Opinio Juris and the Essential Role of States


| Feb 11, 2021

Opinio Juris - UN. USAID


Scholars, jurists, and practitioners have increasingly come to rely on non-State expressions to explain the customary law of armed conflict. This trend is problematic because it runs counter to how customary international law is created and should be understood. As recognized by the International Court of Justice, determining what constitutes customary international law is and remains the province of States.

Undoubtedly, State silence regarding customary law can create ambiguity, and that ambiguity can, in turn, invite speculation about the law. However, while it may be tempting to accept non-State pronouncements for settled law, those applying the law must resist mistaking the opinions of non-State actors for what States consider to be their legal obligations under the customary law of armed conflict. Substituting non-State assessments for State opinio juris when interpreting customary international law is not only legally unsound, doing so can also disrupt the norm creation process in ways that ultimately undermine the law.

As used here, the term “non-State actor” is intended to refer more narrowly to individuals and organizations—including scholars, advocacy groups, and non-governmental organizations—not affiliated with a national government that commonly opine on the law. It is not meant to include non-State armed groups or their members.

Opinio Juris 

Customary international law has long been recognized as one of three primary sources of international law. Article 38(1) of the Statute of the International Court of Justice states that international law derives from international conventions, international custom, and general principles of law.

The second source, customary international law, is commonly agreed to develop from a general and consistent practice of States followed by them from a sense of legal obligation.[1] Accordingly, customary international law is understood to consist of two parts: State practice and States’ subjective understanding of their legally binding obligations—otherwise known as opinio juris. In the formulation of the ICJ in its North Sea Continental Shelf judgment, opinio juris is “a belief that [a] practice is rendered obligatory by the existence of a rule of law requiring it.”

Significantly, evidence of State practice alone is insufficient to identify customary international law. Understanding opinio juris is essential to determining whether certain practices have developed into norms of customary international law. States may engage in behaviors for a variety of non-legal reasons, including political expedience. Because these non-legal reasons may influence their behavior, opinio juris cannot simply be inferred from State practice. As a result, discerning State opinio juris is not always so straightforward.

Ideally, States make clear—through official statements or diplomatic exchanges, for example—which practices they consider legally obligatory. State expressions of opinio juris, however, are not always forthcoming. This absence of opinio juris can create uncertainty about the law, which non-State actors—including legal scholars, humanitarian advocacy groups, and non-governmental organizations—may be eager to resolve. Differentiating between non-State opinion and genuine opinio juris, however, is critical to understanding the actual state of customary law.

Reasons for State Silence

As noted above, various considerations may influence States to remain silent about opinio juris. State silence may be a calculated decision. Military operational concerns, for instance, may necessitate a certain degree of reticence, and States may remain quiet to avoid revealing protected information. Early operations in cyberspace highlight how delicate the decision to assert opinio juris can be—and why States may hesitate to announce their positions, particularly regarding emerging areas of warfare.

The failure to articulate opinio juris may also reflect a deliberate determination that no obligatory rule of law has yet emerged. State silence, then, is in fact indicative of the absence of an extant norm. Alternatively, a State may refrain from articulating a position while the law develops and the State considers its options for compliance.

Lastly, State silence may reflect internal disagreement within a State. The municipal law of the State may require coordination among various governmental agencies before a final decision can be made. Resolving interagency differences can take time, and even when internal consensus has been reached, the nature of governmental bureaucracy could delay the announcement of an official opinion.

Distinguishing Non-State Opinion from State Opinio Juris

In the absence of opinio juris, non-State actors have not hesitated to issue opinions interpreting the law in ways that, unsurprisingly, conform to their particular values and perspectives.

