ad Bellum, Jus In Bello and the Issue of Proportionality’ (2007) 11 Max Planck YB UN L 99.
(p. 1399) A. Evolution and Significance of the Right of Self-Defence*
1 The development of the right of self-defence should be viewed against the background of the development of international law towards the prohibition of war, and, eventually, of the use of force. Until the beginning of the twentieth century the right of self-defence was of only modest significance.1 Public international law permitted States to wage war freely,2 so that no justification for doing so was needed. When, in spite of this, States explicitly invoked their right of self-defence, their purpose was merely political.3 Even so, the right of self-defence played a certain role as a legal justification with regard to hostilities not occurring in a state of war, since the use of force in times of peace was in fact restricted by international law.4 The content and scope of the right of self-defence were relatively unclear, however, and extended well into the sphere of self-help.5
2 At the beginning of the twentieth century, when the freedom to resort to war became more and more restricted,6 the right of self-defence gained in significance. This development first culminated in the conclusion of the Briand-Kellogg Pact in 1928. The general prohibition of war laid down in Art. I of the Pact was subject only to the reservation of the right of self-defence.7 Consequently, it was solely in the exercise of the right of self-defence that war could still be lawful. Various incidents and legal developments subsequent to the entry into force of the UN Charter led to a further increase in the importance of the right of self-defence. Today, the right of individual or collective self-defence is invoked with regard to almost every unilateral use of military force.8
3 The prohibition of the use of force embodied in Art. 2 (4) not only proscribes war, but any use or threat of force. Apart from the now obsolete clauses concerning the former enemy States,9 the UN Charter contains only two exceptions to the prohibition of force, namely SC enforcement actions pursuant to Chapter VII, and the right to individual and collective self-defence laid down in Art. 51. International legal practice since 1945, (p. 1400) contrary to the intentions of the authors of the Charter, has continued to see a significant amount of unilateral use of force by States. Yet in this respect the Charter provides in Art. 51 for a regulation which allows individual States the threat or use of force, in principle, only under the conditions stipulated therein.10 The right of self-defence laid down in Art. 51 of the UN Charter has therefore become the pivotal point upon which disputes concerning the lawfulness of the use of force in interstate relations usually concentrate. Such disputes not only raise questions of legal interpretation, but even more often questions of fact, as shown by several decisions of the ICJ.11
4 The interpretation and the development of Art. 51, like the law on the use of force in general, remain subject to the applicable rules of international law. The UN Charter is to be interpreted according to the general rules on treaty interpretation which are codified in Arts 31–33 VCLT, and which also reflect customary international law, taking duly into account the character of the Charter as the constitutive document for the organization of the international community of States.12 The general rule on treaty interpretation of Art. 31 (3) (b) VCLT requires in particular that ‘any subsequent practice in the application of the treaty which establishes the agreement of the parties regarding its interpretation’ and any ‘established practice of the organisation’ (Art. 2 (f) VCLTIO as a reflection of customary international law) be taken into account. In principle, the rules on treaty interpretation and on the sources of international law do not exclude the possibility that Art. 51 is reinterpreted, including on the basis of subsequent practice.13 Theoretically it is even possible that an additional exception to the general prohibition of the use of force could develop alongside Art. 51 by way of superseding customary international law.14 In view, however, of the fundamental importance of the right to self-defence for the Charter system of collective security, the conditions for the recognition of any significant reinterpretation of, or superseding exception to, Art. 51 are strict. This is true regardless of whether the Charter is seen as possessing a special constitutional status, or as only having the quality of an ordinary treaty. It is therefore not sufficient that one or more ‘incidents’ lead a State or a group of States to assert a certain reinterpretation of Art. 51, or to postulate another exception to the general prohibition of the use of force (Art. 2 (4)), even if such assertions are not immediately, clearly, or broadly opposed by a number of other States. It would rather be necessary that such assertions (p. 1401) of a reinterpretation or a new exception are ‘generally accepted’ or that they constitute an ‘established practice’.
5 A significant amount of current disagreement over the proper interpretation of Art. 51, both among States and among commentators, can ultimately be traced to underlying differences of opinion over the interpretation and application of the rules on the sources of international law, as well as to different assessments of what the respective historical situation (with its characteristic threat scenarios) requires.15 If properly identified, such underlying differences of opinion should not prevent an appropriate interpretation of Art. 51.
