Each of these requirements will be considered in turn.
1. Armed Attack
9 Whatever may once have been the position in customary international law, it is now well established that the right to use force in self-defence exists only in response to an armed attack (though whether that armed attack must have commenced before resort to force in self-defence is undertaken is considered in paras 41–51 below). Self-defence does not give a State the right to resort to force in response to non-military actions and threats, such as economic coercion, no matter how damaging they may be to that State’s rights and interests; any response to such actions must also be of a non-military character.
10 The term ‘armed attack’ is not, however, defined in the UN Charter. Nor does it appear in the principal provisions with which Art. 51 is associated: Art. 2 (4) refers to the ‘threat or use of force’, while Art. 39 speaks of threats to the peace, breaches of the peace, and acts of aggression. The precise relationship of ‘armed attack’ with these other concepts is, therefore, far from easy to determine. Moreover, the French text of Art. 51 uses the term ‘agression armée’. Art. 39, in which the word ‘aggression’ appears in both English and French texts, treats aggression as more serious than a ‘breach of the peace’ and UN General Assembly Resolution 3314 (XXIX) ‘Definition of Aggression’ of 14 December 1974, proceeds on the same assumption. It follows that, if the term ‘agression armée’ in the French text of Art. 51 were to be treated as equivalent to ‘aggression’ in Art. 39, then the French text would point to a very restrictive concept of self-defence which would exclude self-defence in response even to large-scale uses of force. That does not appear, however, to have been the approach taken in international practice. The Iraqi invasion of Kuwait in 1990, for example, was characterized by the Security Council as a breach of the peace, rather than an act of aggression, in UN Security Council Resolution 660 of 2 August 1990 (SCOR 45th Year 19; a proposal to characterize it as an act of aggression having been rejected in consultations on the draft resolution). Nevertheless, the preamble to UN Security Council Resolution 661 of 6 August 1990 (SCOR 45th Year 19) expressly reaffirmed the right of self-defence of Kuwait in the face of that breach. It seems, therefore, that the term ‘agression armée’ is not in practice equated with ‘aggression’ in Art. 39 and should not be regarded as more restrictive than the English ‘armed attack’.
(a) The Nature and Scale of an Armed Attack
An armed attack need not, however, necessarily take the form of action by regular armed forces
. In its judgment in the Nicaragua Case
, the ICJ held that: Similarly, the UN Security Council treated the terrorist attacks on targets in the United States on 11 September 2001, which took the form of crashing hijacked civil airliners into the World Trade Center in New York and the Pentagon, as an armed attack. (see UNSC Res 1368  [12 September 2001] SCOR [1 January 2001–31 July 2002] 290 and UNSC Res 1373  [28 September 2001] SCOR [1 January 2001–31 July 2002] 291)
12 In its judgment in the Nicaragua Case, the ICJ also stated that the use of force would not amount to an armed attack for the purposes of the right of self-defence unless it was of a particular scale and effect, and it contrasted an ‘armed attack’ with a ‘mere frontier incident’ (at para. 195). This approach opens up a gap between the use of force, which is prohibited by Art. 2 (4) UN Charter, and the armed attack which gives rise to a right to use force in self-defence. According to the Court, if a use of force did not rise above the level of a ‘mere frontier incident’, then, even though it constituted a violation of Art. 2 (4) UN Charter, the victim of that violation was not entitled to respond by way of action in self-defence, although it was entitled to take countermeasures, with the Court avoiding any clear stance on whether such countermeasures might involve the use of force. This conclusion has proved controversial with both States and commentators. While it is true that there are differences between the language of Arts 2 (4) and 51 UN Charter, there is no evident textual reason for holding that the concept of armed attack excludes some attacks which, though armed, fall below an unspecified threshold of intensity of violence. Nor was such a distinction apparent in State practice prior to the decision of the Court. There is an understandable concern not to open the door to excessive military action in response to minor incidents, but the requirement that action taken in self-defence must be necessary and proportionate already precludes such an excessive response from being brought within the ambit of self-defence. Moreover, the judgment in the Nicaragua Case gives little guidance regarding where the line is to be drawn between those uses of force which are sufficiently serious to constitute armed attacks and those which are not.
13 That dividing line is particularly important when considering whether terrorist attacks constitute an armed attack for the purposes of the law of self-defence. Some such attacks (eg those which took place on 11 September 2001) may be of such severity that they plainly cross the threshold established in the Nicaragua Case. In other cases, however, no one individual incident may be of sufficient gravity to cross that threshold, but if one looks at the totality of such incidents, then the picture changes. In such circumstances, it has often been suggested that the assessment of whether or not there is an armed attack has to be based on the series of terrorist incidents taken as a whole, rather than by examining each one in isolation.
