Collective self-defence

add the UN-authorized actions in Korea and Iraq as further examples of collective self-defence.6

The above list includes some episodes where collective self-defence was invoked and foreign troops were introduced into the ‘victim’ state requesting assistance, but force was not used in actual conflict, or was not used beyond the national border of the victim state. States have invoked collective self-defence as a justification for inviting in foreign troops before any armed attack has occurred, in case collective self-defence is needed in the future; that is, as a deterrent or as a precaution. The sending of troops and the provision of other aid has been much more common than the use of those troops in actual fighting against an attacking state. The US use of force against North Vietnam, Cambodia, and Laos in the name of the collective self-defence of South Vietnam and the US use of force against Nicaragua (in the name of collective self-defence of El Salvador, Costa Rica, and Honduras) are exceptional in that the USA used force outside the ‘victim’ state.

That is, although in theory there is a distinction between collective self-defence and assistance in reply to an invitation by a government to respond to external intervention against that government, in practice the line may not be a clear one.7 The states sending in their troops make choices as to the justification they offer. They may invoke collective self-defence before it is actually necessary and conversely they do not always expressly invoke collective self-defence even when a case could be made for it on the basis that there has been or might be an armed attack.

Thus, for example, Ethiopia, the USSR and Cuba all tended to play down the presence of Soviet and Cuban troops in Ethiopia (1977–78) even though there had been an armed attack by Somalia into the Ogaden region of Ethiopia after the overthrow of Emperor Haile Selassie, and the armed response by Ethiopia was limited to driving out the invading forces.8 Also with regard to the collective self-defence of Angola against attacks by South Africa, Cuba at first simply stressed that it had been invited in by the MPLA, which subsequently formed the government on the coming to independence of Angola in 1975. Angola referred to Article 51 in relation to the presence of the Cuban troops only from 1983, after the issue of ‘linkage’ had become more prominent. On the basis of ‘linkage’, South Africa argued that its withdrawal from Namibia was linked to that of Cuba from Angola, thus implying an equivalence between the two situations. Angola replied that the Cuban presence had been requested by the legitimate government of Angola for the clear and express objective of repulsing the open and flagrant invasion by South Africa. The first invasion (in 1975) was repulsed by the Angolan people with the assistance of Cuban troops, but South African aggression had continued. There was a continued need for the assistance of Cuban forces in full conformity with Article 51, as every state has the right to individual or collective self-defence.9

All the episodes listed above pre-date the judgment in the Nicaragua case.10 After the judgment the USA occasionally again invoked collective self-defence against Nicaragua in Central America.11 And subsequently claims to collective self-defence to justify the use of force in defence of Kuwait,12 Tajikistan,13 and the DRC14 again reveal the complexity of such claims. The legality of the third state use of force was controversial in almost all these cases, both those before and those after the Nicaragua case, but the disagreements between states on the legality of these uses of force have generally centred on the facts rather than the law. In almost all these cases the controversy concerned the question whether there had been an armed attack and also whether there had been a genuine request for help by the victim state. In contrast, Operation Enduring Freedom in Afghanistan did raise fundamental legal questions about the concept of ‘armed attack’.15

On the whole, however, the states directly involved and those responding to their use of force through international organizations have not disagreed as to the content of the applicable law. This may seem surprising, given that the theory of collective self-defence has been controversial since the debate over its express inclusion in the UN Charter. Collective self-defence was included at the instance of the Latin American states to make clear the compatibility of the existing American system and the new UN system. After prolonged debate, collective self-defence was included in Chapter VII on the powers of the Security Council rather than in Chapter VIII on regional arrangements.16 There has subsequently been controversy as to whether collective self-defence was a new concept when it was included in the Charter in 1945.17 Some of the judges in the Nicaragua case took the view that it was an innovation. Thus, for example, Judge Oda said that the term ‘collective self-defence’ was unknown before 1945 and therefore expressed doubt as to whether it was an inherent right.18 Judge Jennings agreed that it was a novel concept.19 Whether or not collective self-defence was a totally new concept, the post-1945 practice has been crucial in the crystallization of the concept.