ICRC Study on Customary International Law

In 2005, for example, the International Committee of the Red Cross (ICRC) released its study on Customary International Humanitarian Law. The result of ten years of intensive research and consultation, the study fulfilled a mandate “to prepare … a report on customary rules of IHL [international humanitarian law] applicable in international and non-international armed conflicts” in an effort to “promot[e] full respect for and compliance with IHL.” Few States, however, responded directly either in support of or in opposition to the study’s conclusions. In the absence of State responses, many of the report’s findings have gained acceptance as authoritative statements of customary international law.

Notably, the United States was one of the few States to comment directly on the conclusions of the Customary International Humanitarian Law study. Among other things, the U.S. response criticized the report’s approach to identifying opinio juris. The United States noted the study tended to merge the State practice and opinio juris requirements into a single test and that doing so was not an appropriate methodological approach. The U.S. response observed that “[a]lthough the same action may serve as evidence both of State practice and opinio juris, the United States does not agree that opinio juris simply can be inferred from practice.”

The United States further commented that the study fell “far short of identifying the level of positive evidence of opinio juris that would be necessary to justify concluding that the rules advanced by the Study are part of customary international law and would apply to States even in the absence of a treaty obligation.”

Despite the United States’ reservations and the methodological problems outlined in the U.S. response, scholars, jurists, and even State legal advisors have demonstrated an uncanny willingness to treat the study’s conclusions as definitive expressions of customary international law supported by opinio juris.

ICRC’s Interpretive Guidance on the Notion of Direct Participation in Hostilities

The ICRC’s Interpretive Guidance on the Notion of Direct Participation in Hostilities provides another example of how non-State commentary has come to fill a vacuum left by State silence. Published in 2008, the Interpretive Guidance explains up front that it is intended “to provide recommendations concerning the interpretation of international humanitarian law (IHL) as far as it relates to the notion of direct participation in hostilities.” It further emphasizes that its recommendations and commentary “do not endeavour to change binding rules of customary or treaty IHL, but reflect the ICRC’s institutional position as to how existing IHL should be interpreted in light of the circumstances prevailing in contemporary armed conflicts” (emphasis added).

Despite clear statements of its nature and purpose, the Interpretive Guidance has gained an outsized influence on how important concepts relating to direct participation in hostilities are interpreted today. States bear much of the blame for the subsequent focus on the ICRC’s report and its position as a baseline for legal discourse in this area. Although some States have commented on the report’s findings, many have not or have responded only to some of its conclusions. The United States did, however, subsequently incorporate its views on several aspects of direct participation into the DoD Law of War Manual, and notably rejected significant parts of the Interpretive Guidance as reflective of customary law (see here for more detail).

Ultimately, by not addressing the Interpretive Guidance more explicitly and clarifying their positions on the direct participation of civilians in hostilities, many States have essentially allowed the ICRC’s interpretive views to fill the vacuum left by the absence of State opinio juris on the subject.

Regardless of whether States respond to non-State commentaries, those responsible for applying the customary law of armed conflict must always distinguish between speculation and actual opinio juris. As a matter of law, only States’ understanding of their binding legal obligations constitute opinio juris and may contribute to the formulation of customary international law. The temptation to accept non-State studies and commentaries as expressive of prevailing customary law must be avoided when States stay silent and opinio juris is unclear.

Risking Balance and Perspective in the Law of Armed Conflict

Substituting the interpretive views of non-State actors for opinio juris can upend the State-centric process of norm creation and damage the workability of the law. Accordingly, jurists and practitioners must avoid doing so. The law of armed conflict seeks to balance the interests of military necessity and humanity. Achieving the appropriate balance depends on State input and the representation of State interests. Consequently, to fill gaps in the customary law of armed conflict with speculation risks skewing the balance between military necessity and humanity. Allowing non-State assessments to define the law and drive its development would frustrate its position as a body of law dedicated to governing relations between States during armed conflict.

