Principles of international law on the use of force by states in self-defence

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International Law
ILP WP 05/01


This publication contains:

I Principles of International Law on Self-Defence

II The list of participants

III The responses to the Questionnaire

Elizabeth Wilmshurst

October 2005

Chatham House is an independent body which promotes the rigorous study of international questions and does not express opinions of its own. The opinions expressed in this publication are the responsibility of the authors.

© The Royal Institute of International Affairs, 2005

This material is offered free of charge for personal and non-commercial use, provided the source is acknowledged. For commercial or any other use, prior written permission must be obtained from the Royal Institute of International Affairs.

In no case may this material be altered, sold or rented.


The purpose of this study is to provide a clear statement of the rules of international law governing the use of force by states in self-defence. The rules are being challenged in the light of what are seen as new threats from terrorism and from the possession of weapons of mass destruction, and there has been controversy as to whether they need revision or redefinition. The study was prompted by various statements and actions by states, recent developments in the United Nations and by decisions of the International Court of Justice.

In the resolution incorporating the Outcome of the World Summit in September 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, and has reaffirmed the authority of the Security Council to mandate coercive action to maintain and restore peace and security. But the resolution 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.
This study was undertaken because we believe that, in the light of current challenges, it is of importance to world order that there be clarity and understanding about the relevance and application of international law to the use of force by states.
A questionnaire was sent to a small group of international law academics and practitioners and international relations scholars in this country, asking for their views on the criteria for the use of force in self-defence. At a meeting at Chatham House the participants discussed a paper which had been drawn up on the basis of the responses to the questionnaire.

Following that meeting a set of Principles was prepared by the International Law Programme at Chatham House. They are put forward here with the intention of contributing to discussion and comment. Readers are encouraged to communicate any views and reactions. Depending upon the outcome of this stage of the study, further meetings may be held and the Principles further refined.

While the Principles are intended to give a clear representation of the current principles and rules of international law, the law in this area is politically and legally contentious, and the interpretation of the Principles and their application to particular cases will rarely be without difficulty.
The Principles do not necessarily represent the views of all the participants in the study.
Comments are invited on the Principles. Any comments should be addressed to Iwona Newton at Chatham House (


Sir Franklin Berman QC is a barrister at Essex Court Chambers and Visiting Professor of International Law at the Universities of Oxford and Cape Town; formerly Legal Adviser at the Foreign & Commonwealth Office.
Daniel Bethlehem QC is Director of the Lauterpacht Centre for International Law at the University of Cambridge, and barrister at 20 Essex Street, London.
James Gow is Professor of International Peace and Security, and Co-Director of the International Peace and Security Programme, King’s College London.
Christopher Greenwood QC is Professor of International Law at the London School of Economics and Political Science, and a barrister at Essex Court Chambers.
Vaughan Lowe holds the chair of Chichele Professor of Public International Law, is a Fellow of All Souls College, University of Oxford, and a barrister at Essex Court Chambers.
Sir Adam Roberts holds the chair of Montague Burton Professor of International Relations and Fellow of Balliol College, University of Oxford.
Philippe Sands QC is Professor of Law and Director of the Centre of International Courts and Tribunals at University College London, and is a barrister at Matrix Chambers.
Malcolm Shaw QC is Sir Robert Jennings Professor of International Law at the University of Leicester, and is a barrister at Essex Court Chambers.
Gerry Simpson is a Reader in the Department of Law at the London School of Economics and Political Science.
Colin Warbrick is Professor of Law at the University of Durham.
Nicholas Wheeler is Professor in the Department of International Politics at the University of Wales, Aberystwyth.
Elizabeth Wilmshurst is senior fellow, international law, at Chatham House.
Sir Michael Wood is the Legal Adviser at the Foreign & Commonwealth Office.


The Charter of the United Nations prohibits the use of force against another state except where the Security Council has authorised the use of force to maintain or restore international peace and security; and where a state is exercising its inherent right of individual or collective self-defence recognised by Article 51 of the Charter1.
The principles set out below are intended to provide a clear statement of international law regarding the inherent right of self-defence.
All the principles need to be read together.
Even in a case where a state is legally entitled to use force, there may be reasons of prudence and principle not to exercise that right.

  1. The law on self-defence encompasses more than the right to use force in response to an ongoing attack.

Article 51 preserves the right to use force in self-defence “if an armed attack occurs”, until the Council has taken the necessary measures. On one view, the right is confined to circumstances in which an actual armed attack has commenced.2 But the view that states have a right to act in self-defence in order to avert the threat of an imminent attack - often referred to as ‘anticipatory self-defence’3 - is widely, though not universally, accepted.4 It is unrealistic in practice to suppose that self-defence must in all cases await an actual attack.