Early debates on Article 51 of the UN Charter focused on whether collective self-defence was an autonomous right allowing any third state to use force in defence of the victim of an armed attack, or whether it was a collection of rights to individual self-defence only to be exercised if the third state was itself a victim or if the interests of the third state were somehow engaged.20 What were the conditions for its exercise: did it require a pre-existing treaty arrangement for collective action? Some argued that there was a need of prior agreement for collective self-defence, otherwise the use of force would be contrary to the spirit of Article 51;21 other writers like McDougal insisted on a common interest rather than a pre-existing treaty.22


The ICJ decision in the Nicaragua case on the legality of the US use of force and intervention in Nicaragua renewed the passion of the debate on the scope of collective self-defence. The judgment has been much attacked and much misinterpreted. It plays a crucial role in this area. The Court’s decision, its first extended discussion of the law on the use of force, was based on customary international law because of the US reservation to its Optional Clause acceptance. The Court found that the US multilateral treaty reservation prevented it from applying the UN Charter and other multilateral treaties, such as the OAS Charter and the Rio Treaty, which in fact bound the parties. However, the reservation did not stop the Court from deciding the case on the basis of customary international law, which continued to exist alongside treaty law.23 Moreover, the Court could properly adjudicate because the provisions of multilateral treaties did not diverge from customary international law to such an extent that a judgment of the Court on custom would be a wholly pointless exercise. The Court went on to say that, although it had no jurisdiction to determine whether the conduct of the USA constituted a breach of the Charter of the UN and that of the OAS, it could and must take them into account in ascertaining the content of customary international law.24

The Court’s exposition of the law on collective self-defence, the justification used by the USA to support its use of force and intervention in and against Nicaragua, was relatively brief. The parties, in view of the circumstances in which the dispute had arisen, had relied only on the right of self-defence in the case of an armed attack which had already occurred; the lawfulness of a response to the imminent threat of armed attack was not raised.25 Also the parties agreed that any exercise of self-defence must be necessary and proportionate. The Court accordingly went on to define the other specific conditions which had to be met for the exercise of collective self-defence.26

First, the Court considered what constituted an armed attack: the sending of armed bands rather than regular army could constitute an armed attack, provided that the scale and effects of the operation were such as to be classified as an armed attack and not a mere frontier incident. Assistance to rebels in the form of the provision of weapons or logistical or other support could amount to a threat or use of force or intervention, but did not constitute an armed attack.27 Second, ‘it is also clear that it is the State which is the victim of the armed attack which must form and declare the view that it has been so attacked. There is no rule in customary international law permitting another state to exercise the right of collective self-defence on the basis of its own assessment of the situation. Where collective self-defence is invoked, it is to be expected that the State for whose benefit this right is used will have declared itself to be the victim of an armed attack.’28 Third, the Court held that ‘there is no rule permitting the exercise of collective self-defence in the absence of a request by the State which regards itself as the victim of an armed attack’.29 The Court also held that the requirement in Article 51 of the UN Charter that the state claiming to use the right of individual or collective self-defence must report to the Security Council was not a customary law requirement, although ‘the absence of a report may be one of the factors indicating whether the state in question was itself convinced that it was acting in self-defence’.30

The Court was criticized for its treatment of collective self-defence in the separate and dissenting opinions on contrasting grounds. Judge Ruda said that the Court should not have gone into the topic at all, given that it had held that there was no armed attack.31 Judge Oda said that if it was going to consider collective self-defence, it had been far too brief on this controversial topic.32

The judgment on the merits in Nicaragua attracted strong criticism, especially from US writers.33 They were unhappy at the brevity of the Court’s reasoning on collective self-defence and at its approach to customary international law. Or it may be more accurate to say that, because some of the writers were unhappy with the substantive conclusions of the Court that the USA had illegally used force and intervened in Nicaragua, they therefore attacked its legal reasoning. How far were the Court’s conclusions on collective self-defence justified on the basis of customary international law and compatible with treaty law? To what extent were they based on sound policy considerations?


