A. Consent

Where a territorial State consents to another State targeting non-State actors on the former's territory with drones, no issue in principle should arise under the ius ad bellum, for Article 2(4) of the UN Charter will not have been violated.Footnote ¹⁸ Indeed, consent has been given by Pakistan, Yemen and Somalia which has then been relied upon by the U.S. to justify the use of drones in those territories; regarding Pakistan, this consent was subsequently withdrawn, whereas regarding Yemen and Somalia, though consent has not been withdrawn, the authority of those governments might call the validity of that consent into question.Footnote ¹⁹

While consent would seem a simple test that would result in a particular drone strike in foreign territory being consistent with the ius ad bellum, in practice there are issues of considerable difficulty surrounding consent, including who may give consent, whether consent must be given publicly and explicitly or could instead be implicit, and when consent could be considered to have been vitiated by coercion.Footnote ²⁰

With regard to who may give consent, the fundamental nature of the prohibition of the use of force for State sovereignty means that only the State's highest authorities may validly give consent—that is, have the authority to give consent—to a use of force.Footnote ²¹ One cannot simply rely on the notion of attribution under the secondary rules on State responsibility by analogy. Although the acts of all government officials, acting in that capacity, are attributable to the government, not all parts of the government are entitled to give consent with respect to the use of force. In particular, it is not sufficient to obtain consent from regional authorities or from particular agencies or departments of government. The International Law Commission has made it clear that:

The answer to the question of which domestic authority is entitled to give consent to what would otherwise constitute a violation of international law depends on the interplay between international and domestic law. In the first place, one needs to consider the nature of the international rule in question and the manner in which that rule is generally applied.Footnote ²³ As already stated, the importance of the prohibition on the use of force suggests that consent to a departure from this rule will usually need to be established at the highest levels of government. Secondly, one will need to consider the domestic arrangements made with respect to the matter at hand.Footnote ²⁴ Where domestic law or domestic constitutional arrangements are such as to give responsibility on these issues to lower level officials this may be taken into account.

However, it would be natural that where there is a difference of view between the highest authorities in government and lower-level officials, it is the view of the higher-level officials that should be taken as determinative. This arises in part because international law itself assumes that certain organs of government have the capacity to represent the State in international affairs. Thus, the Head of State, Head of Government and the Foreign Minister are presumed, as a matter of international law, to have plenary competence to conclude treaties on behalf of the State.Footnote ²⁵ In addition, these senior government officials are accorded immunity ratione personae from the jurisdiction of other States because of their privileged position in conducting international relations on behalf of the State.Footnote ²⁶ Thus, it can be assumed that it is the view of these officials, where expressed, that should be determinative of whether the State gives consent or not to the use of force on the State's territory. Importantly, the view that consent to the use of force must be given by the highest authorities in the central government of a State accords with the bulk of international practice.Footnote ²⁷

Regarding the issue of publicity and consent, while there does not appear to be a requirement that consent be made public, it must nevertheless be clear as between the States concerned that consent is being given to the use of force, and the parameters of that consent should also be made clear. Consent must be given in advance.Footnote ²⁸ Moreover:

Consent on so serious a matter as the use of force is not to be implied. Furthermore, where force exceeds the limits of the consent given, that force will be a violation of Article 2(4).Footnote ³⁰ In addition, once consent to the use of force is withdrawn, the State conducting targeting operations is bound by international law to refrain from conducting any further operations from that moment onwards (unless there is a separate justification for using force).Footnote ³¹

B. Self-Defence

Where no consent is given, Article 2(4) will be engaged by a drone strike on another State's territory, requiring resort to one of two exceptions to the prohibition contained in the UN Charter. The most common exception invoked by States using drones in these situations is self-defence.Footnote ³² International law poses stringent conditions on the use of force in self-defence. Under Article 51 of the UN Charter and customary international law, a State may invoke self-defence to justify its use of force to target individuals on another State's territory where an armed attack against it occurs or is imminent (on which see below). The International Court of Justice (ICJ) has confirmed that, for an attack to constitute an ‘armed attack’ and thus enable the State's right to use force in self-defence, the scale and effects of the attack must reach a certain threshold of gravity.Footnote ³³ Thus, not any use of force against a State will necessarily justify a response in self-defence; rather, only grave uses of force can do so.

In addition to the requirement of an armed attack, the State claiming to be acting in self-defence must also satisfy the dual requirements of necessity and proportionality, grounded in customary international law.Footnote ³⁴ These requirements, as defined in the context of the ius ad bellum, are closely linked to the important issue of the aim of an act of self-defence. Thus, ‘necessity and proportionality mean that self-defence must not be retaliatory or punitive; the aim should be to halt and repel an attack’.Footnote ³⁵ In other words, action taken lawfully in self-defence (eg the use of drones to target individuals in another State's territory) must serve the purpose of halting and repelling an armed attack and must be both necessary and proportionate to this end.Footnote ³⁶ Action taken after an armed attack has ended, and which in reality seeks to retaliate against that armed attack, would not constitute a lawful exercise of self-defence, but rather an armed reprisal in violation of Article 2(4) of the UN Charter.Footnote ³⁷ In considering the elements relevant to determining whether a particular action is necessary and proportionate to the aim of self-defence, the ICJ in the Oil Platforms case placed emphasis on the nature of the objects targeted by the US and their role in staging any initial armed attacks.Footnote ³⁸

The requirements of necessity and proportionality also help to define the limits of the right to self-defence. The right persists only for so long as it is necessary to halt or repel an armed attack and must be proportionate to that aim. However, that is not to say that, in determining what is necessary to bring an attack to an end and what is a legitimate objective for self-defence, States are entitled to continue to act in self-defence until the absolute destruction of the enemy is achieved such that the enemy poses no long-term threats. International law cannot permit States to act until the elimination of long-term threats is secured. The law of self-defence permits responses to emergency situations in which States face immediate or imminent risks. Once the immediate or imminent threats of (continued) attack are resolved, States will need to find other methods of securing long-term peace and stability.

To permit force to be used as a means of achieving long-term security where the threat of immediate or imminent attacks has receded, is to permit perpetual warfare. Therefore, even when drones are used in self-defence, consideration needs to be given to the moment when the group against which drones are being used is sufficiently disrupted such that it no longer poses an immediate or imminent threat.

