1 These Principles were first published in a working paper on the Chatham House website.<www.chathamhouse.org.uk/pdf/research/il/ILPForce.doc>.
2 Waldock, CMH ‘The Regulation of the Use of Force by Individual States in International Law’ The Hague Recueil Des Courts, vol 81 (1952–11), 461.
3 Report of the Secretary-General's High-level Panel on Threats, Challenges and Change ‘A More Secure World: Our Shared Responsibility’ (2004).
4 High-level report, Synopsis of Part 3. In the resolution incorporating the Outcome of the World Summit in Sept 2005 the UN General Assembly affirmed that the relevant provisions of the UN Charter are sufficient to address the full range of threats to international peace and security, but did not deal with the question as to when it is lawful for a state to use force in the exercise of its inherent right of self-defence.
5 The question whether there is also a right to take action in exceptional circumstances of humanitarian emergency, or to protect fundamental rights, is not dealt with here; nothing in this paper can be regarded as prejudicing the question one way or the other. Although Art 51 mentions the right of collective self-defence, this study deals only with individual self-defence.
6 The International Court of Justice (ICJ) expressly left open the issue of the lawfulness of a response to the threat of an imminent armed attack in the Case concerning Military and Paramilitary Activities in and against Nicaragua (Nicaragua v USA) (Merits)  ICJ Reports, para 194. When the question of the existence of an armed attack featured in the Court's overall reasoning on the law of self-defence, it appeared before the treatment of the principles of necessity and proportionality. The same framework was followed by the Court some 17 years later in the Oil Platforms Case (Case Concerning Oil Platforms (Islamic Republic of Iran v United States of America)  ICJ Rep, where it first investigated the existence of an armed attack (paras 61–64 and 72) before it turned to the application of the principles of necessity and proportionality (paras 73 and 74).
7 For the purposes of this document the term ‘anticipatory’ self-defence is preferred over ‘preemptive’ self-defence, although the latter is also in current use, for example in the report of the United Nations Secretary-General's High-level Panel on Threats, Challenges and Change (n 3) para 189.
8 The United Nations Secretary-General's response ‘In Larger Freedom’ to the High-level Panel report mentioned above states: ‘Imminent threats are fully covered by Article 51, which safeguards the inherent right of sovereign states to defend themselves against armed attack. Lawyers have long recognized that this covers an imminent attack as well as one that has already happened’ (para 124). In the resolution adopting the World Summit Outcome the UN General Assembly reaffirmed that ‘the relevant provisions of the Charter are sufficient to address the full range of threats to international peace and security’ and reaffirmed ‘the authority of the Security Council to mandate coercive action to maintain and restore international peace and security’ but did not comment on the meaning of Art 51.
9 As in the Caroline incident, and in the case of the intervention in Afghanistan in 2001, which was categorized by the US and the UK as the exercise of the right of anticipatory self-defence (see UN Doc S/2001/946 and UN Doc S/2001/947).
11 See commentary to Principle D below.
12 This study does not, however, deal with the rescue of citizens abroad, which raises different issues.
13 There are statements by the International Court of Justice which suggest that there may be instances of the use of force which are not of sufficient gravity as to scale and effect to constitute an armed attack for the purpose of self-defence. See Nicaragua case (n 6) paras 191 and 195 and Oil Platforms case (n 6) paras 51, 63–64 and 72. But these statements are not generally accepted.
15 Legal Consequences of the Construction of a Wall in the Occupied Palestinian Territory (Advisory Opinion)  ICJ Rep para 139.
16 The criterion of ‘necessity’ if force is legally to be used in self-defence can be traced back to the language of the Caroline formula:
‘[i]t will be for … [Her Majesty's] Government to show a necessity of self-defence, instant, overwhelming, leaving no choice of means, and no moment for deliberation’ and the action must not be ‘unreasonable or excessive; since the act, justified by the necessity of self-defence, must be limited by that necessity, and kept clearly within it.’
