On the Question of opinio juris in Nicaragua vs. Colombia (Judgement 13 July 2023)

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The recent judgment of the International Court of Justice on the Question of the Delimitation of the Continental Shelf between Nicaragua and Colombia beyond 200 Nautical Miles from the Nicaraguan Coast (Nicaragua v. Colombia) is, so to speak, making waves, despite the holiday period (eg here and here). The Court had previously decided that, prior to addressing any technical and scientific matters related to the question of the delimitation of the continental shelf between 200 nm, it was crucial to decide “certain questions of law”. The first asked whether, under customary international law, a “State’s entitlement to a continental shelf beyond 200 nautical miles from the baselines from which the breadth of its territorial sea is measured [may] extend within 200 nautical miles from the baselines of another State”. The second asked “[w]hat are the criteria under customary international law for the determination of the limit of the continental shelf beyond 200 nautical miles from the baselines from which the breadth of the territorial sea is measured” and whether paragraphs 2 to 6 of Article 76 of the UN Convention on the Law of the Sea reflect customary international law.

The Court found in favour of Colombia, seemingly establishing that distance is “a stronger basis of entitlement […] than natural prolongation and/or geophysical criteria” (Woker, 2023). As the Court answered the first question negatively, it had no need to address the second.

Already other commentators have discussed some of the shortcomings of this judgement, described as “surprising” and “questionable”, and I refer to these for detailed presentations of the case and of some of the salient issues that the case raises.

Opinio Juris, Motives and Method

The key question of whether there exists a norm of customary international law regarding the issue at hand is discussed by the Court in paragraph 77, where it considered that:

“in practice, the vast majority of States parties to the Convention that have made submissions to the CLCS have chosen not to assert, therein, outer limits of their extended continental shelf within 200 nautical miles of the baselines of another State”.

The Court considered that, given the information available:

“[t]aken as a whole, the practice of States may be considered sufficiently widespread and uniform for the purpose of the identification of customary international law”.

I will not discuss the question of State practice, even though that also may raise some important questions, some of which were raised indeed by the separate opinion of Judge Shue and discussed in more details in another commentary. I will focus only on the element of opinio juris (a “most controversial” topic) which appears to be, methodologically, the weakest aspect of the decision. The Court, still in paragraph 77, considered that the relevant State practice “is indicative of opinio juris, even if such practice may have been motivated in part by considerations other than a sense of legal obligation” on the basis that “given its extent over a long period of time, this State practice may be seen as an expression of opinio juris”. To further buttress its decision, the Court referred to Gulf of Maine, where it had found it is possible to demonstrate opinio juris “by induction based on the analysis of a sufficiently extensive and convincing practice” ( para 111)”.

However, it is arguably “essential to distinguish between practice which is considered to be legal, on the one hand, and practice based on courtesy, morality or fairness” (Elias, p. 502). In other words, a key question is the motives underpinning or fuelling a practice. As the Court acknowledges, there may have been other motives underpinning the practice of States in relation to their submission to the CLCS than the belief of behaving in accordance with a legal obligation. Yet there was no further discussion of this arguably crucial point in the decision, and the Court was satisfied to quote itself and rely on its characterization of State practice as widespread and uniform, to then demonstrate opinio juris inductively from that. Judge Tomka, however, in his dissenting opinion observed that motives underpinning the conduct of States may vary (para 53), and underscored how it is crucial to establish the right motive, before arriving at the conclusion that a practice is an expression of a customary international norm, and not of something else. He continued observing how States may have wished:

“to put off a diplomatic row; (b) to avoid the objection procedure of the CLCS, which would result in blocking or seriously delaying the consideration of its submission; or (c) because a given area may not be worth claiming” (para 53).

States, in other words, “have adopted various strategies to avoid the possibility of their submissions being blocked by a neighbour” (para 54). Tomka then concluded that the inference of the Court that all submissions refrained from making the claim on the basis of a belief of acting in accordance with a legal rule is “a perilous leap” (para 58).

Inevitably, the motivation underpinning State practice is an important factor to consider, as only the belief of conforming to a legal rule is capable of contributing to the formation and consolidation of a norm of customary international law (and here I mean to take no side in the “highly controversial” debate between voluntarist and intellectualist theories of opinio juris, as the issue of the motive underpinning or fuelling a conduct is arguably equally relevant regardless of one’s position in that debate).