14 In recent years a particular focus of debate has been the possibility of so-called ‘cyber-attacks’, in which computer technology is used to bring down a State’s computer systems, causing extensive economic and social harm (Cyber Warfare). It has sometimes been suggested that such action should be regarded as a modern form of armed attack which would give rise to a right, on the part of the victim State, to take military action in response. Such suggestions need to be treated with considerable caution. The planting of a virus or the use of other computer techniques to undermine, for example, the computer systems regulating a State’s financial system or immigration controls is difficult to see as an armed attack. Although the consequences of such conduct may be very serious, it seems closer to the concept of economic coercion. On the other hand, if such action were used to produce results similar to those which could otherwise be achieved only by the use of armed force, for example, causing aircraft to crash or dams to open and flood areas of a State’s territory, then the argument that such action should be treated as a form of armed attack is more plausible.
(b) The Source of an Armed Attack
15 Controversy has also surrounded the question of whether an armed attack must be attributable to a State in order to give rise to a right on the part of the victim State to take action in self-defence. This question has assumed increased importance as a number of States have resorted to the use of force in response to terrorist attacks (Terrorism).
16 There is no doubt that a terrorist attack against a State which is carried out by the organs of another State, or for which that other State is in some other way responsible, is capable of amounting to an armed attack. The ICJ has stated, in its judgments in the Nicaragua Case and in the Armed Activities on the Territory of the Congo (Democratic Republic of the Congo v Uganda), as well as in Legal Consequences of the Construction of a Wall in the Occupied Palestinian Territory (Advisory Opinion), that such acts of terrorism amount to armed attacks provided that they are of sufficient intensity to cross the threshold identified in the Nicaragua Case and discussed in paras 11–14 above. On the other hand, the Court at times appears to suggest that acts of terrorism for which no State is responsible are not capable of constituting armed attacks, even if the level of violence is such as to cross the Nicaragua Case threshold (see, in particular, Legal Consequences of the Construction of a Wall in the Occupied Palestinian Territory [Advisory Opinion] para. 139; Armed Activities on the Territory of the Congo para. 146).
17 Nevertheless, the legal basis for such a limitation of the right of self-defence is unclear. The correspondence between the United Kingdom and United States Governments in the Caroline incident, which is generally treated as the earliest formulation of the right of self-defence in customary international law, runs counter to the existence of any such limitation. That correspondence proceeded on the basis that the attacks from United States territory against the British authorities in Canada, which were the basis for the claim to self-defence, were not the responsibility of the United States. Yet that fact was not seen by either government as excluding the possibility that the right of self-defence might have been triggered. Nor does any such limitation appear in the text of Art. 51 UN Charter, which refers to the need for there to be an armed attack against a State but makes no comment on the source of such an attack. It is true that the practice of States in relation to self-defence focussed, for most of the 20th century, on self-defence against armed attacks which emanated from States. However, that would not have sufficed to cut down the right of self-defence identified in the Caroline incident by limiting it to such a case. For such a limitation to have been grafted onto the right of self-defence, it would have been necessary for that practice to have reflected widespread opinio iuris that international law precluded resort to self-defence in response to other armed attacks. The response of the Security Council to the terrorist attacks against the United States on 11 September 2001 is also instructive. UN Security Council Resolutions 1368 of 12 September 2001 and 1373 of 28 September 2001 condemned the attacks and expressly reaffirmed the right of self-defence but made no mention of whether or not the attacks were in some way the responsibility of a State. In taking military action against Al Qaeda in Afghanistan shortly after the September 2001 attacks, the United States and those States allied with it claimed to be acting in self-defence. Their claim was not contested by the overwhelming majority of other States even though attributing the attacks by Al Qaeda to Afghanistan on 11 September was by no means established at the time.
18 Whether an armed attack, for the purposes of the right of self-defence, must be the responsibility of a State should therefore be regarded as unsettled. On the one hand, the increasing capacity of groups acting outside the responsibility of a State to engage in acts of extreme violence suggests that any such limitation would be an unreasonable restriction on the right of the victim to defend itself (see also Non-State Actors). Contemporary State practice supports the notion that no such broad restriction exists. On the other hand, there is an understandable concern that a State which has been the victim of an attack by a group unconnected with any other State should not inevitably be free to take action against that group in the territory of other States. It is possible, however, that this latter concern can be met by a proper application of the principle that action in self-defence must be limited to what is necessary and proportionate (see paras 25–29 below). If the State, in whose territory a group which has perpetrated a terrorist attack against another State is located, is prepared to take effective action against that group, then military action in that territory by the victim of the terrorist attack cannot be regarded as necessary. Only if the former State has shown itself to be unwilling (or, perhaps, unable) to act effectively against the group it can be said that military action in its territory in the exercise of the right of self-defence meets the criterion of necessity.