The views of humanitarian organizations like the ICRC, which are intimately familiar with conflict and impressively knowledgeable about the law, are critical to achieving that balance. Their understanding of the humanitarian impact of armed conflict can provide much needed perspective as customary norms develop and evolve. However, humanitarian organizations and other non-State entities invariably view the law through a particular prism. Their focus may be the protection of victims of conflict or the alleviation of suffering in war rather than the achievement of specific national goals. Consequently, their interests are likely to diverge from those of States, and their viewpoints must be considered in context.

Those responsible for applying the customary law of armed conflict, including judges of international courts and State legal advisors, must be particularly careful to recognize the limitations of non-State reports and commentaries and endeavor to seek out the input of States before opining on customary rules.

In addition to identifying three primary sources of international law as discussed above, Article 38(1)(d) of the Statue of the ICJ further indicates that “judicial decisions and the teachings of the most highly qualified publicists” may serve as “subsidiary means for the determination of rules of law.” Because judicial decisions may be used as secondary evidence of international law, international court judges must ensure States’ views of their legal obligations, not the speculation of non-State actors, inform their understanding of the customary law of armed conflict (see, for example, the recent Attack Symposium on Articles of War).

International court decisions can be useful in clarifying State understandings of the law. For example, the Tadić case decided by the International Criminal Tribunal for the former Yugoslavia is frequently cited as a case that has had a positive impact on the law of armed conflict. The Tadić Appeals Chamber decision was informed by the views of States, several of which submitted amicus curiae briefs to the tribunal, and State input may account for its broad acceptance and longevity. In contrast, other international court decisions have been criticized for declaring interpretations of customary law unsupported by State practice or opinio juris. These cases, some of which are discussed here, highlight the danger of accepting non-State opinions for statements of opinio juris.

It is crucial to note, that the objective of international tribunals differs from that of States. Their adjudicative function favors dispute settlement. States, however, may seek ambiguity in the law for important reasons, such as to serve diplomatic purposes. When compelled to evaluate claims, courts usually devise firmer and clearer articulations of law than would States. And although the legal mechanism of non liquet is available to acknowledge unclear law, courts seldom resort to that tool. Courts then often place a finer point on law than do States. This may be allowable in light of their adjudicative mandate. But their work and conclusions, particularly with respect to customary international law, should be regarded cautiously. Article 38(1)(d) notwithstanding, their work is not legislative in nature.

Like international courts, State legal advisors must exercise caution when advising on customary law. As Mike Meier recently reiterated here, it is critical for law of war practitioners to get the law right even when non-State commentaries obscure the difference between what the law is and what some may want it to be. Although Meier’s article focused more specifically on methodological approaches to treaty interpretation, his general admonition—that military practitioners and State legal advisors more broadly must understand and apply the law correctly—is equally applicable to customary international law.


To ensure the law remains workable in armed conflict, States should commit to affirming their views of the law whenever practicable. However, in the absence of such expressions, those responsible for applying the law of armed conflict, including international courts and law of war practitioners, must be vigilant not to mistake non-State opinions for the views of States. In the absence of clear State opinio juris, accepting speculative statements about customary law may seem appealing, but ensuring the law remains workable and relevant in armed conflict requires that States, not private individuals or non-State organizations, remain the focus of law creation.

Michael Schmitt and Sean Watts have observed that “State opinio juris remains the critical bellwether for the degree of consensus, acceptance, and therefore effectiveness and legitimacy of any international legal rule.” Permitting non-State views to substitute for opinio juris can upset State consensus on the law of armed conflict and disrupt the balance between military necessity and humanity needed to ensure its effectiveness. The determination of State obligations under the customary law of armed conflict, therefore, must not be left to non-State entities, whether intentionally or otherwise.


LTC Ronald Alcala is an Assistant Professor in the Department of Law at the United States Military Academy at West Point and Managing Editor of Articles of War.



[1] Some States, however, have suggested that international organizations may also play a role in the formation of customary international law. For example, in the International Law Commission’s Fifth Report on the Identification of Customary International Law, several states acknowledged the practice of international organizations could contribute to customary law (paras 37-38, A/CN.4/710/Rev.1).