6 Arts 51 and 2 (4) do not exactly correspond to one another in scope, ie not every use of force contrary to Art. 2 (4) may be responded to with armed self-defence. The UN Charter did not intend to exclude self-defence entirely, but has restricted its scope considerably. A comparison of the different terms used in the two provisions illustrates that, remaining uncertainties apart, ‘armed attack’ is a narrower notion than ‘threat or use of force’.16 If Art. 51 is thus read in connection with Art. 2(4), the—at first sight—counter-intuitive conclusion is that any State which is affected by another State’s unlawful use of force that does not reach the threshold of an ‘armed attack’, is bound, if not exactly to endure the violation, at least to respond only by means falling short of the use of cross-border force. One possible means of defence is, of course, the use of force by a State on its own territory. While such means may sometimes not be fully effective, this result is intended by the Charter, since the unilateral use of force is meant to be excluded as far as it is bearable for States, in view of the typical dangers of escalation which are connected with mutual uses of armed force. Until an armed attack occurs, States are expected to renounce forcible self-defence. Because of the pre-eminent position of the SC within the Charter system of collective security, the affected State can in that situation merely call upon the SC to qualify the violations of Art. 2 (4) as constituting a breach of the peace and to decide on measures under Chapter VII. Only if and when the prohibited use of force rises to an armed attack can the State concerned unilaterally resort to forcible measures for its defence. But even this authority is limited in two ways: first, the State acting in self-defence must observe the principle of ‘proportionality’;17 secondly, it has to report immediately to the SC the measures taken, and it has to discontinue (p. 1402) them as soon as the latter has itself taken the measures necessary for the maintenance of international peace.
7 The view of the present authors that there exists a gap between Arts 2 (4) and 51 not only corresponds to the prevailing view in international legal writings18 but has also been confirmed by the ICJ’s Nicaragua19 and Oil Platforms20 judgments, as well as, indirectly, in the Congo v Uganda decision.21 It clearly follows from the ICJ’s observations that not every use of force is necessarily to be considered an armed attack.22
8 This conclusion has, as a matter of policy, given rise to objections, for it means that there is not always effective protection against States violating the prohibition of the use of force, as long as they do not resort to an armed attack. Therefore, critics in international legal doctrine have made efforts to deny or to fill the lacuna existing between Arts 2 (4) and 51. Some do not regard Art. 51 as being more restrictive than the term ‘use of force’ in Art. 2 (4)23 and others would permit ‘proportionate defensive measures’ against any smaller-scale ‘unlawful use of force “short of” an armed attack (“agression armée”) within the meaning of Article 51’.24 Such approaches may at first sight appear more satisfactory policywise than the prevailing view. It is true that a State which has violated the prohibition of the use of force has at least provided the primary cause for the response. And it must be admitted that if States are bound to endure acts of force that do not reach the intensity of an armed attack, they may remain devoid of fully effective protection until the SC has taken remedial measures, which may not always come about. These considerations do not, however, override the concern which underlies the prevailing view that the danger of escalation which is inherent in most forms of transboundary uses of force justifies the rule that States deal with small-scale uses of force against them by using force on their own territory or by using non-violent means, and thus in a way which does not involve the use of cross-border force. The concern that an escalation, or even a full-scale war, could be the consequence of a State responding in self-defence to slight uses of armed force, such as a small frontier incident, cannot be dispelled by (p. 1403) responding that force used in self-defence must never be disproportionate25 since the principle of proportionality is not determinate enough to provide sufficiently clear guidance in such situations. The fact that the Charter has not explicitly banned uses of force on a smaller scale does not mean that self-defence against such acts is permissible. On the contrary, it can be claimed equally well that, by deliberately not pronouncing upon the small-scale use of force in the context of self-defence and by allowing self-defence only in response to an armed attack, the UN Charter has expressed the position that no small-scale forcible self-defence is permissible against small-scale uses of force. Thus the critique of the prevailing position merely leads back to the question of whether, and to what extent, there is a broader general right of self-defence apart from Art. 51, but it does not provide an independent foundation for a right to ‘small-scale self-defence’.
9 What has been shown so far is only that Arts 2 (4) and 51 do not entirely coincide in their scope. But whether the UN Charter really does allow for forcible self-defence only in the case of an armed attack also depends on whether and to what extent a general right of self-defence exists apart from Art. 51, and whether Art. 51 is intended to abolish that right, or at least to restrict it as far as possible. This point is the subject of a long-standing controversy.