The question of ‘imminence’ raises a further controversial point of considerable importance to the use of armed drones abroad. Article 51 recognizes the right to self-defence where ‘an armed attack occurs’, but also refers to self-defence as an ‘inherent’ right of States. This has given rise to arguments that the right to self-defence under customary law is not displaced by the Charter. Importantly, the argument that an anticipatory attack against an imminent threat is permissible rests on this basis.Footnote ³⁹ There is a significant debate about the legality of action in self-defence prior to an actual armed attack. However, as the Report of the High-Level Panel Established by the UN Secretary-General noted: ‘Long-established customary international law makes it clear that States can take military action as long as the threatened attack is imminent, no other means would deflect it, and the action is proportionate.’Footnote ⁴⁰ The UK, for example, takes the view that action in self-defence can be lawful where an armed attack is imminent.Footnote ⁴¹

Importantly, at most anticipatory self-defence could be lawful only in response to an existing threat. It may not be employed pre-emptively to prevent a threat from arising in the future. The necessity of self-defence, according to the well-known construction, ‘must be instant, overwhelming, and leaving no choice of means, no moment of deliberation’.Footnote ⁴² The body of opinion and State practice that rejects the concept of anticipatory self-defence altogether should also be noted, and serves at least as a confirmation of the limited scope of the exception.Footnote ⁴³

Finally, it must be emphasized that the imminence requirement in IHRL which stipulates that intentional lethal force may be used only to protect life, is not to be conflated with the requirement of imminence in the law governing the use of force on foreign territory under Article 51. The former is a condition required for all uses of lethal force to be lawful under IHRL. The latter applies under the doctrine of anticipatory self-defence and the legality of a use of force on another State's territory under the ius ad bellum.Footnote ⁴⁴ Meeting the requirement of imminence under one regime may not necessarily satisfy the test under the other. This is consistent with one of the key themes of the present article, that separate assessments of the legality of drone strikes under each applicable branch of international law must be undertaken.

A further controversy that must also be considered here concerns self-defence measures taken in a foreign State against non-State actors who do not act on behalf of that foreign State. This question is of particular importance given that non-State actors are currently being targeted by drones in States that do not necessarily bear responsibility for the acts of those actors.Footnote ⁴⁵ Prior to 9/11, the claim that force could be used in self-defence in response to an armed attack by a non-State group whose acts were not attributable to a State was controversial at best.Footnote ⁴⁶ The International Court of Justice rejected this view of the law in the Nicaragua case.Footnote ⁴⁷

However, State practice since the events of 11 September 2001 suggests that international law now permits such a notion of self-defence. The shift began with the near universal support for the US and UK's response in Afghanistan to the 9/11 attacks, which was based on a claim to be acting in self-defence against a non-State group operating from a State where the actions of that group could not be said to be attributable to the territorial State using ordinary rules of attribution under the law of state responsibility.Footnote ⁴⁸ Indeed, following the events of 9/11, the Security Council adopted Resolutions 1368 and 1373, both of which recognized ‘the inherent right of individual or collective self-defence in accordance with the Charter’.Footnote ⁴⁹ This would suggest that a State may use force in self-defence on another State's territory, where that first State has been the victim of an armed attack by non-State groups operating on the latter's territory, even where that attack is not attributable to the ‘host’ State.

In contrast, the ICJ in its Israeli Wall advisory opinion subsequently implied that inter-State force cannot be used in self-defence in response to an armed attack by a non-State actor:

Nonetheless, State practice relating to the use of force abroad against non-State actors, since 2001, is now broad and extensive, and indeed it extends beyond the immediate post-9/11 context.Footnote ⁵¹ Following this practice, and despite academic controversy,Footnote ⁵² the law on this matter, appears to have changed.

However, even if States may, in certain circumstances, lawfully exercise self-defence in response to an armed attack against a non-State actor, this only means that the armed attack requirement can be fulfilled without attribution of the attack to a State. Other customary international law requirements would still need to be fulfilled before a State can respond with force in self-defence on the territory of another State. In particular, it must be necessary and proportionate to respond with force to an armed attack (or imminent armed attack) from a non-State actor. In the context of a use of force against a non-State group, it is suggested that the necessity condition would only be satisfied where the territorial State itself is either unable or unwilling to prevent continued attacks.Footnote ⁵³ It is worth emphasizing that this test of whether the territorial State is unable or unwilling to act is not an independent legal standard but merely an aspect of the application of the long-standing criterion of necessity. In determining whether a State is unable or unwilling to take action, the State acting in self-defence might be required to request permission for such action prior to the commencement of acts taken in self-defence.Footnote ⁵⁴

Finally, Article 51 of the UN Charter makes it clear that measures adopted by States in exercise of self-defence must be reported to the UN Security Council.Footnote ⁵⁵ This can be seen as posing an obligation of transparency and justification to the international community, placing the issue formally on the agenda of the Security Council and recognizing its role. All member States of the UN have an obligation under its founding treaty to submit such reports. While failure to report will not render unlawful an otherwise lawful action taken in self-defence, ‘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’.Footnote ⁵⁶ According to Article 51 the right to exercise self-defence shall continue ‘until the Security Council has taken measures necessary to maintain international peace and security’. This demonstrates that the end point of an action in self-defence is not only determined by the principles of necessity and proportionality, noted above, but may also be determined by the Security Council.Footnote ⁵⁷

In addition to its transparency function, it could be argued that the rationale for this reporting requirement is to contribute towards the protection of the legal rights of sovereignty by the international community, since the State using force is required to offer its justification for that use of force. By extension, it must be concluded that a State must report afresh when the material facts have changed—for example, where self-defence is used as a basis for the use of force on the territory of a new State, or where new parties are added to the conflict.

