Much of the public commentary concerning the UK’s targeted strike in Syria against a British national who had joined ISIS (along with other individuals with him at the time) began with the false assumption that the UK was relying on the US legal rationale in carrying out the drone strike. The commentary overlooked what is unprecedented and extraordinary in the UK’s claims and obscured the fundamental question whether the UK’s new rationale for justifying a military strike targeting one of its nationals (or anyone else) is consistent with international law.

The Aug. 21 strike that killed Britons Reyaad Khan and Rahul Amin marked the first time since 9/11 that a government has claimed the right to deliberately kill a suspected terrorist outside an armed conflict in self defense and then done so. Prime Minister Cameron explained that the strike was carried out under a claim of self-defense under Article 51 of the UN Charter, which permits states to use force in another state in self-defense in response to an “armed attack.” The Charter requires a state to inform the Security Council of any such actions and the UK has now sent a letter doing that. Cameron did not claim that the targeted strike was undertaken as part of an armed conflict; as Noam Lubell notes, Cameron implied just the opposite. While the UK letter to the Security Council leaves open the possibility that the UK may still make a claim that the strike was part of an armed conflict, unless and until it does so, the UK strike raises new questions about the legality of such targeted killings outside the context of an armed conflict.

The extraordinary nature of the UK’s action was initially obscured by inaccurate comparisons between this drone strike and US drone strikes. The US claims that it carries out strikes against belligerents as part of an armed conflict between the US and al-Qaeda and its affiliated forces (including ISIS). At least since President Obama took office, the US has not carried out any drone strikes in any instance where it does not claim that it is doing so as part of an armed conflict. For example, the Justice Department OLC’s opinion approving the targeted killing of US citizen Anwar al-Aulaqi is tied to its conclusion that the operation would “constitute the ‘lawful conduct’ of war,” that al-Aulaqi had become part of the forces of an enemy with which the United States is engaged in an armed conflict, and that al-Aulaqi thus was a lawful target under the law of armed conflict. While many objections have been raised to the way in which the US applies the legal proposition that a State is entitled to target and kill members of an enemy force in an armed conflict, the legal proposition itself is not subject to serious challenge. These objections include the arguments that: there is no armed conflict with the US; even if there is an armed conflict, the drone strikes are outside the scope of that conflict; and US strikes are not in fact carried out consistent with law of armed conflict requirements. These objections to the US strikes, whatever their merits in a particular case, are irrelevant to the UK’s claim that it has the legal right under Article 51 to target and kill a British national with ISIS in Syria without regard to the existence of any armed conflict.

Some commentators, in particular Lubell, have now recognized important differences between these legal justifications and begun to tease apart various questions not yet answered by the UK government concerning its legal theory or the facts supporting its action.

Some issues raised by the UK Article 51 legal theory are less controversial than others. The US and other states understand customary international law to include the right to use military force in self-defense against armed attacks, and claim the right to use military force under Article 51 outside of an armed conflict. As Lubell has noted, there is support for reading Article 51 as justifying the use of military force against non-state actors. There is broad consensus that there is a right to use military force in self-defense when the host country is unable or unwilling to stop the attack. And there is even support for reading it to allow force in advance to prevent an attack, although there is controversy over how imminent an attack must be before it can justify military force.

But there are additional less-examined questions about relying upon Article 51 as a justification for the targeted killings of individuals outside the context of armed conflict. Article 51 is limited to self-defense against “armed attacks” and there has been little public discussion concerning when an attack being planned by an individual terrorist would rise to the level of an “armed attack” justifying the use of anticipatory military force.

This question is especially salient if the constraints of the Charter in Articles 2(4) and 51 on State uses of force are intended to do more than preserve state sovereignty — if they are also designed as a means of limiting uses of military force and advancing peace more broadly. I am no scholar of the history of the Charter, but certainly it is an important question whether the requirements of Article 51 can be satisfied simply by addressing sovereignty concerns, such as by establishing host-state consent or through a determination of unwillingness or lack of capacity by the target state, and by limiting uses of military force to instances where there is a truly imminent threat. Doesn’t Article 51 in its reference to an “armed attack” require an additional showing that the targeted threat is substantial enough to justify the use of military force (beyond the ad bellum requirement that any force in response to the threat be proportional and necessary)? (For a brief discussion of some of these issues, see this article.)

If the Charter is meant to provide real constraints on the use of military force and give teeth to the concept that States’ use of such force should be limited to instances where it is both proportional and necessary while providing for the right of self-defense, we need much more substantive discussion about when planned terrorist attacks rise to the level justifying military force under Article 51. The UK strike makes it critical to engage this question, in a way in which the US legal claim, based on the claim of an armed conflict, has not.

Of course, the US also has claimed the right to use military force under Article 51 outside an armed conflict, for example against al-Qaeda after its attacks on American embassies in 1998. The justification for those strikes, however, was based on fundamentally different facts than those the UK now relies upon. In particular, and critically, the US used force there only after al-Qaeda had already demonstrated the willingness and capacity to engage in large-scale attacks on the US. By contrast, the UK strike was undertaken before Khan undertook any attacks specifically upon the UK — let alone any large-scale attacks. (The ISIL attack in Tunisia killing many British tourists may complicate this analysis, but the UK has not offered any explanation of any relationship between that attack and its use of force against Khan.)

Nor has there been sufficient discussion and examination of how any use of military force to target and kill individuals planning terrorist acts is consistent with international human rights law. It’s noteworthy that the Justice Department’s OLC opinion carefully cabins its conclusion that the targeted killing of al-Aulaqi was consistent with international law (and thereby permissible under US law), by indicating that using force against members of enemy forces in an armed conflict, in a manner consistent with the law of armed conflict, satisfies human rights obligations (footnote 37). That rationale would not apply to UK strikes outside the context of an armed conflict.

As Lubell suggests, application of law enforcement rules to justify the targeted killings of terrorists overseas is also questionable. On their face, those rules apply to actions by law enforcement officials, not to the use of military force. And there are many significant differences between the two kinds of operations that should give human rights proponents pause before arguing that these rules are somehow appropriate or feasible to justify the use of military force against an individual. For example, while there are law enforcement precedents where a State has gone into another country in order to seize an individual and bring him back for trial, it would be extraordinary for law enforcement to claim the right to go into another country to simply kill someone.

The assumed equivalence between the US and UK legal positions has obscured the critical question whether the UK position represents a dangerous expansion of claimed legal authorities used to date in the military actions against al-Qaeda and ISIS. For while there is much disagreement about the extent (or even existence) of an armed conflict between the US and al-Qaeda and ISIS as a matter of fact, the claim of authority to kill enemy combatants in an armed conflict is not new. Application of the US theory is cabined to instances in which a country claims the existence of an armed conflict. And while the law of armed conflict allows the deliberate killing of enemy combatants, it also requires significant measures to provide humanitarian protections. The UK’s new claim of authority to use military force to kill suspected terrorists including its own nationals apparently outside the context of any armed conflict is crucially different. Not only the source, but the limits of such authority remain unclear.