1.International law prohibits States from using force in the conduct of their international relations: Article 2(4) of the UN Charter provides:
All Members shall refrain in their international relations from the threat or use of force against the territorial integrity of political independence of any state, or in any other manner inconsistent with the Purposes of the United Nations.
2.As well as being recognised in the UN Charter, this general prohibition is so well established as to be generally agreed to be part of customary international law.
3.While international treaties and other sources of international law are not generally part of UK law until they have been transformed into domestic law by, for example, incorporation by statute, customary international law is automatically part of UK law.
4.In keeping with its status as a norm of customary international law, the International Court of Justice has interpreted the general prohibition on the use of force in Article 2(4) of the UN Charter very strictly.
5.The international law prohibition on the use of force by states in the territory of another state is, however, subject to certain exceptions. There are three well established exceptions:
(1) Consent (or invitation)
(2) UN Security Council authorisation
6.Each of these three exceptions is explained in more detail below. The International Court of Justice insists on the exceptions being interpreted very narrowly, and is generally resistant to arguments that would widen the scope of the exceptions. The most relevant exception for the purposes of the Committee’s inquiry is the third exception, self-defence, as this is the justification relied on by the Government for its new policy on the use of lethal force abroad.
7.A fourth exception, humanitarian intervention, is arguably emerging, but there continues to be disagreement amongst states, and between international lawyers, as to whether or not it has attained the status of a recognised exception. The UK Government claims that the use of force for humanitarian intervention is a lawful exception. Indeed, this was the basis of the Government’s argument in favour of air strikes against the Assad regime in Syria in 2013, to protect the Syrian people against the future use of chemical weapons. Humanitarian intervention has not, however, been invoked by the Government to justify its use of lethal force abroad by drones and it is therefore not considered further here.
8.In international law, the government of a state can consent to another state using force on its territory, or can invite another state to use force on its territory.
9.This is the legal basis on which the UK Government is already using force against ISIL/Da’esh in Iraq, following the request of the Iraqi government for international assistance to combat ISIL/Da’esh. It also appears to be the basis on which Russia is using force in Syria, at the request of President Assad. The UK Government has not, however, been asked for assistance by Syria to defend itself against ISIL/Da’esh, nor, it appears, has the Government asked the Syrian Government to agree to the use of force on its territory.
10.Consent of the state on whose territory the force is used may be implied from that state’s acquiescence in the use of force on its territory. However, while there may have been a period during which Syria was arguably acquiescing in the use of force on its territory against ISIL/Da’esh by coalition forces, it is clear that this has not been the case since September 2015 when Syria expressly invited Russia to assist. Consent, express or implied, therefore cannot be the legal basis for the use of force by the UK Government in Syria.
11.The UN Security Council can authorise states to use force under Chapter VII of the UN Charter (Articles 39 to 51). Once the Security Council has determined that there is a threat to the peace, a breach of the peace or act of aggression, it can authorise states under Article 41 of the UN Charter to use force “to maintain or restore international peace or security” when it considers that other measures would be inadequate or have proved to be inadequate. Such Security Council resolutions usually authorise states to “take all necessary measures” rather than explicitly authorise the use of force.
12.There is currently no Security Council resolution authorising the use of force in Syria or Iraq. UN Security Council Resolution 2249 (2015) on ISIL/Da’esh in Syria and Iraq, which was passed by the Security Council on 20 November 2015 in the wake of the attacks in Paris, is not a Chapter VII resolution authorising the use of force and therefore does not provide a legal basis for the use of force in Syria. In this resolution, the Security Council determines that ISIL/Da’esh “constitutes a global and unprecedented threat to international peace and security” and “calls upon Member States that have the capacity to do so to take all necessary measures, in compliance with international law, in particular with the United Nations Charter, as well as international human rights, refugee and humanitarian law, on the territory under the control of ISIL/Da’esh in Syria and Iraq, to redouble and co-ordinate their efforts to prevent and suppress terrorist acts committed specifically by ISIL/Da’esh […] and to eradicate the safe haven they have established over significant parts of Iraq and Syria.”
13.The resolution therefore does not purport to authorise the use of force, but provides the Security Council’s implicit support for the other legal basis states are invoking to justify their use of force in Syria and Iraq: self-defence.
14.The Government does not rely on Security Council Resolution 2249 as providing legal authority for using force in Syria. Both the Prime Minister and the Attorney General have clearly stated that the legal basis for using force in Syria is not the Security Council resolution but the right of self-defence, which is “underscored” or “underlined” by the Security Council in its resolution.