The ICJ held in the Nicaragua case (n 6) that ‘the specific rule whereby self-defence would warran only measures which are proportional to the armed attack and necessary to respond to it’ was ‘a rule well established under customary international law’ (para 176), and re-affirmed this in its Advisory Opinion on the Legality of the Threat or Use of Nuclear Weapons  ICJ Rep para 41)
17 This formulation leaves open the question whether greater mechanised force can be justified by the reduction in risk to the lives of the defending State's forces, a question which is more normally dealt with by the rules of international humanitarian law.
18 In its decision in the Oil Platforms case (n 6), the ICJ elaborated on the ‘necessity’ criterion. It held that ‘the requirement of international law that measures taken avowedly in self-defence must have been necessary for that purpose is strict and objective, leaving no room for any “ìmeasure of discretion.”’(para 73). In practice of course the assessment of the necessity of a particular action is far from straightforward, and can be undertaken only on the basis of the facts available at the time, but with a good faith assessment of those facts.
19 Nicaragua case (n 6) para 176; see also, para 41 of the Advisory Opinion on the Legality of the Threat or Use of Nuclear Weapons (n 16).
20 For example, the United Kingdom Attorney General stated in the House of Lords on 21 Apr 2004: ‘the force used must be proportionate to the threat faced and must be limited to what is necessary to deal with the threat.’(Hansard HL col 370).
22 (n 15) para 139: ‘Article 51 of the Charter thus recognizes the existence of an inherent right of self-defence in the case of armed attack by one State against another State.’ But the European Union statement made upon the adoption of General Assembly resolution ES–10/18 (concerning the Wall advisory opinion) suggests that EU Member States and the other States associated with the statement would not accept the possible implication of the Opinion that self-defence is not available unless the armed attack is by a State. ‘The European Union will not conceal the fact that reservations exist on certain paragraphs of the Court's advisory opinion. We recognise Israel's security concerns and its right to act in self-defence.’ The matter came up again in a recent case in the ICJ; the Court stated that in the absence of attribution of the armed force to a State there is no right of self-defence against that State. (Case concerning Armed Activities on the territory of the Congo (Democratic Republic of the Congo v Uganda) (Merits)  ICJ Rep paras 146, 147)). In line with the Wall advisory opinion this should not be read as prohibiting action in self-defence against non-state actors as such.
23 While certain writers have argued that Art 51 concerns only responses to aggression against another State, their argument based on the French text is not persuasive. True, the French text of Art 51 uses the term agression armée, and agression is also the term used in Art 39, but the French Government accepted during the debates on the definition of aggression that aggression in Art 39 was not the same concept as agression armée in Art 51; further, the English, Chinese, and Spanish texts of the Charter use different terms for Arts 39 and 51.
24 It should however be noted that Security Council resolution 1368(2001) does not settle the matter entirely, as in that case there was already significant evidence of a degree of responsibility of a State (Afghanistan) for the continuing ability of the terrorists to carry out attacks.
25 The ICJ Judgment in the Case concerning Armed Activities on the territory of the Congo ((n 22) paras 146 and 147) implies that unwillingness or inability of a State to deal with irregular forces on its territory is insufficient to create a right in self-defence against the State. However, the Court does not answer the question as to the action a victim State may take in the case of an armed attack by irregular forces, where no involvement of the State can be proved. According to Judges Kooijmans and Simma the occurrence of an armed attack is sufficient to create a right of action in self-defence, whether or not the actions are attributable to a State (Separate Opinions of Judge Kooijmans, paras 26–30 and of Judge Simma, paras 7–12).
26 It is in this context (rather than that of an attack by a State itself) that it is relevant to consider the ICJ's remarks in the Nicaragua judgment (n 6) para 195. The Court stated that: ‘it may be considered to be agreed that an armed attack must be understood as including not merely action by regular armed forces across an international border, but also the sending by or on behalf of a State of armed bands, groups, irregulars or mercenaries, which carry out acts of armed force against another State of such gravity as to amount to (inter alia) an actual armed attack conducted by regular forces, “or its substantial involvement therein.” … The Court sees no reason to deny that, in customary law, the prohibition of armed attacks may apply to the sending by a State of armed bands to the territory of another State, if such an operation, because of its scale and effects, would have been classified as an armed attack rather than as a mere frontier incident had it been carried out by regular armed forces’ (italics added).