(c) The Target of an Armed Attack
19 Art. 51 UN Charter preserves the right of self-defence ‘if an armed attack occurs against a Member of the United Nations’. For the reasons already given, the right of self-defence is applicable to any State, irrespective of whether it is a member of the UN. What is required, therefore, for a State to be entitled to use force in individual self-defence is that there is an armed attack against that State. That in turn raises the question of what is to be considered as constituting ‘the State’ for these purposes.
20 There is no dispute that the use of force (provided that it meets the criteria in the preceding sections) against the territory of a State is an armed attack against that State. That is so not only in the case of an attack upon the metropolitan territory of a State but also an attack upon islands and overseas territories which may be a considerable distance from the metropolitan territory. For example, most States treated the use of force by Argentina against the Falkland Islands/Islas Malvinas and Falkland Islands Dependencies (overseas territories of the United Kingdom) in 1982 as an armed attack against the United Kingdom.
21 It is also generally agreed that the use of force against the organs of a State outside the territory of that State constitutes an armed attack upon the State. For example, an attack upon units of the armed forces of a State lawfully stationed or operating in the territory of another State can constitute an armed attack upon the State of those forces (the sending State) as well as the State on whose territory the attack occurs (the host State; Military Forces Abroad). Similarly, an attack upon the naval forces or other State ships of a State lawfully operating on the high seas can constitute an armed attack upon that State.
22 A more difficult problem arises with attacks upon privately owned merchant ships flying the flag of a State or upon nationals of that State outside its territory (see also Flag of Ships). Neither are organs of the State and it has therefore been argued that the use of force against them is not an armed attack against the State and cannot, therefore, give rise to a right on the part of the State to take action in self-defence.
23 So far as merchant vessels are concerned, however, the practice of many maritime States has been to use force to protect merchant vessels flying their flag from attacks by the forces of other States and to justify such action by reference to the State’s right of self-defence. That right was frequently invoked in, eg, the Iran-Iraq War (1980–88). In the Oil Platforms Case (Iran v United States of America), the US contended that an attack on a US registered oil tanker, the Sea Isle City, entitled the US to exercise the right of self-defence by taking measures against Iran, whom it blamed for the attack. While the Court rejected that argument, it did so because it concluded that the United States had failed to prove that the attack was attributable to Iran. The judgment appears to accept that an attack by Iran on the Sea Isle City was capable of amounting to an armed attack upon the United States (at para. 64).
24 There is greater controversy surrounding the question whether attacks upon a State’s nationals abroad entitle the State to take action in self-defence. There is an understandable reluctance to see the kind of gunboat diplomacy practised in the 19th century revived under the auspices of self-defence. Nevertheless, the possession of a ‘population’ is one of the requirements of statehood and a case can certainly be made that an attack of sufficient violence upon a substantial number of a State’s nationals, especially where those nationals are selected as victims on account of their nationality and, in particular, where they are attacked in order to harm, or put pressure upon, their State of nationality, is a more serious assault upon the State than some forms of attack upon its territory. Thus the rescue of nationals abroad may well fall within the ambit of the right of self-defence, where the territorial State itself is unable or unwilling to act.
2. Necessity and Proportionality
25 The fact that a State is entitled to take action in self-defence does not mean that it is entitled to employ unlimited force. It has long been an established principle of customary international law that force used in self-defence must not exceed what is necessary and proportionate. As the ICJ explained in the Nicaragua Case, there is a ‘specific rule whereby self-defence would warrant only measures which are proportional to the armed attack and necessary to respond to it’ (at para. 176). While this important limitation on the right of self-defence is not mentioned in Art. 51 UN Charter, the ICJ has recognized that it applies to measures of self-defence taken under that provision (Legality of the Threat or Use of Nuclear Weapons [Advisory Opinion]  ICJ Rep 226 para. 41; Armed Activities on the Territory of the Congo para. 147).
26 The concepts of necessity and proportionality have often been equated in the literature on self-defence. While closely related, they are, however, separate requirements. The use of force, and the degree of force used, must be necessary to respond to the armed attack and must be proportionate to the threat posed by that attack. In its judgment in the Oil Platforms Case, the ICJ considered the two requirements separately.
