Revisiting Customary IHL Series – Study 2.0 Featuring the Martens Clause: Tool Rather Than Subject

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. 

As a codification project, the International Committee of the Red Cross (ICRC) Study on Customary International Humanitarian Law can only provide a snapshot of a body of law under constant evolution. Hence, it does not come as a surprise that the introduction to the Study envisages the possibility of a future update. It is noteworthy that the introduction itself even suggests certain topics as conceivable candidates for such a future edition. One of the proposals mentioned is the codification of the Martens clause (introduction, p. xxxvi). This idea can be found in the literature as well (e.g. MacLaren and Schwendimann p. 1241, Scobbie p. 18). As our workshop aims at opening a discussion on the need to update the Study, this post discusses the role of the Martens clause in such an endeavour.

In a Nutshell: The Martens Clause

The so-called Martens clause is named after Fyodor Fyodorovich Martens, who drafted the clause as a Russian delegate to the Hague Peace Conference in 1899 to overcome a diplomatic deadlock.

In its original form in the Preamble to Hague Convention (II) on the Laws and Customs of War on Land, the clause establishes that

until a more complete code of the laws of war is issued, the High Contracting Parties think it right to declare that in cases not included in the Regulations adopted by them, populations and belligerents remain under the protection and empire of the principles of international law, as they result from the usages established between civilized nations, from the laws of humanity, and the requirements of the public conscience.

Since then, this clause has been included in military manuals, in several international treaties and their preambles (see e.g. art. 63(4) GC I, art. 142(4) GC III, art. 1(2) AP I) and it has played a central role in a number of proceedings before (inter)national courts. As a result, the clause is often considered to reflect customary international law (see Legality of the Threat or Use of Nuclear Weapons, para. 84) and praised for playing a key role in humanising international humanitarian law (IHL). This development notwithstanding, there remains an enduring debate on the different interpretations of the clause. This disagreement over the precise content of the clause apparently prompted the ICRC to decide in 2005 that it was not “developed in sufficient detail for inclusion in this edition“ (introduction, p. xxxvi).

Indeed, the diverging interpretations raise the question whether the existence of a norm as customary international law can be established despite its main content remaining unclear. Even if one of the proposed interpretations can be generally accepted, it seems questionable whether the rule is the right candidate for a codification in a future edition of the Study. This becomes apparent when one takes a closer look at the interpretations under discussion.

Anything but Ordinary: The Martens Clause as a Secondary Norm

Interpreted narrowly, the Martens clause can be used to rule out arguments a contrario, in cases where no applicable provision can be found in a treaty or other codification document (see e.g. Schwarzenberger, p. 261-62, Abi-Saab, p. 274-75). Three different variations of this narrow interpretation can be envisaged.

First, the clause could confirm that customary international law remains binding on States alongside international treaties, providing default rules for matters not regulated by treaty law. Second, the clause could clarify that the exclusion of a rule from a codification treaty does not necessarily preclude its identification as custom (for the significance of a codification treaty as evidence of customary international law see Conclusions 6, 10, 11 of the International Law Commission (ILC) Draft Conclusions on Identification of Customary International Law). Third, the clause could be interpreted as a reverse Lotus principle. According to this interpretation, the clause would caution against conceiving States’ freedom of action as a necessary consequence of a gap in treaty or customary IHL. However, inasmuch as this raises the question how a remaining gap should be addressed, this variation resembles certain broader interpretations of the Martens clause (discussed by Salter, p. 422).

These broader interpretations invoke the clause as a justification for modifying the criteria to identify customary IHL (for example, with an emphasis on opinio juris) or advocate for the clause as an interpretative tool for a humanitarian interpretation of the law (see Cassese, p. 212-13, Meron, p. 88, Crawford, p. 20). Finally, the most far-reaching interpretations refer to laws or principles of humanity and public conscience as a separate source of international law, alongside customary international law (see dissenting opinion of Judge Shahabuddeen in Legality of the Threat or Use of Nuclear Weapons, p. 408-09; for further discussion see Cassese, p. 213-14).