A. The Threshold for Non-International Armed Conflict

Where a State targets non-State actors abroad, that act may, depending on the fulfilment of other conditions, take place in the context of a non-international armed conflict,Footnote ⁵⁸ and thus be governed by the rules of IHL applicable in that type of conflict. Classification of situations as international or non-international armed conflicts depends primarily on the question of who are the parties to the conflict. Conflict between States, are, according to Common Article 2 of the 1949 Geneva Conventions, international armed conflicts. Violence between a State and an organized non-State armed group, or between two or more such groups, may be a non-international armed conflict. It is generally accepted that a non-international armed conflict may take place across State boundaries, with the phrase ‘non-international’ referring not to the territorial scope of the conflict but to the status of the parties.Footnote ⁵⁹

Drones have thus far typically not been used in inter-State conflict. The question thus arises whether they are being used in non-international armed conflicts (NIACs). For violence to amount to a non-international armed conflict there must be ‘protracted armed violence between governmental authorities and organized armed groups or between such groups within a State’.Footnote ⁶⁰ Two cumulative criteria must thus be satisfied in order for a particular situation to be classified as a non-international armed conflict to which IHL would apply. These two criteria relate to: ‘the intensity of the conflict and the organisation of the parties to the conflict’.Footnote ⁶¹ Thus, an armed group will only be considered to constitute a party to a NIAC if it is sufficiently organized. International jurisprudence has determined the relevant indicative criteria, which include, inter alia, the existence of a command structure, headquarters, and a group's ability to plan and carry out military operations.Footnote ⁶²

Moreover, for a conflict to qualify as a NIAC, armed violence must also reach a certain threshold of intensity which is higher than that of internal disturbances and tensions.Footnote ⁶³ The armed violence should not be sporadic or isolated but protracted.Footnote ⁶⁴ The requirement of protracted violence ‘refers more to the intensity of the armed violence than its duration’.Footnote ⁶⁵ Like the condition of organization, the intensity of the armed violence is an issue which is determined on a case-by-case basis.Footnote ⁶⁶

In the context of drones, these requirements mean that IHL will not apply where the threshold levels of violence or organization are not present, leaving IHRL to govern the situation alone. It should be noted that individual drone strikes by themselves are not likely to meet the necessary threshold of violence for a NIAC to come into existence, leaving IHL inapplicable unless a drone strike is used in the context of a pre-existing NIAC.

A number of controversies have, however, arisen with regard to the scope of non-international armed conflicts and the threshold criteria for their existence, as a result of claims made by particular States when employing armed drones abroad. For example, where a State employs armed drones against members of the same non-State group in a number of different territories, the question arises as to whether the violence between the State and non-State group should be treated as a single, global non-international armed conflict.Footnote ⁶⁷ The consequence of following such an approach would be that, when assessing whether the intensity threshold for a non-international armed conflict had been met, one would aggregate the entirety of the violence between the State and non-State group across the globe. It might be argued that where a State targets in different States, members of the same (sufficiently organized) non-State armed group, with which it is already in a non-international armed conflict, a single, global non-international armed conflict exists, with the relevant rules of IHL applying to all such strikes.Footnote ⁶⁸ If this approach is followed, the intensity requirement could be met by aggregation of violence, rather than by examining the level of violence in each country. The result of such an approach is that, on some views, the less protective rules of IHL will apply in such cases to those targeted and exclude or modify the more protective rules of IHRL. Under this model, unsuspecting communities, far away from any battlefield, may lawfully be significantly affected so long as they are not disproportionately so (on proportionality, see below).

This result does, of course, depend on one's view of the interaction between IHL and IHRL in this area. If the ICJ's approach to the interaction between IHL and the right to life under the International Covenant on Civil and Political Rights (ICCPR) is applied equally in non-international armed conflicts,Footnote ⁶⁹ then the right to life of the person targeted would not be violated so long as IHL is complied with.

The possibility of a non-international armed conflict which spans across more than one country could certainly only exist where the individuals against which the State uses force in those several countries are actually members of the same non-State group.Footnote ⁷⁰ This is because the violence can only be part of the same armed conflict if it is between the same parties. Where violence is between entities that are not parties to the same conflict, that violence cannot be part of a single non-international armed conflict. Thus, in a supposed global non-international armed conflict, the fulfilment of the organized armed group criterion would have to be tested on a global basis.Footnote ⁷¹ Where there is no single organization that groups together all those fighting then the test for an organization would not be met, and one could not aggregate all the violence together for the purposes of the intensity criterion.

There is clearly an incentive for States that are engaged in military clashes to define the group with whom they are engaged so broadly as to capture those not really members thereof, so as to bring them all within the same non-international armed conflict. The way in which the existing organization criterion under IHL has been interpreted to some extent helps to protect against this. An armed group is a party to a non-international armed conflict if, and only if, it is sufficiently organized in accordance with the demands of IHL. An armed group will only be sufficiently organized to be a party to a non-international armed conflict, if there is ‘a sufficient body of evidence pointing to the … [group] being an organized military force, with an official joint command structure, [and] headquarters …’.Footnote ⁷² Moreover the group should have ‘a unified military strategy’ and be ‘able to speak with one voice’.Footnote ⁷³ Importantly, this is a factual, objective test, immune from the political considerations of the attacking State; it is not, therefore, for the State party to a NIAC to define for itself what constitutes the armed group against which it is fighting.

A related controversy arising from the use by certain States of drones against non-State armed groups abroad involves claims by certain States that force may be used not only against an organized armed group in a situation that meets the above requirements but also against its co-belligerents (or ‘affiliates’ or ‘associates’).Footnote ⁷⁴ Co-belligerency is a concept that applies in international armed conflicts and entails that a sovereign State becomes a party to a conflict either through formal or informal processes.Footnote ⁷⁵ A treaty of alliance may be concluded as a formal process, while the informal process could involve providing assistance to or establishing a ‘common cause’ with belligerent forces.Footnote ⁷⁶

Transposing the concept of co-belligerency into non-international armed conflicts has been met with resistance because it ignores the significant differences between the two types of armed conflict and opens the door for an expansion of targeting without clear limits.Footnote ⁷⁷ Where the individuals targeted are not part of the same command and control structures as the organized armed group or are not part of a single military hierarchical structure, they ought not to be regarded as part of the same group, even if there are close ties between the different groups.Footnote ⁷⁸ As a result, one cannot aggregate the violence between the State and these different armed groups as part of the determination as to whether the threshold for non-international armed conflicts has been met. Violence by different organized armed groups against the same State can amount to non-international armed conflicts only where the intensity of violence between each group and the State individually crosses the intensity threshold.