10 The prevailing view considers Art. 51 to exclude any self-defence, other than that in response to an armed attack, referring, above all, to the purpose of the UN Charter, ie to restrict as far as possible the use of force by individual States.26 According to this view, the designation in Art. 51 of the right of self-defence as ‘inherent’ simply means that the right is also vested in States other than UN members, and that UN members may give assistance to a non-member falling victim to an armed attack.27 This position is supported, though by no means unambiguously, by most commentators28 in that it restricts the right of self-defence to cases in which ‘an armed attack occurs against (p. 1404) a Member of the United Nations’. The view that Art. 51 has superseded any previous customary right of self-defence does not, however, exclude that subsequent practice concerning the scope and the limits of the right of self-defence needs to be taken into account when interpreting the provision.29
11 Another approach regards the customary right to self-defence as not being affected by Art. 51, but rather as having only received a particular emphasis, in a declaratory manner, for the case of an armed attack.30 Above all, this approach is intended to serve as a justification for certain traditional forms of self-defence, even of self-help, in particular cases. For example, the protection of the lives and property of a State’s own nationals abroad,31 and even the forcible protection of certain economic interests in a foreign country,32 have been claimed to be justified on the grounds of self-defence not prohibited by Art. 51. The latter case aptly illustrates that this view of Art. 51 is highly questionable.
12 The content and scope of a customary right of self-defence are unclear and could extend far into the spheres of self-help in such a way that its continuing existence would, to a considerable extent, reintroduce the unilateral use of force by States, the far-reaching abolition of which is intended by the UN Charter.
In the case of individual self-defence, the exercise of this right is subject to the State concerned having been the victim of an armed attack. Reliance on collective self-defence of course does not remove the need for this.35…
The Court has recalled above (paragraphs 193 to 195) that for one State to use force against another, on the ground that that State has committed a wrongful act against a third State, is regarded as lawful, by way of exception, only when the wrongful act provoking the response was an armed attack. Thus the lawfulness of the use of force by a State in response to a wrongful act of which it has not itself been the victim is not admitted when this wrongful act is not an armed attack. In the view of the Court, under international law in force today—whether customary international law or that of the United Nations system—States do not have a right of ‘collective’ armed response to acts which do not constitute an ‘armed attack’.36
… there are two levels to be distinguished: there is, first, the level of ‘armed attacks’ in the substantial, massive sense of amounting to ‘une agression armée’, to quote the French authentic text of Article 51. Against such armed attacks, self-defence in its not infinite, but still considerable, variety would be justified. But we may encounter also a lower level of hostile military action, not reaching the threshold of an ‘armed attack’ within the meaning of Article 51 of the United Nations Charter. Against such hostile acts, a State may of course defend itself, but only within a more limited range and quality of responses (the main difference being that the possibility of collective self-defence does not arise, cf. Nicaragua) and bound to necessity, proportionality and immediacy in time in a particularly strict way.42
While Judge Simma’s position has found some support among commentators, the prevailing view, in particular among States, does not accept this position.43
14 Since the right of self-defence embodied in Art. 51 is restricted to the case of an armed attack, and in the absence of further exceptions to Art. 2 (4) allowing for the use of force by individual States,44 the exercise of force for the enforcement of a vested right (p. 1406) or for the purpose of ending another State’s unlawful behaviour is prohibited.45 Not even arbitral awards or judgments by the ICJ46 may be enforced by means of forcible self-help. It is of particular importance that countermeasures (formerly reprisals), once the most frequently used form of force, are today likewise only admissible in so far as they do not involve the use of armed force.47
15 For example, the firing of 23 cruise missiles, on 26 June 1993, by US warships in the Red Sea and the Persian Gulf upon Iraqi intelligence headquarters in Baghdad, in response to an unsuccessful Iraqi attempt to murder former US President Bush during his visit to Kuwait from 14 to 16 April 1993, could not be justified as an act of self-defence, as US President Clinton claimed on the evening of the raid.48 Even if the attempted assassination of former President Bush were to be qualified as an armed attack, it is quite clear that in this situation, such an armed attack no longer existed more than two months later, nor did the threat of such an attack. On 27 June 1993, Madeleine Albright, US Permanent Representative to the UN also referred to Art. 51 to justify the raid, but added that the raid ‘was designed to damage the terrorist infrastructure and deter further acts of aggression against the United States’.49 This suggests that the raid was intended to be a countermeasure but as such it was not compatible with the existing law governing the use of military force.50