15.The third exception to the general prohibition on the use of force is self-defence. This is an “inherent right”, recognised in Article 51 of the UN Charter which provides:
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 measures necessary to maintain international peace and security.
16.Self-defence may be individual (in defence of the State itself) or collective (in defence of another State at its request).
17.The Prime Minister told the House of Commons on 7 September that the drone strike against Reyaad Khan in Syria on 21 August was in defence of the UK against a threat of imminent terrorist attack—that is, in individual self-defence.
18.The UK’s Permanent Representative to the UN told the Security Council by letter dated 8 September that the drone strike was also in the collective self-defence of Iraq.
19.The Government’s position is that the drone strike in Syria on 21 August was a lawful use of force on the territory of another State because it was an exercise of the inherent right of both individual self-defence of the UK and collective self-defence of Iraq. The Government similarly relies on both individual self-defence of the UK against terrorist attack by ISIL/Da’esh and collective defence of Iraq against attack by ISIL/Da’esh to justify future such uses of force in Syria. There is nothing necessarily contradictory about these claims: a single act of self-defence can serve both purposes of individual and collective self-defence.
20.However, a number of conditions must be satisfied for a state to be entitled to invoke the right of self-defence: there must be (1) an “armed attack” which must be (2) actual or imminent and the resort to force must be both (3) necessary and (4) proportionate.
21.For a State to invoke the right of self-defence there must be an “armed attack”. To constitute an “armed attack” for the purposes of the right of self-defence the attack must cross a certain threshold of seriousness or intensity. A series of minor attacks is not necessarily enough to constitute an armed attack. The scale and effect of the attack must reach a certain threshold of gravity.
22.The Prime Minister told the House of Commons that “It is […] clear that ISIL’s campaign against the UK and our allies has reached the level of an ‘armed attack’, such that force may lawfully be used in self-defence to prevent further atrocities being committed by ISIL.”
23.Whether the right of self-defence can be exercised where the threat of armed attack emanates from non-state actors who are not acting under the control or direction of another state is an issue which is not clearly settled in international law. Some international lawyers appear to take the view that the right of self-defence can only be invoked against another State, relying on the judgment of the ICJ in the Israeli Wall case. Others, including the Government, take the view that a State’s inherent right of self-defence extends to attacks originating from non-state actors such as ISIL/Da’esh. State practice since 9/11 certainly supports the view that a State’s right of self-defence includes the right to respond with force to an actual or imminent armed attack by a non-State actor, and the most recent UN Security Council Resolution 2249 (2015) lends support to this view.
24.To be entitled to rely on self-defence against non-state actors, the State from whose territory the armed attack is being launched or prepared for must be unable or unwilling to prevent the attack. Again, the legal status of the unwilling or unable test is disputed.
25.The Government’s position is that the right of self-defence can be invoked against non-state actors operating in another state which is unwilling or unable to prevent the attack by the non-state actors. The Prime Minister told the Commons that “there is a solid basis of evidence on which to conclude, first, that there is a direct link between the presence and activities of ISIL/Da’esh in Syria and its ongoing attack on Iraq, and secondly, that the Assad regime is unwilling and/or unable to take action necessary to prevent ISIL/Da’esh’s continuing attack on Iraq, or indeed attacks on us.”
26.A State’s right of self-defence can be invoked preventively, to prevent an imminent armed attack. However the meaning of imminence is disputed. At one extreme the US position is that there is a right of pre-emptive self-defence. Under the long established “Caroline test” (so called after a 19th century case on the use of force), the need to use force in self defence must be “instant, overwhelming, leaving not choice of means and no moment for deliberation.” However, others argue that the Caroline test is too narrow in the light of modern conditions.
27.The Prime Minister did not refer to the requirement of “imminence” at all in his Response to the Foreign Affairs Committee Report nor in his statement to the House of Commons on 26 November. Instead, he referred to ISIL’s “ongoing” and “continuing” attack on Iraq. The Government treats the armed attack on Iraq by ISIL/Da’esh as an actual attack which is ongoing. However, the Government appears to rely on the armed attack on the UK being “imminent”. Both the Attorney General and the Defence Secretary have suggested in oral evidence that the Government favours a more expansive definition of “imminence” which would entitle the UK to act in self-defence where an identified individual is involved in an ongoing way in plotting terrorist attacks on the UK.
