(Editor’s Note: This is the first of a two-part series on international legal obligations related to repatriating the thousands of foreign fighters who fought with ISIS in Syria. The first installment considers international law on the question. The second will examine how to strengthen political will to address repatriations.)

The Law of Non-international Armed Conflict is a shadowland – an opaque legal dimension that can pose challenges to those seeking to navigate its terrain. And yet, contemporary armed conflicts are more frequently taking place in circumstances under which this body of law is the relevant legal framework. The uncertainty in this area of law presents obvious challenges, but it also offers opportunities to observe a degree of legal evolution. Shifts can be discerned in State attitudes and behaviors that signal development of what international law will become (lex ferenda).

In this new and dynamic security environment, States will increasingly confront novel and complex situations beyond the ambit of settled international law (lex lata), but careful observation of State practice and opinio juris – the constituent elements of customary international law – can help dispel the juridical darkness and provide legal signposts to guide operational activity. An example of this phenomenon can be witnessed in the international community’s struggles with the ongoing problems associated with large numbers of foreign terrorist fighters detained by non-state actors in Syria.

U.S. Marine Corps General Kenneth F. McKenzie Jr., commander of United States Central Command (CENTCOM), testified before the House Armed Services Committee on March 10 and, like many U.S. officials before him, emphasized the problems associated with the ongoing detention of more than 2,000 foreign fighters of the Islamic State of Iraq and Syria (“ISIS”) by the non-state armed group the Syrian Democratic Forces (SDF). In his testimony, McKenzie said:

Also concerning are near- and long-term implications of SDF detention facilities in Syria and the disposition of foreign-terrorist fighters (FTFs). While CENTCOM and our coalition partners are working to address and mitigate security challenges at the facilities, this serves only as a tactical-level band-aid, not a long-term solution. The United States can mitigate the risks associated with these populations by facilitating repatriations, training and equipping guard forces, and providing the funding required to improve prison infrastructure. Ultimately, the best way to alleviate this problem is to reduce the numbers of detainees through repatriation. The ISIS detainee and IDP populations represent more than 60 nations. While some countries have made efforts to reclaim their foreign fighters, full resolution requires a comprehensive diplomatic and international effort. This problem will not go away by ignoring it, and can only be addressed by the international community working together to accept its shared responsibilities.

The General’s testimony is merely the latest addition to the cacophony of official exhortations highlighting the “detention conundrum” created by the modern reliance of States on non-state actors for counterterrorism operations, and the simultaneous reluctance of many States to accept the return of their nationals who have been captured and detained by those partner groups in the course of those operations. McKenzie correctly noted that “[u]ltimately, the best way to alleviate this problem is to reduce the numbers of detainees through repatriation.” Indeed, SDF partners have signaled that they do not have the capacity or authority to continually detain these foreign terrorist fighters. The risk is the release of dangerous terrorists.

So what are the “shared responsibilities” of the international community to which McKenzie referred? Is this mere rhetoric or are actual international legal obligations being ignored by the countries where the foreign fighters originated? In a recent article in the Harvard National Security Journal, I argue the latter, and seek to illuminate the international and comparative legal issues associated with the detention of terrorists by non-state armed groups and to clarify the legal issues relating to the repatriation of detained foreign terrorist fighters by the SDF in Syria. This post will elaborate further on certain points, specifically the degree to which international law requires States to repatriate their foreign fighters and steps that can create more favorable environments for the repatriation of such high-risk individuals.

Can Non-State Armed Groups Engage in Battlefield Detention under International Law?

A threshold legal issue to be confronted when exploring issues related to the detention of foreign terrorist fighters is whether the SDF (as a non-state group) has any authority to detain under international law. This issue is far from settled, with some legal scholars arguing that non-state armed groups have no international legal authority to detain during the course of a non-international armed conflict, and others conversely positing that international law does provide such authority or at least assumes such authority and focuses instead on the imperative of regulating detainee treatment.

Recent state practice, however, may provide an inflection point on this question. This is because the United States and members of the Global Coalition to Defeat ISIS — a coalition that is now 74 nations strong — have helped support SDF-run detention centers in Syria. Rather than generating disapproval, this facilitation of non-state detention has generally been viewed as a necessary and beneficial contribution to the mission to defeat ISIS. Notably, the Ministers of the Global Coalition To Defeat ISIS issued a joint public statement in February 2019 declaring, “For those detained foreign terrorist fighters that remain in the region, the Coalition should seek to enable their continued secure, fair, and humane detention.”

One can therefore see that a striking degree of international consensus is forming in favor of the permissibility of non-state detention of terrorist subjects in the course of a non-international armed conflict. The development of lex ferenda favoring an evolution toward more permissible action would be consistent with the overall evolution of international law, which has slowly but perceptibly begun to fill gaps in ways that more effectively permit counterterrorism operations. The legality of the detention is highly relevant in the context of repatriations, in which States may be loathe to accept custody of an individual—even a national—whose initial detention from the outset may have lacked legal authorization.

Is There a Duty to Repatriate Detained Foreign Terrorist Fighters?