To illustrate the points made above, one may make reference to the possibility of a single, global non-international armed conflict with al-Qaeda, for which it was stated that ‘[t]here is little evidence that the various terrorist groups that call themselves AQ or associate themselves with AQ possess the kind of integrated command structure that would justify considering them a single party involved in a global NIAC with the US … Indeed, even the US government rejects the idea that AQ is a unified organization, dividing AQ into three separate tiers.’Footnote ⁷⁹ The view of the International Committee of the Red Cross (ICRC) was that, based on the facts, this type of NIAC was not and has not been taking place. Instead, a case-by-case approach to legally analysing and classifying the various situations of violence that have occurred in the fight against terrorism should be applied. Seen in this light, some of these situations may be classified as an international armed conflict, others as NIACs, while the rest may be outside any armed conflict.Footnote ⁸⁰

A final, important point must be emphasized here, in keeping with the general theme of this article that each applicable legal regime must be addressed separately to determine the legality of a particular drone strike under international law. Even if the concept of a global non-international armed conflict is accepted, IHL does not, on its own, determine whether armed force can be used in foreign States. IHL simply governs how force may be used and does not regulate whether or when armed force may be used. It is the ius ad bellum which determines whether force may be used on foreign territory and which restrains the extraterritorial use of force. Thus, accepting the possibility of a global non-international armed conflict between a State and a non-State group does not in itself give permission to States to use force on foreign territory. Whether a State can carry the fight to a(nother) foreign territory will depend on the law relating to the use of force. The State seeking to carry out a drone attack in a particular foreign State will need to show that there is a sufficient, and indeed, separate justification under the law relating to the use of force for any attacks on each individual foreign State territory. In particular, the necessity of using force in self-defence on the territory of that particular State will need to be assessed separately.

B. Who May Be Targeted under the Law of Non-International Armed Conflict?

Once it has been established that a non-international armed conflict exists, and thus that the rules of IHL applicable in such situations apply in the specific case, the next question concerns who may be targeted. The law of non-international armed conflict is less settled in this area than the law of international armed conflict. It is clear that, as a matter of both treaty and customary law, civilians may not be made the object of an attack unless, and for such time as, they take a direct part in hostilities (DPH).Footnote ⁸¹ This is the principle of distinction and constitutes one of the cardinal principles of targeting under IHL. Where there is doubt as to whether a person is a civilian or is taking a direct part in hostilities, civilian status must be presumed.Footnote ⁸²

The treaty rules applicable in non-international armed conflict, however, do not define ‘civilian’, and there have been debates about whether members of non-State armed groups may be targeted on the basis of their status alone (eg as combatants can in international armed conflicts). In its Interpretive Guidance on Direct Participation in Hostilities, the ICRC has taken the view that ‘civilians’ protected from direct attack in a NIAC are all those who are neither members of a State's armed forces nor members of organized armed groups. The latter are then defined as ‘individuals whose continuous function it is to take a direct part in hostilities (‘‘continuous combat function’’ or CCF)’.Footnote ⁸³ They may be targeted based on their status alone.

Thus, where a drone strike is carried out against an individual with a continuous combat function in an organized armed group with which the attacking State is engaged in a NIAC, this will be consistent with the principle of distinction in IHL, though of course the other applicable rules of IHL also must be observed. However, ‘recruiters, trainers, financiers and propagandists may continuously contribute to the general war effort of a non-State party, but they are not members of an organized armed group belonging to that party unless their function additionally includes activities amounting to direct participation in hostilities.’Footnote ⁸⁴ Such persons, therefore, remain civilians, protected against attack unless they directly participate in hostilities.

In addition to targeting on the basis of status, individuals may be targeted as result of conduct they have undertaken. Civilians lose their protection from direct attack where, and for such time as, they take a direct part in hostilities. According to the ICRC there is a three-stage (cumulative) test for determining when a civilian is directly participating in hostilities and thus may be targeted.Footnote ⁸⁵ In order for activity to qualify as direct participation in hostilities:

1. 1. The act must be likely to adversely affect the military operations or military capacity of a party to an armed conflict or, alternatively, to inflict death, injury, or destruction on persons or objects protected against direct attack (threshold of harm); 2. There must be a direct causal link between the act and the harm likely to result either from that act, or from a coordinated military operation of which that act constitutes an integral part (direct causation); and 3. The act must be specifically designed to directly cause the required threshold of harm in support of a party to the conflict and to the detriment of another (belligerent nexus).Footnote ⁸⁶

The most important criterion with respect to direct participation in hostilities is the requirement that the civilian either carries out acts that directly cause harm or engages in an operation that directly causes harm.Footnote ⁸⁷ The ICRC has defined this requirement narrowly, stating that ‘direct causation should be understood as meaning that the harm in question must be brought about in one causal step’.Footnote ⁸⁸ General production and transport of weapons, or recruiting and training of personnel, is considered too indirect, unless forming an integral part of a specific military operation.Footnote ⁸⁹

The ICRC also emphasizes the rule that civilians lose their immunity from attack only for so long as they directly participate in hostilities, stating that ‘[c]ivilians lose protection against direct attack for the duration of each specific act amounting to direct participation in hostilities’.Footnote ⁹⁰ However, the ICRC Guidance also states that ‘[m]easures preparatory to the execution of a specific act of direct participation in hostilities, as well as the deployment to and the return from the location of its execution, constitute an integral part of that act’.Footnote ⁹¹ As such, a civilian will be participating directly in hostilities both during the commission of the act and for a certain period before and after the act is carried out.

The ICRC's concept of ‘continuous combat function’ may be criticized because of its lack of an authoritative basis in treaty law. However, it has the advantage of answering the question of who is a legitimate target by referring to activity that directly causes harm to belligerents and/or civilians. This provides some objective basis for determining who may be targeted. It is noteworthy that the ICRC's approach to the concepts of ‘members of organised armed groups’ and ‘direct participation in hostilities’ has been followed in recent State practice concerning drone attacks.Footnote ⁹² Indeed, many would accept that members of an organized non-State armed group that is party to a non-international armed conflict are not to be regarded as civilians and are therefore not immune from attack.