28.The use of force in self-defence must also satisfy the requirement of necessity: it must be a last resort after all other options have been exhausted. This requirement can be given effect by building certain conditions into the decision-making process. The US policy, for example, requires the decision-maker to determine that capture is not an option before lethal force is authorised. In the context of the use of force in self-defence against non-state actors, the requirement that the host State is either unable or unwilling to prevent attacks is in effect a specific application of the necessity requirement.
29.The final requirement is that any resort to force must be proportionate: the degree of force used must be no greater than necessary to end the armed attack or to avert the threat of such attack, and the harm caused by the use of force must be in proportion to the scale of the threat that the action was designed to avoid. A use of lethal force which resulted in large numbers of civilian casualties, for example, would be unlikely to be proportionate if the scale of the threatened attack was relatively small.
30.Compliance with international law on the use of force does not exhaust all the legal questions about the lawfulness of the drone strike in Syria on 21 August or the Government’s policy. The fact that a use of lethal force is lawful under the international law on the use of force, for example because it was taken in self-defence, does not mean that the use of force is necessarily lawful under the other relevant international legal frameworks: the Law of War (otherwise known as the law of armed conflict or international humanitarian law) and international human rights law. Any use of force in lawful exercise of the right of self-defence must also comply with those other legal frameworks where they apply. Those legal frameworks must also therefore be addressed, separately and in turn.
31.The Law of War consists of a set of international rules the purpose of which is to limit the suffering caused by armed conflict by regulating its conduct. The Law of War does not determine whether a State does or does not have the right to resort to armed force: that is the business of the branch of international law on the use of force considered in section (1) above. The Law of War governs the way in which armed conflict is conducted. It is premised on the idea that even in wartime some things are not permitted. It seeks to temper military necessity with principles of humanity.
32.The Law of War is to be found in customary international law and various international treaties, the most important of them the four Geneva Conventions of 1949 and their additional protocols. Every State in the world has ratified the Geneva Conventions.
33.There is no specific court to ensure the observance of States’ obligations under the Law of War. The International Committee of the Red Cross is the “guardian and promoter” of the Law of War. It issues interpretive guidance on the Law of War, but it is not a court and its interpretations of the Law of War are therefore not legally binding on States.
34.The Law of war applies where there is an armed conflict (which is why it is often colloquially referred to as “the law of war” or “the law of armed conflict”).
35.Armed conflicts are of two types: international and non-international. The classification of an armed conflict as one type or another depends on the parties to the conflict:
36.Today, most armed conflicts are non-international armed conflicts. A non-international armed conflict can take place across State boundaries: the conflict is “non-international” because one of the parties is a non-State actor, even though the territorial scope of the conflict may cross State boundaries. So, the Government considers itself to be involved in a non-international armed conflict with ISIL/Daesh which is taking place in both Iraq and Syria.
37.The classification of an armed conflict as international or non-international affects the legal standards that apply to the conflict, because the applicable law depends on which type of armed conflict is taking place. A more basic set of rules applies to non-international armed conflicts compared to international armed conflicts. For example, in a non-international armed conflict, there is no prisoner-of-war status, and no combatant immunity.
38.Certain threshold conditions must be satisfied in order for a non-international armed conflict to exist and for the Law of War therefore to apply. The first threshold condition concerns the intensity of the violence. Armed violence should not be sporadic or isolated: there must be “protracted armed violence between governmental authorities and organised armed groups or between such groups within a State.” The second condition is that the conflict must be with an armed group that satisfies the criteria of being sufficiently organised (there must be a command structure, headquarters and an ability to plan and carry out military operations).
39.The Law of War permits targeted killing in an armed conflict, provided certain principles and conditions are complied with.
40.The first Law of War principle that must be complied with is the principle of “distinction”: targeting must distinguish between lawful military targets and civilians. A person is a lawful target in an international armed conflict if he or she is a combatant or a directly participating civilian. A person is a lawful target in a non-international armed conflict if he or she is a member of an armed group or a civilian directly participating in hostilities.
41.The second Law of War principle is the principle of proportionality: a lawful military target can be attacked only after an assessment concluding that the military advantage outweighs the risk of civilian casualties. Civilian casualties are therefore contemplated by the Law of War, provided they are not disproportionate to the military advantage to be gained from the use of force.