This brings us to the second area of interest related to McKenzie’s exhortation: the degree to which States are obligated to seek the repatriation of their nationals in SDF custody. Beyond a few narrow exceptions, nothing in international law requires a State to seek the extradition or repatriation of a fugitive, even if a national. Because such decisions are inexorably intertwined with antecedent questions of sufficient proof, proper legal process, prosecutorial discretion, and the rule of law, etc., it makes sense that most international obligations in this realm are largely phrased in the negative: i.e., obligations to deny extradition where there are substantial grounds for believing that there is a real risk of irreparable harm in the receiving State.

Relatedly, the principle of aut dedere aut judicare — enshrined in multiple multilateral treaties — requires that States must either (a) surrender an alleged perpetrator found within their jurisdiction to a State that wishes to prosecute; or (b) prosecute the offender in their own domestic courts. But what is a detaining authority to do when it can do neither?

I argue that more is required of the international community in terms of an appropriate disposition of foreign terrorist fighters detained by the SDF in Syria. The legal source of these more extensive obligations can be found in various U.N. Security Council Resolutions (UNSCRs), such as UNSCR 1373, adopted in the immediate aftermath of the Sept. 11, 2001, attacks and which mandates that States ensure that terrorists are “brought to justice.”  In addition to that legal obligation, UNSCR 2178 — issued in September 2014 precisely to address the problem of foreign fighters — provides as follows:

  1. Recalls its  decision,  in  resolution  1373  (2001),  that  all  Member  States shall ensure that any person who participates in the financing, planning, preparation or  perpetration of  terrorist  acts  or  in  supporting  terrorist  acts  is  brought  to  justice, and  decides that  all  States  shall  ensure  that  their  domestic  laws  and  regulations establish  serious criminal  offenses  sufficient to  provide  the ability to  prosecute  and to penalize in a manner duly reflecting the seriousness of the offense:

(a)     their  nationals  who  travel  or  attempt  to  travel  to  a  State  other  than  their States  of  residence  or  nationality,  and  other  individuals  who  travel  or  attempt  to travel   from   their   territories   to   a   State   other   than   their   States   of   residence   or nationality,  for  the  purpose  of  the  perpetration,  planning,  or  preparation of,  or participation in, terrorist acts, or the providing or receiving of terrorist training[.]

Even more recently, in 2017, the U.N. Security Council adopted UNSCR 2396, “which updates U.N. Security Council Resolution 2178 (2014), and provides greater focus on measures to address returning and relocating foreign terrorist fighters (“FTFs”) and transnational terrorist groups,” according to a fact sheet issued at the time by the U.S. Mission to the United Nations. The new resolution created new international obligations and other provisions to counter the threat posed by foreign terrorist fighters, including many provisions regarding the exchange of needed law enforcement information and to “ensure appropriate prosecution, rehabilitation, and reintegration of FTFs and their accompanying family members.” The Security Council further:

Recalls its decision, in resolution 1373 (2001), that all Member States shall ensure that any  person who  participates in the financing, planning, preparation or perpetration of terrorist acts or in support of terrorist acts is brought to justice, and further recalls its decision that all States shall ensure that their  domestic laws and regulations establish serious criminal offenses sufficient to provide the ability to prosecute and to penalize the activities described in paragraph 6 of resolution 2178 in a manner duly reflecting the seriousness of the offense.

My Harvard National Security Journal article argues that such obligations should, at a minimum, be interpreted to impose on all member States obligations of undertaking their “best efforts” to investigate, repatriate, and take all lawful measures to incapacitate (or rehabilitate) their nationals who are detained in SDF custody. This is because they are under a binding Chapter VII legal obligation to “ensure appropriate prosecution, rehabilitation, and reintegration of FTFs and their accompanying family members.”

This obligation falls to States, as no other disposition options are available as a practical matter for terrorists detained by the SDF: As a non-state entity, the SDF has no real ability to prosecute and detain for the long term; the International Criminal Court has neither the clear mandate nor the capacity to prosecute the vast majority of detained foreign terrorist fighters; the creation of a separate international tribunal is nowhere on the horizon (and highly unlikely for numerous reasons, including the obvious lack of any existing international facility to detain over 2,000 foreign fighters who might be prosecuted by such a tribunal); and no one State – nor even a subset of States – is willing to take on the daunting, resource-intensive task of prosecuting that many foreign terrorist fighters (note the resistance that countries have shown to prosecuting even their own nationals).

In such circumstances, “best efforts” means doing the one thing that can be done – each State repatriating, investigating, and either prosecuting or otherwise taking steps to reintegrate its own nationals. Because such obligations are imposed by numerous UNSCRs under Chapter VII, the “shared responsibilities” of the international community to which General McKenzie referred are quite real.

IMAGE: A member of the Syrian Democratic Forces (SDF) stands guard in a prison where men suspected to be affiliated with the Islamic State (IS) group are jailed in northeast Syria in the city of Hasakeh on October 26, 2019. – Kurdish sources say around 12,000 IS fighters including Syrians, Iraqis as well as foreigners from 54 countries are being held in Kurdish-run prisons in northern Syria. (Photo by FADEL SENNA/AFP via Getty Images)