The difference between these two schools of thought should not be overstated: many of those in the first school take the view that an attack has commenced when there are active preparations at an advanced stage, if there is the requisite intent and capability; and many of those in the other school require not dissimilar conditions before force in self-defence may lawfully be used in respect of an imminent attack. Further, those who deny the right of anticipatory self-defence may accept that a completed attack is sufficient to trigger a right to respond in anticipation of another attack5.
The requirements set out in the Caroline case6 must be met in relation to a threatened attack. A threatened attack must be ‘imminent’ and this requirement rules out any claim to use force to prevent a threat emerging7. Force may be used in self-defence only when it is necessary to do so, and the force used must be proportionate.

  1. Force may be used in self-defence only in relation to an armed attackwhether imminent or ongoing.

  • The ‘armed attack’ may include not only an attack against a state’s territory, but also against emanations of the state such as embassies and armed forces.

  • Force in self-defence may be used only when: the attack consists of the threat or use of force (not mere economic coercion, for example); when the attacker has the intention and the capability to attack; and the attack is directed from outside territory controlled by the state.

  • In the case of a threatened attack, there must be an actual threat of an attack against the defending state itself.

The inherent right of self-defence recognised in Article 51 of the Charter of the United Nations “if an armed attack occurs” forms an exception to the general prohibition against the use of force under Article 2(4).

For the purpose of Article 51, an armed attack includes not only an attack against the territory of the State, including its airspace and territorial sea, but also attacks directed against emanations of the State, such as its armed forces or embassies abroad. An armed attack may also include, in certain circumstances, attacks against private citizens abroad or civil ships and airliners.8 An ‘armed attack’ therefore is an intentional intervention in or against another state without that state’s consent or subsequent acquiescence, which is not legally justified.

An armed attack involves the use of armed force and not mere economic damage. Economic damage, for example, by way of trade suspension, or by use of a computer virus designed to paralyse the financial operations of a state’s stock exchange or to disable the technology used to control water resources, may have a devastating impact on the victim state but the principles governing the right to use force in self-defence are confined to a military attack. A purely ‘economic’ attack might however give rise to the right of self-defence if it were the precursor to an imminent armed attack.

An armed attack means any use of armed force, and does not need to cross some threshold of intensity.9 Any requirement that a use of force must attain a certain gravity and that frontier incidents, for example, are excluded is relevant only in so far as the minor nature of an attack is prima facie evidence of absence of intention to attack or honest mistake. It may also be relevant to the issues of necessity and proportionality. In the case of attacks by non-State actors, however, different considerations may come into play (see section 6 below).

The term ‘armed attack’ requires the attacker to have the intention to attack. In the Oil Platforms Case the ICJ made reference to this requirement when it inquired into the question whether the US was able to prove that certain of Iran’s actions were “specifically aimed” at the US or that Iran had “the specific intention” of harming US vessels10. But to the extent that this may be read as suggesting that military attacks on a state or its vessels do not trigger a right of self-defence as long as the attacks are not aimed specifically at the particular state or its vessels but rather are carried out indiscriminately, this part of the ICJ’s ruling in Oil Platforms has been criticised as not supported by international law.

An armed attack is an attack directed from outside territory controlled by the State. In its Advisory Opinion on the Legal Consequences of the Construction of a Wall in the Occupied Palestinian Territory11 the ICJ’s observations may be read as reflecting the obvious point that unless an attack is directed from outside territory under the control of the defending state the question of self-defence in the sense of Article 51 does not normally arise.

In the case of a threatened attack, there must be an actual threat of an attack against the defending state itself, whether directed against that state or by an indiscriminate attack.This is an aspect of the criterion of necessity. It addresses the question whether it is necessary for the target state to take action.

