Revisiting Customary IHL Series – Non finito: Should the Study also Identify Norms that Are not Custom?

Editors’ note: This post is part of a series relating to the ICRC’s Customary International Humanitarian Law Study, featured across Articles of War and Völkerrechtsblog. The introductory post is available here. The series highlights presentations delivered at the young researchers’ workshop, Customary IHL: Revisiting the ICRC’s Study at 20, hosted by the Institute for International Peace and Security Law (University of Cologne) and the Institute for International Law of Peace and Armed Conflict (Ruhr University Bochum) on September 18-19, 2025. 

The high and increasing levels of reliance that both academics and practitioners place on the International Committee of the Red Cross (ICRC) Study of Customary International Humanitarian Law (IHL) corroborates its authority as a reference point for IHL rules that have attained the status of customary law.

As it currently stands, the ICRC Customary IHL Study does not pretend to represent an exhaustive list of customary norms, underlining that “choices had to be made.” Not all of these choices are explained in the Study, however. For example, its introduction explicitly mentions that naval warfare was excluded as this was the subject of another recent study. For other questions, the Study leaves a dose of ambiguity. For instance, if an issue is not addressed, is the omission deliberate or because no customary law norm could be identified?

The absence of an authoritative pronouncement inevitably leaves the door open for States to assert the existence of additional customary norms outside the Study, or to claim that their conduct is ungoverned by law. This post illustrates the issue by reference to the relevance of nationality to a detainee’s entitlement to prisoner of war (POW) status.

This post explores the possibility that the Study could explicitly signal issues where State practice is not (yet) consistent enough to demonstrate the existence of a customary norm governing the question. The post argues that the Study has the potential to take up such a role, and not only to tell us what customary IHL is, but also what it is not (yet).

Nationals of the Detaining Power as Prisoners of War

A person’s nationality is irrelevant to their qualification as a POW. This holds true in all cases apart from potentially one: when a State captures its own nationals who had been fighting with the enemy. Legal scholars have posited that persons who have taken up arms against their own State are not entitled to POW status and that this entitlement of the capturing State to deprive its own citizens of POW status stems from customary law. The issue is, however, a subject of decades-long debate, with both the existence and scope of such an entitlement remaining unsettled.

Both sides of the debate draw on the Third Geneva Convention (GC III). Arguments in the dispute stem from the Convention’s Article 4 and take its silence on the question of nationality as their point of departure. Those arguing in favour of the nationality requirement remind us that Article 4 does not set out an exhaustive list of criteria for POW status. To corroborate this argument, they mention the example of the individual obligation on combatants to distinguish themselves from the civilian population. This is another pre-requisite which is not mentioned in this Convention but widely accepted to stem from customary law.

Further, proponents of the nationality requirement flag that other provisions of the Convention clarify Article 4’s ambiguity; Articles 87 and 100 do refer to the detainee’s nationality. Finally, they corroborate their reading of Article 4 by referring to the object and purpose of GC III, namely, to regulate the relations between two belligerents with respect to the treatment of captured forces, while refraining from interference in a State’s treatment of its own nationals.

The opposite camp juxtaposes the absence of any nationality requirement in Article 4 GC III against the explicit reference to nationality in the definition of protected civilians in Article 4 of the Fourth Geneva Convention. This argument also touches upon Articles 87 and 100, responding that these provisions cannot be employed to interpret Article 4, but that they rather refer to a sub-category of POWs who do not hold the detaining power’s nationality. These authors likewise refer to the aims of GC III but understand it differently. In particular, they see this instrument as oriented towards the protection of persons captured by the enemy.

State practice on the matter is rather varied. Examples are drawn from historic accounts of belligerents’ conduct, jurisprudence of domestic courts, as well as military manuals. For instance, Flory mentions Irish citizens who pledged allegiance to the South African Republic in the Boer War and who were treated as POWs. This contrasts with Debuf’s example of Spanish nationals who, as members of the Blue Legion of the German forces, were captured by Soviet forces.

Interestingly, both camps rely extensively on the Koi decision of the UK Privy Council. In the context of the conflict between Malaysia and Indonesia, the decision concerned Malaysia’s capture of its own nationals fighting against it, in support of Indonesia. Those arguing in favour of a nationality requirement rely on the Court’s conclusion that POW status is not owed to one’s own citizens. The opposing side highlights the decision’s ambiguities and challenge how much weight ought to be given to this single decision. They also argue for other possible interpretations of its conclusion. These scholars also invoke other cases such as In re Territo, Ex parte Quirin, or Hamdi v Rumsfeld where U.S. courts have regarded nationality as irrelevant to POW status.

Lastly, even military manuals that refer to nationality are not uniform in their approach. Some explicitly exclude all nationals from POW status while others seemingly confine this exclusion to deserters and defectors. It therefore comes as no surprise that the ICRC Commentary to GC III flags that State practice on the matter is inconsistent. Additionally, many writings point to interesting nuances in the debate which suggest that there are areas where the two opposing sides could potentially reach a middle ground.