However, the ICRC's ‘continuous combat function’ test has also been criticized for not going far enough,Footnote ⁹³ in the sense that it provides advantages to non-State groups as compared to State forces and thus promotes inequality between belligerents.Footnote ⁹⁴ There is an imbalance under the combined combat function approach since members of the State's armed forces without a combat role (with limited exceptions) are lawful targets, whereas their counterparts in non-State groups are not. Furthermore, since the continuous combat function is defined by reference to taking a direct part in hostilities, which is then itself narrowly defined, persons who join an organized armed group and perform military functions that do not constitute direct battlefield activity are exempt from targeting on the basis of status. It has therefore been suggested that an analogy should be drawn between members of regular armed forces and members of organized armed groups such that persons who perform the types of functions performed by members of the armed forces would not be considered a civilian. Although superficially attractive as it promotes equality of arms, the problem with this latter approach is that the functions performed by members of the regular armed forces are very broad indeed. Since non-State groups do not necessarily have formal indicia of membership, one would still need some criteria for identifying the membership of an organized armed group.

One of the critical questions with regard to targeted killing in any armed conflict is how to ensure that the persons being targeted are indeed legitimate targets. A loose test of membership of a group that cannot be applied on the basis of objective evidence is likely to lead to greater civilian casualties or at least a lack of public confidence in the operations. In all cases where a State uses lethal force against individuals in a non-international armed conflict there must be objective evidence to support the claim of membership of an armed group, or alternatively that the persons in question are taking a direct part in hostilities at that point in time. Furthermore, as noted above, where there is doubt as to whether a person is a civilian or is taking a direct part in hostilities, the person is to be presumed as protected from direct attack.Footnote ⁹⁵

Moreover, all feasible precautions must be taken in determining whether a person is a civilian, and, if so, whether the person is taking a direct part in hostilities.Footnote ⁹⁶ This obligation requires parties to the conflict to use all information that is reasonably available in making the determination about whether a person is a lawful target. Where drones are used for targeting it could be argued that there is a greater degree of responsibility to take precautions than might otherwise be the case. This is because the technology and the way in which it is used in many cases make long-term surveillance possible. This means that more information is available about targets and more information can be made available than might otherwise be possible. Therefore, the assessment as to what information is reasonably available should take into account the relative ease with which information can be acquired.

It will not always be the case that evidence that a person is a member of an armed group or is directly participating in hostilities arises from information known about or which reveals the identity of the individual. In some cases, evidence about membership in an armed group or direct participation will arise from the characteristics associated with the individual or the activity that the individual is known to have performed or is performing. For example, where a non-State group has a particular uniform or insignia, evidence of membership may be derived from the wearing of that uniform or insignia. Strikes based on these ‘signatures’ or defining characteristics of an individual are not per se unlawful. What is crucial is whether the signatures or characteristics that are being used in making targeting decisions are the relevant characteristics as a matter of law.Footnote ⁹⁷ The characteristics must be relevant to a determination that a person is a member of an armed group or that the person is taking a direct part in hostilities. The information must be such as to provide a high degree of confidence that the legal tests have been satisfied.

A determination that individuals are military-age males in an area of known terrorist activity does not provide a high degree of confidence that the individual is either a member of an armed group or that the person is taking a direct part in hostilities at that particular time. Indeed, mere presence in a given locality can never, in itself, amount to direct participation in hostilities. Some specific act would need to be engaged in for the person to be considered to be taking a part in hostilities. Furthermore, the fact that a person is armed, even in areas that are known to contain terrorists, is not sufficient to constitute evidence of loss of civilian status or direct participation in hostilities.Footnote ⁹⁸ There are many parts of the world where it is lawful and/or common for individuals to carry personal arms. Carrying of such arms will not be sufficient to displace the presumption of civilian status. The public statements of States that they conduct threat assessments of individuals before targeting them in armed conflict should be welcomed and should be urged, as this helps to raise the floor of protection with respect to legitimate targets.Footnote ⁹⁹ Of course, the situation must be correctly classified as an international or, as is more likely, a non-international armed conflict; if the requirements posed for a NIAC are not met, a loose threat assessment is not enough, and the more rigorous conditions of self-defence under IHRL would then exclusively regulate who can be targeted.

Finally, even in cases where lethal force is targeted at a member of an organized armed group or a civilian directly participating in hostilities, any such strike would need to comply with the principle of proportionality, the second cardinal principle of targeting in IHL. Drones come from the sky, but they leave the heavy footprint of war on the communities they target.Footnote ¹⁰⁰ According to this principle, it is prohibited to carry out ‘an attack which may be expected to cause incidental loss of civilian life, injury to civilians, damage to civilian objects, or a combination thereof, which would be excessive in relation to the concrete and direct military advantage anticipated’.Footnote ¹⁰¹ By implication, where the use of force is not excessive, such losses are regarded as incidental damage and are not prohibited provided the IHL rules have been respected.

The risk to civilians may be exacerbated where drone strikes are carried out far away from areas of actual combat operations, especially in densely populated areas, where unsuspecting civilians may suddenly find themselves in the line of fire. Avoiding harm to civilians requires taking all feasible precautions to prevent or minimize incidental loss of civilian lives and to gather information relating to possible civilian casualties and military advantages that might result from the proposed operation.Footnote ¹⁰²

C. The Possible Existence of an International Armed Conflict

As noted above, where there is a conflict between a State and a non-State group, it may qualify as a non-international armed conflict (should the conditions for a NIAC be met). Drone strikes may also take place in circumstances where there is no NIAC between the State and the non-State group, because the criteria for the existence of a NIAC are not met. However, the absence of a non-international armed conflict between the State and the non-State armed group does not necessarily mean that IHL will not apply to the use of force by a State. This is because the use of lethal force by a State directed at an armed group may still be governed by the law relating to international armed conflicts.