42.The third Law of War principle that applies to the use of lethal force in armed conflict is the principle of “precaution”: constant precautions must be taken to spare civilians when choosing targets, means and methods of attack. This means that care must be taken to minimise the danger to civilians; any use of force must be abandoned if it becomes clear that harm to civilians will outweigh the military objective.
43.Apart from in the extreme case of alleged war crimes there is no obligation to conduct an ex post investigation of the use of lethal force in armed conduct. Most of the legal constraints imposed by the Law of War operate prior to the decision whether or not to use lethal force.
44.Compared to international human rights law (see section (3) below), the Law of War is more permissive towards the use of lethal force. Deliberate targeted killing is permitted, provided the individual in question is a lawful military target and the principles of proportionality and precaution are complied with. Ex post accountability obligations are weak in the Law of War compared to those imposed by human rights law.
45.The human right most relevant to the Committee’s inquiry is the right to life. The right to life is often referred to as the most fundamental human right, or the supreme right.
46.The right to life is proclaimed in the Universal Declaration of Human Rights of 1948, and the rule against the arbitrary deprivation of life is a rule of customary international law. The right is also recognised in the main human rights treaties to which the UK is a party. Article 6 of the International Covenant on Civil and Political Rights (“ICCPR”), for example, provides:
“6(1) Every human being has the inherent right to life. This right shall be protected by law. No one shall be arbitrarily deprived of his life.”
47.Article 2 of the European Convention on Human Rights (“ECHR”) also recognises the right to life. Article 2(1) provides that “Everyone’s right to life shall be protected by law”, while Article 2(2) contains an exhaustive list of grounds on which lethal force may be used:
“2(2) Deprivation of life shall not be regarded as inflicted in contravention of this Article when it results from the use of force which is no more than absolutely necessary:
(a) in defence of any person from unlawful violence;
(b) in order to effect a lawful arrest or to prevent the escape of a person
(c) in action lawfully taken for the purposes of quelling a riot or
48.The right to life in Article 2 ECHR is also part of UK law by virtue of the Human Rights Act. Public authorities are therefore under a statutory duty to act compatibly with the right to life, as interpreted by the European Court of Human Rights, and that duty is directly enforceable against public authorities in UK courts.
49.The common law has also long recognised and protected the right to life, as demonstrated, for example, in the common law criminal offences of murder and manslaughter.
50.International human rights law is referred to by Cristof Heyns, the UN Special Rapporteur on Extrajudicial, Summary or Arbitrary Executions, in his written evidence, as “the default, generally applicable regime.” The International Court of Justice has confirmed in a number of decisions that human rights treaties apply not only in peacetime but also in armed conflict.
51.The general applicability of human rights obligations, even in armed conflict, is shown by the fact that the relevant human rights treaties provide for derogations during times of war or public emergency. Under the ICCPR Article 4, for example, in time of public emergency which threatens the life of the nation and the existence of which is officially proclaimed, States can derogate from their obligations under the Covenant to the extent strictly required by the exigencies of the situation. There can be no derogation from the right to life under the ICCPR, however. States can similarly derogate from ECHR rights “in time of war or other public emergency threatening the life of the nation” (Article 15). However, no derogations are permitted from the right to life in Article 2 ECHR “except in respect of deaths resulting from lawful acts of war” (Article 15(2)).
52.The applicability of the right to life in Article 2 ECHR depends on the victim being within the jurisdiction of the UK. Jurisdiction under the ECHR is primarily territorial, but the ECHR also has extraterritorial application in certain circumstances, including the exercise of power and control over the person in question. On the current state of the case-law, the use of lethal force abroad by a drone strike is sufficient to bring the victim within the jurisdiction of the UK: in the recent case of Al Sadoon v Secretary of State for Defence, the High Court held that “whenever and wherever a state which is a contracting party to the [ECHR] purports to exercise legal authority or uses physical force, it must do so in a way that does not violate Convention rights.” The judge found it difficult to imagine a clearer example of physical control over an individual than when the State uses lethal force against them.
53.The assumed applicability of IHRL to action taken extraterritorially against ISIS/Da’esh is also evident from the terms of the UN Security Council Resolution 2249 (2015) on ISIS/Daesh in Syria and Iraq, which calls on States to take all necessary measures “in compliance with international law, in particular with the United Nations Charter, as well as international human rights, refugee and humanitarian law”.
54.The protection human rights law gives to the right to life is multi-faceted and has been elaborated by the monitoring and enforcement mechanisms which States have created under the human rights treaties to give effect to the human rights recognised in the treaties: in particular the UN Human Rights Committee which monitors compliance with the ICCPR and the European Court of Human Rights which was established to ensure the observance of the obligations assumed by States in the ECHR.