3. Force may be used in self-defence only when this is necessary to bring an attack to an end, or to avert an imminent attack. There must be no practical alternative to the proposed use of force that is likely to be effective in ending or averting the attack.
The criterion of necessity is fundamental to the law of self-defence12. Force in self-defence may be used only when it is necessary to end or avert an attack. Thus, all peaceful means of ending or averting the attack must have been exhausted or be unavailable. As such there should be no practical non-military alternative to the proposed course of action that would be likely to be effective in averting the threat or bringing an end to an attack. Necessity is a threshold, and the criterion of imminence can be seen to be an aspect of it, inasmuch as it requires that there be no time to pursue non-forcible measures with a reasonable chance of averting or stopping the attack.
Necessity is also a limit to the use of force in self-defence in that it restricts the response to the elimination of the attack and is thus linked to the criterion of proportionality. The defensive measure must be limited to what is necessary to avert the on-going attack or bring it to an end.
In applying the test of necessity, reference may be made to the means available to the state under attack; the kinds of forces and the level of armament to hand13 will be relevant to the nature and intensity of response that it would be reasonable to expect, as well as the realistic possibilities of resorting to non-military means in the circumstances.14

4. A state may use force in self-defence against a threatened attack only if that attack is ‘imminent’.

There is a risk of abuse of the doctrine of anticipatory self-defence, and it needs to be applied in good faith and on the basis of sound evidence. But the criterion of imminence must be interpreted so as to take into account current kinds of threat and it must be applied having regard to the particular circumstances of each case. The criterion of imminence is closely related to the requirement of necessity.

  • Force may be used only when any further delay would result in an inability by the threatened state effectively to defend against or avert the attack against it.

  • In assessing the imminence of the attack, reference may be made to the gravity of the attack, the capability of the attacker, and the nature of the threat, for example if the attack is likely to come without warning.

  • Force may be used only on a proper factual basis and after a good faith assessment of the facts.

The concept of ‘imminence’ reflects the Caroline formulation of ‘instant, overwhelming, leaving no choice of means, and no moment for deliberation’. In the context of contemporary threats imminence cannot be construed by reference to a temporal criterion only, but must reflect the wider circumstances of the threat.
There must exist a circumstance of irreversible emergency. Whether the attack is ‘imminent’ depends upon the nature of the threat and the possibility of dealing effectively with it at any given stage. Factors that may be taken into account include: the gravity of the threatened attack – whether what is threatened is a catastrophic use of WMD; capability - for example, whether the relevant state or terrorist organisation is in possession of WMD, or merely of material or component parts to be used in its manufacture; and the nature of the attack – including the possible risks of making a wrong assessment of the danger. Other factors may also be relevant, such as the geographical situation of the victim state, and the past record of attacks by the state concerned.
The criterion of imminence requires that it is believed that any further delay in countering the intended attack will result in the inability of the defending state effectively to defend itself against the attack. In this sense, necessity will determine imminence: it must be necessary to act before it is too late. There is a question as to whether ‘imminence’ is a separate criterion in its own right, or simply part of the criterion of ‘necessity’ properly understood. As an additional criterion however it serves to place added emphasis on the fact that a forcible response in these circumstances lies at the limits of an already exceptional legal category, and therefore requires a correspondingly high level of justification.
To the extent that a doctrine of ‘pre-emption’ encompasses a right to respond to threats which have not yet crystallized but which might materialise at some time in the future, such a doctrine (sometimes called ‘preventive defence’) has no basis in international law. A fatal flaw in the so-called doctrine of prevention is that it excludes by definition any possibility of an ex post facto judgment of lawfulness by the very fact that it aims to deal in advance with threats that have not yet materialised.

Each case will necessarily turn on its own facts. A forceful action to disrupt a terrorist act being prepared in another state might, depending upon the circumstances, be legitimate; force to attack a person who may in the future contemplate such activity is not. While the possession of WMD without a hostile intent to launch an attack does not in itself give rise to a right of self-defence, the difficulty of determining intent and the catastrophic consequences of making an error will be relevant factors in any determination of ‘imminence’ made by another state.

The determination of ‘imminence’ is in the first place for the relevant state to make, but it must be made in good faith and on grounds which are capable of objective assessment. Insofar as this can reasonably be achieved, the evidence should be publicly demonstrable. Some kinds of evidence cannot be reasonably produced, whether because of the nature or source, or because it is the product of interpretation of many small pieces of information. But evidence is fundamental to accountability, and accountability to the rule of law. The more far-reaching, and the more irreversible its external actions, the more a state should accept (internally as well as externally) the burden of showing that its actions were justifiable on the facts. And there should be proper internal procedures for the assessment of intelligence and appropriate procedural safeguards.

5. The exercise of the right of self-defence must comply with the criterion of ‘proportionality’.

  • The force used, taken as a whole, must not be excessive in relation to the need to avert or bring the attack to an end.

  • The physical and economic consequences of the force used must not be excessive in relation to the harm expected from the attack.

In the Caroline formulation, the principle of proportionality was stated to require “nothing 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 has confirmed that it is a well-established rule of customary international law that a use of force in self-defence must be “proportional to the armed attack and necessary to respond to it.” 15

This requires that the level of force used is not greater than that necessary to end the attack or remove the threat. As such it is another way of looking at the requirement of necessity.