As noted earlier, international armed conflicts are limited to conflicts between States. When a State carries out drone strikes on the territory of another State, it will either do so with the consent of the territorial State or without that consent. Where the force is used with the consent of the territorial State, there can only be a non-international armed conflict between the State and the group against whom force is used (or indeed no armed conflict at all). However, where force is used without the consent of the territorial State, one of the present authors has argued elsewhere (as have others and the ICRC) that there would be an international armed conflict between the State using force and the State on whose territory force is used.Footnote ¹⁰³ This would be an international armed conflict between the foreign State and the territorial State. This could be the case because the use of force by the intervening foreign State on the territory of the territorial State, without the consent of the latter, is a use of force against the territorial State. This, so the argument goes, is so even if the use of force is not directed against the governmental structures of the territorial State, or if the purpose of the use of force is not to coerce the territorial State in any particular way.

However, even on the view that there is an international armed conflict between the two States, such a conflict would exist in parallel with the non-international armed conflict between the State and the non-State armed group that is being targeted, if the criteria for a NIAC were met. The two conflicts would thus exist simultaneously. With regard to targeting of the members of the non-State group, it would be the criteria discussed above from the law of non-international armed conflict that would be determinative of the legality of the targeting of such members.Footnote ¹⁰⁴ This is so because, as seen from the Tadić criteria discussed above,Footnote ¹⁰⁵ the hostilities between the State and non-State group constitute the essence of a non-international armed conflict. Failure to apply the law of non-international armed conflict to targeting of members of the group would in effect be a failure to recognize the non-international armed conflict.

In cases where drone strikes occur outside the context of non-international armed conflicts and without the consent of the territorial State, on this view the individuals targeted would necessarily be classified as civilians, given that they would not constitute combatants in the international armed conflict between the two States.Footnote ¹⁰⁶ IHL prescribes that such persons would be protected from direct attack,Footnote ¹⁰⁷ unless and for such time as they take a direct part in hostilities.Footnote ¹⁰⁸ Drones strikes against them would, therefore, on this view only be lawful so long as they are directly participating in hostilities (on which, see above).

Finally, the fact that drone strikes would, according to this approach, take place in the context of an international armed conflict means that additional rules of international law to those discussed thus far would apply. For example, in addition to all the IHL obligations of the State using lethal force applying in such a situation, the applicable principles of international criminal law with respect to war crimes in an international armed conflict will also be relevant in determining individual responsibility for particular strikes. This conception of the relevant legal framework under IHL where drones are used abroad without the territorial State's consent thus has important implications both for substantive primary rules and for secondary rules on the responsibility of the actors involved.

D. Does IHL Require a Capture Rather Than Kill Approach?

Recent debates have raised the issue of whether IHL requires that a party to an armed conflict, under certain circumstances, considers the capture of an otherwise lawful target (i.e. a combatant, a member of an armed group with a CCF or a civilian directly participating in hostilities) rather than targeting with force. The ICRC has taken the view that it does, and in its Interpretive Guidance, it states that ‘it would defy basic notions of humanity to kill an adversary or to refrain from giving him or her an opportunity to surrender where there manifestly is no necessity for the use of lethal force.’Footnote ¹⁰⁹

The articulation of this principle has been controversial.Footnote ¹¹⁰ It has been criticized for its alleged misrepresentation of the current lex lata,Footnote ¹¹¹ particularly on the basis that it suggests that the principle of military necessity sits above every rule of IHL in a limiting manner, rather than simply as a consideration that has already been factored into the rules.Footnote ¹¹² In other words, so the argument goes, States have already decided that it is necessary and proportionate to target combatants on the basis of their status, and in some cases, conduct alone.

It is too early to determine in which direction the controversy around this concept will be resolved. The impulse to move in the direction of the approach that in some cases capture rather than kill is required will likely continue in the context of modern anti-terrorism measures where individuals or small groups may be isolated in territory far away from the conflict zone, which may even be controlled by the State party or its allies.Footnote ¹¹³ At least one State that uses drones has stated that, as a matter of policy, it will not use lethal force when it is feasible to capture a terror suspect.Footnote ¹¹⁴ At the same time it should be remembered that drones by their very nature will in most cases not allow capture.

E. Investigation and Accountability for Violations

The modern concept of human rights is based on the fundamental principle that those responsible for violations must be held to account. A failure to investigate and where applicable punish those responsible for violations of the right to life in itself is increasingly understood to constitute a violation of that right.Footnote ¹¹⁵ Legal, and indeed political, accountability are dependent on public access to the relevant information.Footnote ¹¹⁶ Only on the basis of such information can effective oversight and enforcement take place. The first step towards securing human rights in this context is transparency about the use of drones. A lack of appropriate transparency and accountability concerning the deployment of drones undermines the rule of law and may threaten international security.Footnote ¹¹⁷ Accountability for violations of IHRL (or IHL) ‘is not a matter of choice or policy; it is a duty under domestic and international law’.Footnote ¹¹⁸

The various components of transparencyFootnote ¹¹⁹ require that the criteria for targeting and the authority which approves killings be known, and drone operations be brought within institutions which are able to disclose to the public the methods and findings of their intelligence, criteria used in selection of targets and precautions incorporated in such criteria. One of the criticisms levelled against the current drone programmes has been the absence of an official record regarding the persons killed. States must also give guarantees of non-repetition of unlawful drone strikes and realize the right of victims to reparations.

Drone victims, like any other human rights victims, and the society at large, have a right to access information related to allegations of human rights violations and their investigations.Footnote ¹²⁰ Under IHRL, the UN Human Rights Committee has emphasized the need for transparency, highlighting victims’ right to know the truth about the perpetrators, their accomplices and the motives thereof.Footnote ¹²¹ Likewise, during an armed conflict, relatives of persons killed or missing have the right to know the fate of their relatives.Footnote ¹²²

A parallel obligation to investigate and where appropriate punish those responsible in respect of cases of alleged war crimes exists under IHL.Footnote ¹²³ Whenever there are reasons to query whether violations of IHL may have occurred in armed conflict as a result of a drone strike, such as the incorrect designation of persons as targetable, or disproportionate civilian harm, full adherence to accountability demands at least a preliminary investigation.Footnote ¹²⁴ Civilian casualties must be determined and should be disclosed.