55.The right to life imposes on States a negative obligation, not to deprive a person of their life arbitrarily. The use of lethal force may be lawful under human rights law, but only as a last resort: it must be absolutely necessary in the sense that there is no other means, such as capture or non-lethal incapacitation, of preventing the threat; and it must be proportionate in the sense that it is strictly required to avert an imminent threat to life. The State is also under an obligation to ensure that any law enforcement operation which may culminate in the use of lethal force is subject to careful planning and control.
56.Some of the principles derived from international human rights law’s protection of the right to life have been distilled into the UN Basic Principles on the Use of Force and Firearms by Law Enforcement Officials. Principle 9 provides that “intentional lethal use of firearms may be made only when strictly unavoidable in order to protect life.” Under human rights law, therefore, the intentional, premeditated killing of an individual outside of armed conflict will only be lawful if it is the only way to protect against an immediate threat to life, such as in a hostage situation in which the life of a hostage is threatened.
57.The right to life also imposes various positive obligations on States. The obligation to respect and secure the rights enshrined in human rights treaties such as the ICCPR and the ECHR places States under an obligation to protect the lives of individuals against threats from other individuals such as criminals and terrorists. The duty to protect the right to life by law also requires the State to have a legal framework which provides effective protection for the right to life, for example by criminalising murder, and also regulating the use of lethal force by law enforcement officials.
58.In armed conflict, although human rights law applies, its substantive protections, including for the right to life, are to be read in the light of the requirements of the Law of War. Compliance with the Law of War in armed conflict is therefore likely to be sufficient to discharge the State’s obligations under human rights law.
59.The right to life also imposes procedural obligations on the State: to carry out an independent and effective investigation into any credible allegation that there has been an unlawful deprivation of life, capable of giving rise to accountability for any such violation of the right to life. This obligation requires State to investigate deprivations of life following use of lethal force by the State.
60.Compared to the Law of War (see section (2) above), international human rights law is more widely applicable: like the Law of War it applies in armed conflict, but it also applies outside of armed conflict (when the Law of War does not apply). It provides higher protection for the right to life by imposing higher legal standards which must be satisfied before lethal force can be used: life can only be deliberately taken if it is absolutely necessary in order to protect life and if the degree of force used is strictly proportionate to the threat to be averted. Civilian casualties are therefore more difficult to justify under international human rights law. International human rights law also imposes a stronger procedural obligation on the State to carry out an independent and effective investigation and to hold to account for any unlawful deprivations.
215 For a useful overview see , Briefing Paper, House of Commons Library, November 2015
216 See summary of AG’s opinion, 2013.
217 See letter to UN Security Council September 2014.
218 See to the Foreign Affairs Select Committee’s Second Report of Session 2015-16: The Extension of Offensive British Military Operations to Syria (26 November 2015); Prime Minister HC Deb 26 Nov 2015 ; and Attorney General HC Deb 26 Nov 2015 .
219 HC Deb 26 Nov 2015
220 International Law Commission Commentary to Article 21 of the Articles on the Responsibility of States for Wrongful Acts.
221 For a useful introductory guide to The Law of War, see , Briefing Paper 7429, House of Commons Library 8 January 2016
222 See (ICRC/IPU, 1999)
223 IHL is often referred to as the ius in bello (the law which applies in war), in contrast to the ius ad bellum (the law which determines whether a State is entitled to resort to force).
224 See, for example, UN Human Rights Committee General Comment on the Right to Life in Article 6 ICCPR.
225 Article 3 UDHR provides: “Everyone has the right to life …”.
226 Christopf Heyns, Dapo Akande, et al, (), The International Law Framework Regulating the Use of Armed Drones.
227 See e.g. the ICJ’s advisory opinions on Legality of the Threat or Use of Nuclear Weapons  ICJ Rep, para. 25, and Legal Consequences of the Construction of a Wall in the Occupied Territory  ICJ Rep 136 para. 106.
228 Al Skeini v UK, applied by the UK Supreme Court in Smith v MOD.
229  EWHC 715 (Admin) para. 106 (17 March 2015). The Government is appealing against the judgment to the Court of Appeal.
230 McCann v UK.
231 Al Skeini v UK, applied by the UK Supreme Court in Smith v MOD
9 May 2016