16 The term ‘armed attack’ (French: ‘agression armée’) represents the key notion of the concept of self-defence pursuant to Art. 51. Its interpretation decides how far unilateral use of force is still admissible and is of utmost significance for the effectiveness of the rules of international law on war prevention. The more clear-cut and unambiguous the definition of an ‘armed attack’, the more plausible attempts by States to justify illicit uses (p. 1407) of force as self-defence can be successfully discredited. In addition, since the facts which give rise to an allegation that an armed attack has occurred are mostly disputed51 it is also important to note that the alleged victim State has the burden of proving the existence of an armed attack if it wishes to justify any use of force in self-defence.52
17 The UN Charter uses the terms ‘attack’ and ‘aggression’ in Arts 1 (1), 39, 51, and 53, albeit without defining them precisely. Following futile attempts during the League of Nations era, the UN has been unsuccessfully striving since 1950, first in the ILC, then in four subsequent Special Committees of the GA, for a definition of these terms.53 The adoption of GA Res 3314 (XXIX) (14 December 1974)54 constituted a significant indirect contribution to this effort. The ‘Definition of Aggression’ contained therein, however, apart from constituting a mere recommendation, formally only claims to specify the notion of ‘act of aggression’ as it is embodied in Art. 39 of the Charter, and not that of ‘armed attack’ as used in Art. 51.55 Pursuant to paras 2 and 4 of the Preamble, as well as according to Art. 6, the Definition does not contain an interpretation of the right of self-defence in response to an armed attack.56 The travaux préparatoires of the Definition also confirm that a definition of the notion of ‘armed attack’ was not intended.57 In the Special Committee which worked out the Definition, the United States, supported by other Western States, strongly emphasized that the task of the committee was to elaborate a definition of aggression pursuant to Art. 39 of the UN Charter, without prejudice to other provisions of the Charter, especially Art. 51.58 Like the Soviet Union,59 the US expressed the view that the notions of ‘act of aggression’ and ‘armed attack’ are not identical.60 While it must therefore be assumed that the notions of ‘armed attack’ (‘agression armée’) and ‘act of aggression’ (‘acte d’agression’) do not necessarily coincide fully and (p. 1408) that ‘armed attack’ is the narrower concept of the two,61 the difference between the two is so small that it is often overlooked.62
18 The jurisprudence of the ICJ has brought about only a modest amount of clarification in this respect. On the term ‘armed attack’,63 the ICJ simply remarked in the Nicaragua judgment that ‘[t]here appears now to be general agreement on the nature of the acts which can be treated as constituting armed attacks’.64 This statement, albeit fairly surprising in view of the unsuccessful previous efforts towards agreeing on a definition of ‘armed attack’, has not led the ICJ to provide a more specific definition of the term. Instead, the ICJ merely gave one example in order to illustrate the existence of an armed attack in a specific situation and stipulated that the notion not only comprises unspecified cross-border actions by regular forces, but also the participation of a State in the use of force by unofficial armed bands, as described in Art. 3 (g) of the Definition of Aggression.65 The Court reaffirmed this approach in 2005 in its Congo v Uganda judgment.66 Such use by the ICJ of GA Res 3314 demonstrates the (indirect) importance of the definition contained therein for the purpose of defining the concept of armed attack. In its Oil Platforms judgment, the ICJ avoided precisely determining whether several (established and alleged) instances of uses of force in a maritime context, individually or together, constituted an armed attack.67 The jurisprudence of the ICJ, while leaving many controversial questions unanswered, demonstrates that the ascertainment of the specific factual circumstances of each alleged instance of an exercise of the right of self-defence, and the corresponding attribution of the burden of proof, are often more decisive for the determination of a situation, in particular the outcome of a judicial decision, than the resolution of certain questions of legal interpretation.
19 GA Res 3314 has proved to be of continued value as an (indirect) indication of whether certain forms of uses of force constitute ‘armed attacks’ in the sense of Art. 51 and this (p. 1409) seems to have been accepted by the vast majority of States.68 The lasting significance of GA Res 3314 has been reaffirmed in 2010 when it served as the basis for the definition of the crime of aggression for the purposes of the Statute of the International Criminal Court.69
20 As has been pointed out earlier,70 the notion of ‘armed attack’ has a narrower meaning than the term ‘threat or use of force’ in the sense of Art. 2 (4). Whereas an ‘armed attack’ always includes a use of force in the sense of Art. 2 (4), not all such uses of force constitute an ‘armed attack’. The latter only exists when force is used on a relatively large scale, is of a sufficient gravity, and has a substantial effect.71 Thus, mere frontier incidents, such as the incursion of an armed border patrol into another State’s territory, may well be characterized as a use of force contrary to Art. 2 (4), but not necessarily as an ‘armed attack’. This exclusion of frontier incidents from the concept of armed attack has been confirmed by the ICJ.72 A fortiori, a mere ‘threat’ of the use of force is not, in principle, an ‘armed attack’, but would have to be significantly qualified to be so regarded.73
21 The criterion of gravity need not necessarily be applied in a temporally isolated manner or be confined to one particular act. The accumulation of several smaller attacks, each of which does not as such reach the necessary gravity, can under certain circumstances trigger the right of self-defence of