A. General Considerations

The final substantive area that must be considered in assessing the legality of the use of armed drones under international law is international human rights law (IHRL). It is under IHRL that the right to life is most clearly protected, as set out in the various international and regional human rights treaties,Footnote ¹²⁵ and the rules of customary international law.Footnote ¹²⁶ The right to life is sometimes described as the ‘supreme right’.Footnote ¹²⁷ Indeed, certain violations of the right to life are considered to be war crimes or crimes against humanity.Footnote ¹²⁸ As discussed in the next subsection, IHRL applies not only in peacetime but also in armed conflict, both international and non-international. As such, the right to life under human rights law forms the default legal norm applicable to the protection of people affected by drone strikes.

As a general rule, international human rights law requires that any deprivation of life must be non-arbitrary.Footnote ¹²⁹ The non-arbitrariness standard defining the scope of the right to life has generally been interpreted in human rights jurisprudence to require that intentional lethal force be used only as a last resort in order to protect life.Footnote ¹³⁰ Thus, Principle 9 of the UN Basic Principles on the Use of Force and Firearms by Law Enforcement Officials provides that ‘intentional lethal use of firearms may only be made when strictly unavoidable in order to protect life’. Indeed, Article 2 of the European Convention on Human Rights (ECHR) contains, rather than a general prohibition of arbitrary deprivation of life, an exhaustive list of grounds on which it is permissible to use lethal force.Footnote ¹³¹

Under IHRL, any force must be necessary, that is, the least harmful means of achieving the particular legitimate purpose of protecting life. Whether that is the case is a factual assessment. Other avenues should be explored first and only when they are shown to be inadequate can there be resort to force.Footnote ¹³² Force must also be proportionate (that is, the potential harm in using force does not outweigh the legitimate protective goal pursued). This entails making a value judgement. It is important to note that proportionality in this human rights law sense is a different test to the principle of proportionality in IHL (on which, see above). Moreover, intentionally lethal or potentially lethal force can only be used where strictly necessary to protect against an imminent threat to life: ‘[T]he police may shoot to kill only when it is clear that an individual is about to kill someone (making lethal force proportionate) and there is no other available means of detaining the suspect (making lethal force necessary).’Footnote ¹³³

Proper precautions must also be taken to prevent, where possible, the resort to force. In McCann and others v UK, the European Court of Human Rights (ECtHR) held that the killing of members of the Provisional Irish Republican Army by State agents was a violation of their right to life as they could have been arrested on their arrival in Gibraltar, where the operation took place.Footnote ¹³⁴

Whether a particular drone strike satisfies these conditions must be examined on a case-by-case basis. It is important to emphasize, however, that, under these rules, alternatives to lethal force would need to be explored and shown to be inadequate for a drone strike to be lawful. In addition, it would need to be concluded that the use of lethal force is necessary on the basis that the person to be targeted constitutes an imminent threat to others.Footnote ¹³⁵ It is here that conflation of different legal standards across different regimes often poses problems. It is important not to confuse this imminence requirement in IHRL with similar conditions under the ius ad bellum. As discussed above, according to the doctrine of anticipatory self-defence, a State can use force against another State where there is an imminent threat of armed attack emanating from the latter State. This, however, is a different legal threshold to the imminence requirement in IHRL. Whether a particular transnational drone strike is lawful would depend on demonstrating imminence in both of these senses in cases where the attacking State is claiming a right of anticipatory self-defence.Footnote ¹³⁶

B. Application of Human Rights in Armed Conflict

It is now well established that IHRL continues to apply in time of armed conflict, alongside IHL.Footnote ¹³⁷ This has been confirmed in a number of decisions of the International Court of Justice (ICJ).Footnote ¹³⁸ Indeed, the general applicability of human rights obligations in armed conflict is confirmed by the presence of provisions relating to derogations in certain human rights treaties. Those provisions permit State parties to derogate in time of war or public emergency from some of their human rights obligations arising under those treaties.Footnote ¹³⁹ Such provisions confirm that, absent derogation, human rights obligations continue to apply in time of war or armed conflict. Thus, a State's conventional human rights obligations will only cease where that State validly suspends the rights in accordance with the treaty's derogation provisions. However, with the exception of the ECHR, the right to life is regarded as a non-derogable right under most major human rights treaties.

Where a particular drone strike falls within the context of an armed conflict, therefore, it will be governed by both IHL and IHRL. In certain cases, the right to life standard in IHRL might be interpreted in accordance with the relevant rules on the conduct of hostilities under IHL, explored above. Thus, the ICJ has stated that, while the right not arbitrarily to be deprived of one's life continues to apply in situations of armed conflict, what is an arbitrary deprivation of life under the ICCPR depends on the circumstances at hand and, in armed conflict, should be considered by reference to the IHL rules on the conduct of hostilities.Footnote ¹⁴⁰ According to this view, where a State targets an individual in a drone strike that falls within an armed conflict, whether that State has violated its obligation not arbitrarily to deprive that individual of their life depends on whether the State has acted consistently with those IHL rules discussed above.

However, the European Convention on Human Rights, as noted, does not express the right to life by reference to arbitrary deprivation of life. Rather, the ECHR provides for a closed list of permissible bases for depriving a person of their life.Footnote ¹⁴¹ The extent to which one can read into this provision the IHL rules on the conduct of hostilities is, consequently, more limited than in the case of the arbitrariness standard found in the ICCPR and ACHR.Footnote ¹⁴² However, it is important to note that the European Court of Human Rights has interpreted the right not to be deprived of one's liberty under Article 5 of the European Convention by reference to the IHL rules on detention in international armed conflicts.Footnote ¹⁴³ This was done despite the fact that the relevant provision, like the provision on the right to life, is not expressed by reference to arbitrary deprivation, but rather a closed list of permissible bases for deprivation of liberty.Footnote ¹⁴⁴ A similar interpretation of the right to life would allow the European Court to read the IHL rules on the conduct of hostilities into the right to life under the European Convention. In any case, Article 15(2) ECHR permits derogation from this provision ‘in respect of deaths resulting from lawful acts of war’. This provision should, therefore, be used where States consider it necessary to resort to more permissive rules of IHL.