Comparing these different interpretations reveals that they can all be classified as secondary norms in one way or another (legal presumptions, guidelines on law identification and interpretation, sources of international law). This character as secondary norms militates against the clause’s inclusion in the Study, as the Studymainly concerns the codification of primary norms (apart from the rules on responsibility, which are sometimes included in a broader definition of the concept of secondary norms). Focusing on questions of methodology and their consequences, the Martens clause seems to be more of a tool than a suitable subject for a codification project on primary norms. From this perspective, the Martens clause appears to be a better candidate for projects focusing on secondary norms (such as the ILC Draft Conclusions on Identification of Customary International Law), where its specific features would be embedded in a broader methodological context.

Two Conceivable Objections Against the Argument

Two objections to the aforementioned argument are conceivable.

First, there are further interpretations of the clause that emphasise its character as a directly applicable, substantive provision (e.g. by providing a minimum standard in IHL), thus giving the impression of the clause as a primary norm. However, the proponents of this interpretation seldom clarify the precise content of such a primary norm. It appears as if the clause either does not add value to specific existing rules (see e.g. Ticehurst, p. 129) or laws or principles of humanity and public conscience are conceptualised as abstract principles that allow for the inference of an indeterminate and evolving number of more specific norms (see e.g. Salter, p. 421-23, Stapleton-Coory, p. 480-82). However, this latter deductive approach converges with the interpretations discussed above, which perceive the clause as a modification of the customary international law process or a new independent source of international law.

Second, it might be argued that the inclusion of the clause in a codification of primary norms of IHL is far from unusual, as demonstrated by the repeated references to the clause in several codification treaties. However, in these cases, references to the clause were often confined to the preamble. When included as substantive provisions, they were introduced as “general principles and scope of application” at the beginning of the treaty or were part of the treaty’s denunciation clause. It seems reasonable to distinguish between codification in the form of a treaty agreed upon by States and the ICRC’s Study, which represents codification by a non-State actor. In the latter case, there is no need for a corresponding provision on the scope of application or denunciation. Hence, it seems advisable to limit the substantive rules identified in a new edition to the Study to the primary norms of IHL and, if necessary, to refer to the Martens clause in the Study’s introduction.

Conclusion

Given its character as a secondary rule under all interpretations discussed, the Martens clause does not appear to be a promising candidate for a possible new edition of the Study, regardless of the extent to which its controversial content would allow the establishment of its customary international law status. However, this does not deprive the clause of any significance for a future edition. The clause continues to play an important role in the development of IHL, particularly in view of new technologies and cyber warfare. If the ICRC is inclined to adopt one of the broader interpretations of the clause (the ICRC’s reference to the clause in its position paper on IHL and cyber operations during armed conflicts could be interpreted this way), it might be appropriate to include the clause in the introduction to the Study, not only to illustrate the enduring importance of custom, but also to explain the methodological choices made.

Although the Study’s methodology has been subject to criticism (see here, here and here) its position has been considered rather moderate compared to approaches that draw directly on the Martens clause (see here,here and here). The Study’s introduction describes its approach to custom-identification as a “classic one” requiring the presence of two elements, namely State practice and opinio juris. Nevertheless, the introduction to the Study explains that “it is not usually necessary to demonstrate separately the existence of an opinio juris” and that a rule could exist when it “is a desirable one … provided that there is no important contrary opinio juris” (introduction, p. xlvi, xlviii).

While this presumption of opinio juris was based on a balanced relationship between State practice and opinio juris in the face of certain values (the so-called sliding scale approach) as well as the International Law Association’s London Principles on Formation of General Customary International Law, it has to be examined whether it finds support in the ILC’s Draft Conclusions on Identification of Customary International Law. To preserve and further consolidate its more flexible concept of State practice and opinio juris in a future edition, the Study might resort to the Martens clause.

Of course, this assumes that a new edition of the Study would adopt a similar methodology to the first. Although a re-evaluation of the original method seems to be valuable in view of the criticism raised in the past, it needs to be discussed whether deviations from the previous method could risk undermining the authority of the rules identified in the 2005 Study. Discussions about a possible new edition should therefore not only focus on its prospective content but also methodological questions. Looking at the Martens clause invites us to do both.

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Paulina Rob is a doctoral candidate and research fellow at the Institute for International Peace and Security Law at the University of Cologne.

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. Mariah Aguilar