An Iraqi policeman fires a machine gun at ISIS fighters in Mosul, Iraq. Image: Carl Court/Getty

As ISIS’ self-proclaimed caliphate disintegrates in Iraq and Syria, Western governments face the question of where their citizens who joined ISIS as foreign fighters will now go. Contemplating the likelihood that foreign fighters will return to their countries of origin, officials are calling for their armed forces to kill them on the battlefield abroad before they can make their way home.

Last week, Britain’s new Defence Secretary, Gavin Williamson, provoked an outcry by saying that Britain should “destroy and eliminate” the threat posed by ISIS terrorists rather than allowing them to return to Britain. But the Defence Secretary was far from alone in his statement – in fact, he was merely repeating an earlier call for a similar approach from the International Development Minister, Rory Stewart.

Meanwhile, the French Defence Minister, Florence Parly, has argued that if French jihadists die in Iraq and Syria, “I’d say it’s for the best”, and there have been reports that France gave a “kill list” of French fighters to Iraqi soldiers earlier this year. Finally, US anti-ISIS envoy Brett McGurk said last summer that the coalition’s mission was to make sure that any foreign fighter who had joined ISIS “will die here in Syria.”

These statements are jarring because they highlight the apparent gulf that exists between the ways that governments (especially those in Europe) believe they can treat ISIS followers on foreign battlefields and at home. If they encounter ISIS members in a theater of military operations, Western governments claim the right to target them under the laws of war unless they are hors de combat (no longer in the fight) through being wounded or attempting to surrender. However, if the fighters manage to sneak back to their home countries in Europe, governments must treat them according to the methods of law enforcement.

Self-evidently, that means they cannot be shot unless there is no other way to prevent a truly imminent attack involving loss of life. It also means they cannot be imprisoned without having first been prosecuted, although both France and the United Kingdom have passed laws allowing them to restrict the movement of terrorist suspects without trial in some circumstances. And prosecution is not always easy: in many cases it can be hard to prove that foreign fighters travelled to Iraq and Syria with the aim of participating in terrorism or that they did in fact commit crimes while there. The British government admitted last year that only 54 out of an estimated 350 foreign fighters who had returned to Britain had so far faced charges. In these circumstances, it’s hard to avoid the suspicion that the calls to kill foreign fighters abroad are not rooted so much in essential reasons of national security, as they are in the desire to avoid the pragmatic problems, imposed by a commitment to the rule of law, in dealing with citizens who return from ISIS territory.

Following the Supreme Court’s decision in the Hamdi case, the United States may have the power under domestic law to detain its citizens, whether captured at home or abroad, as enemy combatants without trial. However there are practical reasons to doubt that the government would see this as a plausible long-term option in the case of ISIS members.

But if the apparent policy of targeting foreign fighters to stop them coming home seems morally questionable, is it also unlawful? There are two bodies of law that are relevant. First is international humanitarian law (IHL), which governs the conduct of military operations once they reach a certain level of intensity. According to a widely agreed interpretation, IHL does not prohibit attacks on people who regularly perform the function of a combatant for armed groups like ISIS except when they are hors de combat and thus no longer pose a threat. It also allows attacks against civilians who are taking a direct part in hostilities at the time of attack, even if that isn’t their normal activity or function. Therefore the crucial question under IHL is whether Western forces are distinguishing between those who are actually fighting for ISIS and may be targeted, and those who performed other functions in ISIS’s fledgling pseudo-state and therefore aren’t lawful targets for attack. (Click here for a closer look into this sometimes murky distinction.) There is reason to believe that some foreign volunteers have been assigned non-combatant roles such as prison guards; without further evidence, Western forces should not simply assume that all foreign men who joined ISIS are playing a role in hostilities, even if we loosely refer to them as “foreign fighters.”

The second set of international legal rules that Western armed forces have to consider asks whether their use of force is necessary and proportionate to a lawful purpose. Here the question is not whether force is being used in line with the rules for the conduct of hostilities, as established in IHL, but rather whether force should be used at all. Since Syria hasn’t consented to the coalition’s use of force on Syrian territory, the UN Charter restricts coalition forces to conducting attacks there only as far as is necessary to prevent ISIS’s core group in Syria from orchestrating violence in other countries. That threshold could soon require a cutback or even halt to Western attacks in Syria: the Iraqi prime minister has said his country has essentially defeated ISIS, and it’s hard to believe that the group retains much capacity to plan foreign attacks among its scattered ranks in Syria. In assessing this capacity, we should consider attacks that are actually directed or organized from Syrian territory, not those merely inspired by ISIS’ ideology.

Aside from the UN Charter, human rights law should also be understood as limiting the use of force against individual ISIS fighters, as I argued in a recent article in International Law Studies. That would mean that fighters could be attacked only where it was necessary and proportionate to suppressing a threat to other peoples’ lives or to public order – not simply on the basis that they were members of ISIS’s armed forces. Many of those who have joined ISIS in Iraq and Syria appear to have received only cursory military training and may not have taken part in any fighting. Moreover, research suggests that the factors have led foreigners to join ISIS in defending its territory do not in many cases carry over into a commitment to perpetrate terrorist attacks internationally. There is simply no basis to assume that in all cases, foreign fighters are now so committed to carry out further acts of violence on behalf of ISIS that killing them is a necessary and proportionate response. This is particularly true because in some cases it may be possible for Western or allied forces to capture foreign fighters rather than targeting them in the field.

Government officials’ talk of eliminating ISIS members on the battlefield may simply be a way to sound tough in the face of public concerns about the return of foreign fighters. But it reinforces a dangerous and flawed vision of military action against terrorist organizations that equates armed conflict with a license to kill all members of an opposing group. Such a vision is not compatible with the understanding of the international rule of law that Western nations should be committed to uphold.