Where a drone strike is conducted outside the context of an international or non-international armed conflict, IHL will not apply; in such situations, there can be no claim that a State's human rights obligations must be interpreted in accordance with IHL. Instead, those obligations will apply according to their ordinary meaning as developed in human rights jurisprudence. That said, the arbitrariness standard remains context-specific and indeed has been applied in ways that are realistic to the context.Footnote ¹⁴⁵ There are, however, limits to this. Importantly for our current enquiry, the view that mere past involvement in planning attacks is sufficient to render an individual targetable even where there is no evidence of a specific and immediate attack, distorts the requirements established in IHRL.Footnote ¹⁴⁶

The above discussion relates to the potential responsibility of the State using the drone. But what about the State on whose territory it is used? It should be remembered, as noted at the outset of this article, that States cannot consent to the violation of their IHL or IHRL obligations. A State that consents to the activities of another State on its territory remains bound by its own human rights obligations, including the obligations not actively to violate the rights of those within its jurisdiction itself, and also to ensure respect for human rights within its jurisdiction and thus to prevent violations of the right to life, including by other States operating on its territory, to the extent that it is able to do so.Footnote ¹⁴⁷

C. Extraterritorial Application of the Right to Life

The fact that drones have been used with lethal effects by States against individuals outside their territory raises the additional question of whether States can be held accountable, under those human rights treaties to which they are party, for their extraterritorial actions. Reference was made earlier to the status of the right to life, and the prohibition of the arbitrary deprivation thereof, as a norm of customary international law. In its customary form, at least the negative obligation not arbitrarily to deprive someone of their life appears not to be limited to application within a State's territory. Indeed, the Universal Declaration of Human Rights does not contain a limitation clause on its geographical application and simply states that ‘[e]veryone has the right to life’.Footnote ¹⁴⁸

In addition, in certain circumstances, States are bound by those human rights treaties to which they are a party even in relation to conduct they take outside their territory. The fact that human rights treaty obligations can apply in principle to the conduct of a State outside its territory has been confirmed, inter alia, by the International Court of Justice,Footnote ¹⁴⁹ the UN Human Rights Committee,Footnote ¹⁵⁰ the Inter-American Commission on Human RightsFootnote ¹⁵¹ and the European Court of Human Rights.Footnote ¹⁵² The applicability of such treaties is normally limited to those individuals within the ‘jurisdiction’ of a State party.Footnote ¹⁵³ It is clear that all persons finding themselves within the territory of a State are presumed to be within its territorial jurisdiction.Footnote ¹⁵⁴ Moreover, it is clear from the case law that, where a State is exercising effective control over a part of another State's territory, the former will be exercising ‘jurisdiction’ such that its obligations under those human rights treaties to which it is a party will apply.Footnote ¹⁵⁵ Jurisdiction for the purposes of extraterritorial application of human rights treaties will additionally exist where, absent such territorial control, the State exercises, through its agents, ‘authority or control’ over the specific individual abroad.Footnote ¹⁵⁶

One of the key difficulties posed by drones is that the attacking State can engage in targeted killing without exercising effective control over territory or without having the individual in custody, leading to questions over whether such persons fall within the ‘jurisdiction’ of the attacking State for the purposes of their human rights treaty obligations. There is limited case law on this matter. In the Alejandre case, the Inter-American Commission concluded that the shooting down of two private US registered air planes by Cuban military aircraft in international airspace violated the right to life of the passengers.Footnote ¹⁵⁷ At the same time, however, in Bankovic, the European Court held that persons killed during aerial bombings by NATO of a radio station in Serbia did not fall within the ‘jurisdiction’ of the participating States for the purposes of establishing whether they have violated the right to life.Footnote ¹⁵⁸ The broad sweep of this decision has, however, increasingly been narrowed in subsequent cases of the Court,Footnote ¹⁵⁹ and it is not clear that the position can be sustained. Indeed, the English High Court has held that ‘whenever and wherever a state which is a contracting party to the [European] Convention [of Human Rights] purports to exercise legal authority or uses physical force, it must do so in a way that does not violate Convention rights’.Footnote ¹⁶⁰

It has been argued that the implication of this broader view of when an individual is within the jurisdiction of a State for the purposes of human rights treaties is that the deliberate killing of selected individuals through extraterritorial drone strikes is likely to bring the affected persons within the jurisdiction of the operating State.Footnote ¹⁶¹ Pursuing this line of reasoning, where a State targets individuals abroad with lethal force, one can argue that it intends to exercise ultimate control over the individuals concerned, resulting in those actions being governed by the State's human rights treaty law obligations.Footnote ¹⁶² Marko Milanovic is similarly of the view that the human rights treaty obligations of a State would apply to the deliberate killing of an individual through extraterritorial drone strikes.Footnote ¹⁶³ However, Milanovic's reasoning is that the negative obligations owed by States under IHRL, eg the obligation not arbitrarily to kill, should extend unbound by territorial constraints. This argument is based on the view that, in treaties that contain a jurisdiction clause, the negative obligation to respect rights is to be treated differently from the obligation to ensure those rights and that texts can and should be interpreted such that only the latter is confined to matters within the ‘jurisdiction’ of the State as that term has come to be interpreted. This appears to be a normatively desirable and principled basis for holding States to account in such situations, and it is in line with the position as set out above under customary international law. Importantly, to say that a State has human rights obligations with respect to conduct outside its territory does not, therefore, automatically mean that those obligations are the same as those that arise within its territory.Footnote ¹⁶⁴ In principle, while control of territory means that a State has obligations not only to respect but also to ensure and to fulfil the human rights of those on the territory guaranteed by international law, the exercise of authority with respect to an individual by State agents in the absence of territorial control at a minimum triggers the State's obligation to respect the rights of those individuals.Footnote ¹⁶⁵

It has been held that human rights treaties ‘cannot be interpreted so as to allow a State party to perpetrate violations of [the treaty] on the territory of another State, which it could not perpetrate on its own territory’.Footnote ¹⁶⁶ The same must apply to the right to life under both custom and treaty law. In consequence, any positive action by a State, on its own territory or that of another State, should be carried out in compliance with its human rights obligations under both treaty and custom.