Promise and Peril of Relying on Human Rights in the Customary International Humanitarian Law Study

Editor’s Note: This post is part of a symposium relating to the ICRC’s customary international humanitarian law study, featured across Articles of War and Völkerrechtsblog. The introductory post is available here. The symposium 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 International Committee of the Red Cross’ (ICRC) Customary International Humanitarian Law (IHL) Study famously calls itself a ‘photograph’ of customary IHL. The metaphor suggests that it merely restates what the law already is – in a user-friendly format. A lot of its Rules undoubtedly correspond to customary IHL, while others were novel interpretations and developments. The Study is best seen not as an objective directory of customary IHL, but instead a thoroughly researched and well-articulated non-binding document which states what the ICRC at the time considered custom. As with other similar documents, some degree of development is almost inevitable.

In my research, I have identified four categories of developed or substantially modified Rules: i) Rules inspired by international human rights law (IHRL), ii) Rules expanding IAC provisions to NIAC, iii) other Rules driven by ‘humanitarian imperative’ and iv) accountability-focused Rules. Here, I engage with one subset of Rules – IHRL-inspired – as most illustrative.

The Study’s attempts to develop the law do not diminish its importance: it is widely used and generally considered authoritative. A lot of early methodological criticism has proven less decisive. Despite the Study’s relative success, it has proven to be uneven in terms of how States and others use it. ‘Success’ can be defined in various ways; I focus on use by states and other actors in international law as an approximation. If we look closely at this annex from an in-depth exploration of the Study’s authority by Marko Milanovic and Sandesh Sivakumaran, we see that citations by Rules differ widely: for instance, Rule 7 has been cited at least 41 times, whereas Rule 78 only once.

The ICRC’s authority and format of the Study (i.e., treaty-like formulations in one compendium) do not explain such discrepancies: these attributes are shared by all Rules. What would explain these differences in acceptance? Was there something the drafters could have changed to improve the ‘success rate’ of some Rules?

Yes: addressing the structural issues in using IHRL language and content for IHL norms. Looking at the IHRL-inspired Rules, simple reference to or reliance on IHRL is not itself a prerequisite for success or failure. However, we can observe that Rules in this category are cited less often, compared to, say, IAC-to-NIAC Rules, suggesting structural issues at play. I argue that ‘success’ rate of a Rule depends on how well it addresses them.

One Does Not Simply Cite IHRL

I have identified 35 Rules in the Study which were inspired by IHRL, to varying degrees: Rules 87-105, 116, 117, 120, 126, 129, 131-137, and arguably 107, 108, 130 and 150.

The Study’s general approach towards IHRL is itself mixed:

• On the one side of the spectrum, some Rules use explicit IHRL language of ‘rights’, as in Rule 133 (Property Rights of Displaced Persons), or rely heavily on IHRL in content, as in Rule 100 (Fair Trial Guarantees).
• On the other side, some Rules merely acknowledge relevant IHRL provisions in passing (e.g., Rule 130 on Transfer of Own Civilian Population into Occupied Territory) or even ignore IHRL when it could have been relevant (e.g., Rule 75 on Riot Control Agents, since some of them may not be prohibited under IHRL).

Such a disparate approach may suggest that the authors relied on IHRL predominantly when IHL may have been insufficient to prove the existence of a CIHL Rule. This can create problems.

Here, I analyse three Rules which show some of the reasons behind the discrepancies in use: Rule 98 (Enforced Disappearance, cited 9 times) and Rule 133 (Property Rights of Displaced Persons, cited 4 times) as generally ‘unsuccessful’, and Rule 100 (Fair Trial Guarantees, cited 22 times) as a more ‘successful’ example.

Case Study: Rules 98, 133 and 100

A quick note on how the Study has argued for existence of Rules 98 and 133. Rule 98 was established using the IHRL logic: it showed that the act violates several prohibitions (arbitrary detention, torture and ill-treatment, murder), noting that the ‘cumulative effect of these rules is that the phenomenon of “enforced disappearance” is prohibited by international humanitarian law.’

Rule 133 on the property rights of the displaced combined the limited protection of property in IHRL with limited protection of various types of forcibly displaced people. In other words, it amalgamated complex and diverse rules into one. It also relied heavily on a non-binding norm: Principle 21 (3) of the Guiding Principles on Internal Displacement (GPID). Strikingly, the practice section for Rule 133 cites no military manuals whatsoever, the only such case across the Study.

How were these provisions used? Both Rules 98 and 133 were cited in some UN documents, but barely any state used them. The only explicit ‘state citation’ for Rule 98 found was by Malaysia, in a footnote of a concept note for a UN Security Council debate on children and armed conflict. This can barely be considered a state’s position on the law.