The proportionality requirement has been said to mean in addition that the physical and economic consequences of the force used must not be excessive in relation to the harm expected from the attack16. But because the right of self-defence does not allow the use of force to ‘punish’ an aggressor, proportionality should not be thought to refer to parity between a response and the harm already suffered from an attack, as this could either turn the concept of self-defence into a justification for retributive force, or limit the use of force to less than what is necessary to repel the attack.

The force used must take into account the self-defence operation “as a whole”. It does not relate to specific incidents of targeting (which is a matter for international humanitarian law). Thus, in the Oil Platforms Case, the ICJ stated that in assessing proportionality, it “could not close its eyes to the scale of the whole operation”17.

6. Article 51 is not confined to self-defence in response to attacks by states. The right of self-defence applies also to attacks by non-state actors.

  • In such a case the attack must be large scale.

  • If the right of self-defence in such a case is to be exercised in the territory of another state, it must be evident that that state is unable or unwilling to deal with the non-state actors itself, and that it is necessary to use force from outside to deal with the threat in circumstances where the consent of the territorial state cannot be obtained18.

  • Force in self-defence directed against the government of the state in which the attacker is found may be justified only in so far as it is necessary to avert or end the attack, but not otherwise.

There is no reason to limit a state’s right to protect itself to an attack by another state. The right of self-defence is a right to use force to avert an attack. The source of the attack, whether a state or a non-state actor, is irrelevant to the existence of the right. The ICJ Wall Advisory Opinion should not be read as suggesting that the use of force in self-defence is not permissible unless the armed attack is by a state.19 There is nothing in the text of Article 51 to demand, or even to suggest, such a limitation.20

This conclusion is supported by reference to the Caroline case; the criteria in Caroline were enunciated in the context of a marauding armed band, not orthodox state-to-state conflict.

State practice in this field, including the recent practice of the Security Council, gives no support to the restriction of self-defence to action against armed attacks imputable to a state; indeed there is state practice the other way. The action against Al Qaeda in Afghanistan in October 2001 (which was widely supported by states) was action in self-defence of anticipated imminent terrorist attacks from Al Qaeda, not from the Taliban. It was necessary to attack certain elements of the Taliban, in order to pre-empt attacks from Al Qaeda. Security Council resolutions 1368(2001) and 1373(2001) support the view that self-defence is available to avert large-scale terrorist attacks such as those on New York and Washington on 11 September 2001.21 So too do the invocations by NATO and the OAS of their respective mutual defence obligations

The right of states to defend themselves against ongoing attacks, even by private groups of non-state actors, is not generally questioned. What is questioned is the right to take action against the state that is the presumed source of such attacks, since it must be conceded that an attack against a non-state actor within a state will inevitably constitute the use of force on the territorial state. It may be that the state is not responsible for the acts of the terrorists, but it is responsible for any failure to take reasonable steps to prevent the use of its territory as a base for attacks on other states. Its inability to discharge the duty does not relieve it of the duty. But the right to use force in self-defence is an inherent right and is not dependent upon any prior breach of international law by the state in the territory of which defensive force is used.

Thus, where a state is unable or unwilling to assert control over a terrorist organisation located in its territory, the state which is a victim of the terrorist attacks would, as a last resort, be permitted to act in self-defence against the terrorist organisation in the state in which it is located22.

The same criteria for the use of force in self-defence against attacks by states are to be used in the case of attacks by non-state actors, but particular considerations are relevant.

The attack or imminent attack by non-state actors must be large-scale.23

For action in self-defence to be ‘necessary’, it must first be clear that measures of law-enforcement would not be sufficient. To show the necessity of action against the territory of another state not directly responsible for the acts of the non-state group requires, inter alia, the demonstration that there is no other means of meeting the attack and that this way will do so. Terrorist organisations are not easily rooted out by foreign armed forces.

Where, therefore, the attack is not ongoing but imminent, the territorial state is entitled to proceed in its own way against the group on its territory. In this context, the requirement of ‘imminence’ means that action in self-defence by another state may not be taken save for the most compelling emergency.

7. The principles regarding the right of self-defence form only a part of the international regulation of the use of force.

  • Measures taken in the exercise of the right of self-defence must be reported immediately to the Security Council. The Council retains the right and responsibility to authorise collective military action to deal with actual or latent threats.

  • Any military action must conform with the rules of international humanitarian law governing the conduct of hostilities.

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