One stream of interpretations suggests that instead of nationality, it is the allegiance of the captured person that is more relevant to their qualification as a POW. Sassòli and Duss note that the challenges in determining civilians’ allegiance to overcome the nationality requirement for protected civilian status are significantly reduced in the context of combatants. In case of the latter, allegiance can often be easily determined based on the person’s uniform at the moment of capture.

Scholars have elaborated on the circumstances when discrepancies might arise between nationality and allegiance: dual nationals; members who joined the enemy forces for ideological reasons; those who have de facto severed their nationality link; as well as members of national liberation movements. Another case where scholars consider the granting of POW status to be increasingly acceptable arises when a State captures members of an armed group who are its nationals, but where the armed conflict has become international in character based on the overall control exercised by a third State over that armed group.

A further compromise suggestion is to separate questions of qualification for POW status from the possibility of being prosecuted for treason. Authors have suggested that persons who owed a duty of allegiance to the detaining power would still qualify for POW status but could be prosecuted for having breached this duty. In such cases, the set of judicial guarantees enshrined in GC III that they would be entitled to as POWs would act as a crucial safeguard.

One Example Among Many?

Despite the lack of consistency in State practice on the matter, States still rely on customary law to justify depriving their own citizens of POW status. As the preceding analysis demonstrates, however, the law remains unclear. Due to the ICRC’s capacity to access, gather, and analyse State practice, this post argues that the Study is well placed to acknowledge that customary international law in this area is not settled. If it did so, this would help prevent States from exploiting ambiguity, for example to justify the mistreatment of captured fighters, and it would provide support to other States or legal commentators that wish to push back against such claims.

Nationality as a consideration in qualifying for POW status is just one example of an unsettled legal question in IHL. The Study already notes some of these contested areas. For example, it explicitly highlights debate regarding the customary status of the prohibitions on nuclear weapons, as well as on belligerent reprisals against civilians and civilian objects in the conduct of hostilities. In other areas, such as the presumption of civilian status in case of doubt, it notes divergences among State positions on the exact scope of customary law obligations. How could the Study map such contested areas of customary law more systematically?

The Study itself reminds us of the potential for customary law to assist in the interpretation of treaty norms. I suggest that the forms of interaction between customary law and treaties could serve as a compass in selecting which disputed norms the Study could elaborate. o questions provide guidance. First, does the claimed customary law norm complement or alter treaty notions? Second, does the claimed customary norm purport to address an ambiguity in treaty text? And as part of both questions, does the claimed customary law norm make room for more permissive behaviour on the part of the belligerents?

The first question would arguably bring under scrutiny cases where notions which are already defined in treaties are understood so permissively that such interpretations risk amounting to treaty modification. Examples of such broader readings are abundant in the realm of the conduct of hostilities. For instance, we can think of the argument that economic or war-sustaining targets that provide indirect but effective support to the belligerents can qualify as military objectives. Another example is the argument that, within the proportionality assessment, the expected civilian damage could be weighed against the anticipated military advantage “from an operation as a whole,” rather than from each individual attack. In these two cases, and others, the Study concludes that these rules form part of customary law. However, the Study articulates less clearly to what extent divergent State interpretations fit within the scope of the respective norms or rather push interpretation beyond the law.

The second question could address issues that are left undefined in treaty law, where the claimed customary law norm would play a “gap-filling” role and interpret the notions in question. This brings to the forefront fundamental questions relating to the very scope of application of IHL. Scholars have already noted the difficulties in establishing with certainty the definition of an armed conflict. Linked issues arise even once an armed conflict has begun including where does IHL apply and how far beyond the State’s borders can an armed conflict “spill over”?

Conclusion

A piece of art can be deliberately left unfinished and admired in its non-complete state. It stands as art in its current form, and not as a particle of what it would have become if the piece were completed. This post argues that a similar perspective can be adopted for law and the formation of custom. The value of a finding that a norm is not custom will lie precisely in what it reveals: that the alleged customary norm, while perhaps in the process of formation, is still not settled. Acknowledging this legal non finito or incompleteness can be an asset, not a weakness.

It is understandable that the ICRC did not refer to non-established customary norms in the Study, given the 26th International Conference of the Red Cross and Red Crescent mandated it to prepare “a report on customary rules of IHL applicable in international and non-international armed conflicts.” If a future edition embraced a broader understanding of its mission and offered a more comprehensive snapshot of the state of customary law, including its grey areas, this could allow the Study to bring another added value. In addition to being an authoritative point of reference for existing rules of customary international law, domestic and international courts as well as other States could rely on the Study’s findings on issues that remain unsettled as a matter of law. This, in turn, would allow legal practitioners to counter dubious claims that the legality of the conduct in question is based on an undeveloped customary norm.

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Mina Radončić is a PhD candidate at the University of Geneva and a Teaching Assistant at the Geneva Academy of International Humanitarian Law and Human Rights.

The views expressed are those of the author, and do not necessarily reflect the official position of the United States Military Academy, Department of the Army, or Department of Defense. 

Articles of War is a forum for professionals to share opinions and cultivate ideas. Articles of War does not screen articles to fit a particular editorial agenda, nor endorse or advocate material that is published. Authorship does not indicate affiliation with Articles of War, the Lieber Institute, or the United States Military Academy West Point.

Photo credit: U.S. Army, Spc. Sherald McAulay