Rule 98 has been used in several UN documents, such as by FFM Myanmar, among others. All mentions explicitly combine the prohibition of enforced disappearance under IHRL and CIHL, and they usually start with IHRL. It seems that the CIHL reference is used as an additional argument, potentially out of convenience, including in cases where IHRL may not be applicable – either because it concerns a non-party to CPED or a non-state armed group.

There seems to be no similar ‘state citation’ for Rule 133, although it was cited by the Inter-American Court of Human Rights. The Kampala Convention seems to have taken the formulation more from the GPID, rather than Rule 133. The laws of Djibouti, Peru and Nigeria, included in Rule 133 practice, barely mention IHL or armed conflict. Only Georgia’s IDP law (2014) seems directly relevant; while it does not repeat Rule 133 formulation, it follows the same general approach.

Rule 100 on fair trial guarantees, on the other hand, has been much more accepted.

In its formulation, the Study based fair trial guarantees largely on IHL provisions, notably from Additional Protocol (AP) I, which were influenced by IHRL. Arguably, there are some legal issues with Rule 100. It included a longer list from AP I and extended it to NIACs. Corresponding provisions in the ICCPR (Arts. 9 and 14) allow for derogations, unlike IHL. This can create confusing obligations for states in armed conflict derogating from ICCPR but keeping virtually the same obligations under IHL. NSAGs, as non-parties to a treaty, become bound by a stricter regime in terms of fair trial guarantees than states.

Nevertheless, there is general acceptance of Rule 100 and its content, including by the US Supreme Court. Some fair trial guarantees included in Rule 100 have been used by non-state armed groups in Syria and Libya, albeit without direct reference to the Study. While they do not explicitly mention Rule 100, the content of their actions seems to correspond with the Rule. Even if some of this can be traced back to AP I, Rule 100 still facilitated the development of this CIHL norm.

Rule 100 proves that simple reliance on IHRL or inspiration from IHRL language does not preclude further acceptance by parties to the conflict.

What Did (Not) Work?

Both Rules 98 and 133 assumed opinio juris for a norm from other humanitarian norms and principles. Being driven by a humanitarian imperative is, unfortunately, not enough. Neither Rule was originally found in binding law or even non-binding IHL norms. Instead, they seem to have been logically ‘deduced’ from other norms, including IHRL ones – and sometimes even only non-binding, such as GPID. The Study attempted to expand protection of IHL to other areas, in line with the ICRC’s understanding of the purpose and nature of the law.

The biggest issue with these two Rules seems to be the unaddressed structural differences between IHL and IHRL.

Most provisions in IHL and IHRL appear as sharing the same goal – protection of the individual. However, IHL is a regime that governs primarily relations between parties to the conflict, either state or non-state. While most of its provisions are protective towards individuals, the obligations were created as obligations towards other states and parties to the conflict. IHRL, on the other hand, is built on obligations of states towards individuals.

Rules 98 and 133 are construed as obligations of states vis-à-vis individuals, which reflects the IHRL approach, rather than obligations of parties to the conflict towards each other in IHL. The authors have relied heavily on IHRL without addressing these structural differences.

Rule 100 shows a different, more useful – if less ambitious – approach.

First, Rule 100 is largely based on IHL, meaning it leaves the hard exercise of reconciling structural differences between fair trial guarantees under IHL and IHRL to Additional Protocols, Commentaries, states and others, rather than doing it itself. Having existing IHL rules already binding on several states simplifies the task.

Second, the Rule does not purport to expand the existing protection in terms of content (i.e., which fair trial guarantees), but in terms of scope (namely, to expand it to NIACs). This expansion may not pose obstacles for normative development, as we can see from the abovementioned citations – at least because it follows the generally accepted way analogies are used in international law.

These are not the only possible explanations for discrepancies in how the CIHL Study Rules are used. However, they seem to apply across other IHRL-inspired rules (e.g., more widely used Rules 99 and 117, or less ‘successful’ Rules 91 and 92). It is difficult to quantify the impact of addressing structural issues between IHL and IHRL. I also admit that there may be other alternative explanations – some Rules are simply invoked less often. Nevertheless, the evidence suggests that addressing structural issues between IHL and IHRL is crucial to the acceptance of these norms.

Recommendations and Outlook

This brings us to two potential recommendations for drafters of future similar non-binding documents:

• Using norms predominantly within the same branch of international law, or addressing structural differences between different branches of law.
• Not relying solely on the ‘humanitarian imperative’ when developing norms.

Drafters of non-binding documents cannot control how states and other actors will use their suggested norms, but they can still increase chances of normative acceptance. The CIHL Study shows that the author’s decisions on how to integrate IHRL can spell the difference between acceptance and rejection. Even if a handful of Study’s Rules remain ‘unsuccessful’, their lessons can still help us develop IHL in realistic ways – and perhaps even change behaviour on the battlefield.

Disclaimer: The views expressed here are those of the author and do not necessarily reflect the views or positions of the United Nations, the International Committee of the Red Cross, or any other organisation with which the author is or has been affiliated.