27 Both concepts are forward-looking in the sense that compliance with them has to be assessed by reference to the goal which the State acting in self-defence is entitled to seek to achieve. Thus, a State which is the victim of an armed attack is entitled, inter alia, to halt and repel that attack and to recover territory occupied during the attack. It will satisfy the requirement of necessity only if it can demonstrate that it could not have achieved these goals without resort to force and that the degree of force employed did not exceed what was reasonably required for that purpose. It is important, however, to look at the totality of what the State acting in self-defence is entitled to achieve. The fact that the attacking State offers to cease its attack does not render the use of force unnecessary if, for example, the attacking State would thereby be left in occupation of part of the victim State’s territory.
28 In contrast to the concept of proportionality in the law of countermeasures or reprisals, proportionality in self-defence is also a forward-looking requirement. Whether action purportedly taken in self-defence meets the requirement of proportionality is to be assessed not by reference to the degree of force which was employed in the initial armed attack, but rather the threat posed by the armed attack. It is not simply a matter of comparing the number of forces or the types of weapons employed or even the scale of casualties and damage occasioned. For example, an armed attack which benefits from the element of surprise may succeed in gaining control of an area of territory or achieving another of the attacking State’s goals with the employment of comparatively little force. If the attacking State then rapidly reinforces its new positions, then the victim State will be unable to reverse the effects of the attack without employing a far greater degree of force. To prohibit it from doing so would be to reward the initial unlawful attack and place the victim State at a disadvantage which is wholly unreasonable. There is no indication that international law requires such an unjust conclusion. Whether the victim State’s use of force in self-defence meets the criterion of proportionality depends not upon its relation to the force initially used, but upon whether it is required in order to reverse the effects of the armed attack.
29 As the ICJ recognized in the Nuclear Weapons Advisory Opinions, the requirement that the force used in self-defence must be necessary and proportionate may have an impact upon which weapons and methods of warfare the State asserting the right of self-defence is entitled to employ (Warfare, Methods, and Means). It may also affect the geographical scope of operations and other aspects of the conduct of hostilities.
3. Self-Defence and the Security Council
30 Although Art. 51 UN was designed to confirm that the right of self-defence, as it existed in customary international law at the time of the adoption of the UN Charter, was preserved in the era of the UN Charter, there are two aspects in which Art. 51 UN Charter set out to alter that right, at least for States who are members of the United Nations.
31 First, it introduced a requirement that measures taken in self-defence had to be reported to the Security Council. Although failure to comply with this requirement will probably not suffice to make a use of force which meets all the other requirements of self-defence a breach of Art. 2 (4) UN Charter, the fact that a State has not reported measures which it subsequently claims were taken in self-defence is likely to make that claim less plausible. Failure to comply with the reporting requirement is also, of course, a violation of the UN Charter in its own right.
32 Secondly, Art. 51 UN Charter introduced a temporal limitation on the right to take action in self-defence by providing that the right of self-defence endured only ‘until the Security Council has taken measures necessary to maintain international peace and security’. This requirement has assumed far greater significance since the Iraqi invasion of Kuwait heralded a marked increase in the use by the Security Council of its powers under Chapter VII UN Charter. That increase makes it necessary to consider what steps on the part of the Security Council will curtail the right of a victim State to take action in self-defence.
33 It is generally accepted that the Security Council has the power to impose, by means of a binding decision adopted under Chapter VII UN Charter, a ceasefire. If it does so, then all the parties to a conflict, whether or not acting in self-defence, will be obliged to comply. Nevertheless, it is plain that not every action taken by the Security Council during an armed conflict will have this effect. The reference to ‘measures’ suggests that the mere passage of a resolution, even one which contains a decision binding under the UN Charter, is not sufficient. The fact that UN Security Council Resolution 660 of 2 August 1990 condemned the Iraqi invasion of Kuwait and issued a binding demand for Iraq to withdraw from all Kuwaiti territory was not treated as removing from Kuwait (and its allies) the right to take action in self-defence when Iraq did not comply with that demand. Moreover, Art. 51 UN Charter refers to measures which are ‘necessary to maintain international peace and security’. Only when the Security Council has taken such action as is necessary will the right of self-defence lapse.
34 On a number of occasions, the Security Council has itself made clear that the action which it is taking is not intended to limit the right of self-defence (UNSC Res 661 [6 August 1990], which imposed economic and political sanctions upon Iraq for failing to withdraw from Kuwait contained such a provision). In such a case, there can be no doubt that the right of the victim State to take action in self-defence remains, notwithstanding the fact that the Security Council has taken measures. However, the absence of such a provision in a Security Council resolution should not be taken as meaning that the victim State necessarily loses its right to take action in self-defence. Careful analysis of the resolution in light of all the circumstances, including the nature of any continuing armed attack, the scale of the measures taken, and their likely efficacy seems called for.