The actions of armed bands and irregular forces

As was discussed in the previous chapter, the Court asserted that on the central question of what constitutes an armed attack the Definition of Aggression gave guidance. An armed attack included the actions of armed bands where these were imputable to a state.34 This limited use of the Definition of Aggression seems justified in the light of state practice.35

A central issue in all the episodes where collective self-defence was expressly invoked by states was whether there had been an armed attack such as to justify the third state assistance to the victim state. In state practice it has been accepted since the early days of the UN that the actions of armed bands and irregular forces could constitute an armed attack by a state. This has been accepted in the context of collective self-defence as well as individual self-defence.36 During the US intervention in Lebanon in the name of collective self-defence in 1958 there was initially some uncertainty on this issue.37 The USA and Lebanon did not at first mention armed attack, although they both reported to the Security Council that the US intervention was in response to a request by Lebanon under Article 51 of the UN Charter. But subsequently Lebanon expressly argued that there was an armed attack by the United Arab Republic (Egypt and Syria) and that there was no difference between a regular army and irregular forces for the purposes of Article 51.38 China also took this position and no state challenged it.39 The reason why the claim to collective self-defence was controversial in Security Council and General Assembly debates was that states were sceptical as to whether there had in fact been any armed attack, whether by regular or irregular troops; they claimed that the USA was simply trying to protect an unpopular leader from internal unrest at a time of growing Arab nationalism and republicanism.

Again, in the case of Vietnam, the USA argued that the infiltration from North Vietnam amounted to an armed attack justifying collective self-defence of South Vietnam. It famously asserted that from 1959 until 1964 the North infiltrated over 40,000 men into the South. It said that in these circumstances armed attack was not as easily fixed by date and hour as in the case of traditional warfare, but the infiltration of thousands of men clearly constituted an armed attack under any reasonable definition.40 States did not deny that the actions of irregular troops could be attributed to a state, but they doubted whether in fact there was an invasion of one state by armed bands from another rather than an uprising throughout Vietnam.41 Similarly, with regard to its intervention in Afghanistan in 1979 the USSR claimed collective self-defence against at first unspecified ‘foreign intervention’; it was not controversial that the actions of armed bands could constitute an armed attack, but there was doubt as to the existence of such an attack.42

More recently, in Tajikistan there was controversy as to the existence of armed attacks from Afghanistan against Tajikistan.43 After Tajikistan attained independence in 1991 civil war broke out and opposition forces operated against the government from Afghanistan. Russia argued that it was justified in using force in collective self-defence of Tajikistan against these incursions. In 1993–5 Russia and Tajikistan repeatedly accused Afghanistan of involvement in the attacks; Afghanistan denied these claims. This continued even after the conclusion of a border agreement between Afghanistan and Tajikistan, an Agreement on a Temporary Ceasefire and the Cessation of Other Hostile Acts on the Tajikistan/Afghanistan Border and within the Country between the warring parties in Tajikistan, and the creation of a UN observer force (UNMOT) to monitor the border/ceasefire. The UN Secretary-General made various reports on the situation, but did not come to any public conclusions as to the occurrence of armed attacks and the right of Tajikistan and Russia to act in collective self-defence against Afghanistan.

The supply of arms

What proved more controversial than the attribution of the actions of armed bands to a state was the ICJ’s assertion in the Nicaragua case that the supply of arms, financial and logistic support could not amount to an armed attack. The USA contended that Nicaragua had intervened in El Salvador and other neighbouring states in order to foment and sustain armed attacks upon the governments of those states, and that its subversive intervention in the governing circumstances was tantamount to an armed attack. The Court said that such assistance as the supply of arms, financial and logistic support could be regarded as a threat or use of force, or amount to intervention in the internal or external affairs of other states, but that it did not amount to an armed attack.44 The Court gave no authority for this statement and was criticized for its failure to do so by some commentators. But the Court’s choice not to elaborate on the basis for its finding may be explained by the fact that the parties had not disagreed about the meaning of armed attack.45 Rather, the central disagreement was whether, on the application of the law to the particular facts, the actions of the Nicaraguan government amounted to an armed attack. On the facts of the case the Court found that there had been no significant assistance to the opposition in El Salvador since 1981 and that Nicaragua could not be held responsible for the limited assistance that had been given.46 Therefore there was actually no need for the Court to go into the question of the definition of armed attack; the decision on the general question of what counted as an armed attack was not decisive.