Past Practice of the Court

The Court had itself taken in the past a more careful position prior to taking the “leap” Tomka speaks about, showing “greater rigour” (Brownlie, p. 24) precisely in relation to motives. In Lotus, the Court held that it was not possible to infer from the “alleged facts” that States had been “conscious of having a duty to abstain” from instituting proceedings in cases of collisions in the high seas, a position argued for by France. The Court, quite rigorously, considered that:

“[e]ven if the rarity of the judicial decisions to be found among the reported cases were [established on the basis of available facts] it would merely show that States had often, in practice, abstained from instituting criminal proceedings, and not that they recognized themselves as being obliged to do so; for only if such abstention were based on their being conscious of having a duty to abstain would it be possible to speak of an international custom” (p. 29, emphasis mine).

In fact States may have abstained from instituting proceedings motivated by other reasons, such as “lack of interest or lack of statutory authority”, as Scharf noted. The Court in Lotus thus showed that it “was not ready to accept continuous conduct as evidence of a legal duty and required a high standard of proof of opinio juris” (Brownlie, p. 24). Judge Robinson relied precisely on Lotus in his dissenting opinion to observe how “[t]here can be no presumption that a State’s abstention is motivated by a legal obligation”.

In North Sea Continental Shelf, the Court, similarly, felt necessary to stress how understanding the motivations of a conduct is crucial in order to ascertain whether the practice of States is supported by the belief of acting in accordance with a legal rule. Thus, the Court observed how:

“[t]he frequency, or even habitual character of the acts is not in itself enough. There are many international acts, e.g., in the field of ceremonial and protocol, which are performed almost invariably, but which are motivated only by considerations of courtesy, convenience or tradition, and not by any sense of legal duty” (para 77, emphasis mine).

A similarly more “exacting” position the Court took in Diallo (esp. para 90), where it “noted the inconclusiveness and insufficiency of mere practice” (Brownlie, p. 25), while in Immunity, the Court relied on “assertions” of relevant States claiming the existing of a particular rule, for example through existing domestic jurisprudence, to ascertain opinio juris.

Ultimately, while the Court has considered that a sufficiently widespread and uniform practice raises a presumption of opinio juris, it is arguably disingenuous or sloppy to do so without ascertaining explicitly the motives underpinning or fuelling State practice, or without, crucially, discussing the reasoning and evidence leading to the inference. Pomson suggests the Court should have used a preponderance test, and certainly spending some time elucidating how the inference was reached would have been warranted, to avoid leaving the impression of a sleight of hand.

And if the Court “will often infer the existence of opinio juris [also] from a general practice” (Brownlie, p. 24), in other cases where the Court may have proceeded inferring opinio juris from practice, there were arguably significant difference in context and circumstances. Thus in the Nuclear Weapons advisory opinion, the Court merely considered that, in relation to the role of General Assembly resolutions towards the formation of customary international law, “a series of resolutions may show the gradual evolution of the opinio juris required for the establishment of a new rule”, with the caveat, however, that it is crucial to look at content and circumstances of adoption (para 70). In Nicaragua, the Court, while considering that opinio juris can be inferred by a sufficiently widespread and general practice, it also stateed that it “must satisfy itself that the existence of the rule in the opinio juris of States is confirmed by practice” (para 184, emphasis mine). Indeed, also in Nicaragua the Court cited North Sea Continental Shelf, to note how the conduct of States must give “evidence of a belief that this practice is rendered obligatory by the existence of a rule of law requiring it” (para 207).

Thus, motives remain arguably crucial, and the nonchalant observation of the Court that the practice in question “may have been motivated in part by considerations other than a sense of legal obligation” falls short of giving satisfaction, at least to the present author, that the inference of the Court is warranted. And even if it is indeed reasonable to consider, like Judge Iwasawa does citing the International Law Commission in his separate opinion, that State practice and opinio juris may be intertwined” and that in certain circumstances “the same material may be used to ascertain practice and acceptance as law”, the key question is not whether the inference is legitimate as such, but what is sufficient evidence for the inference to be satisfactory, and that question, in my view and also in light of the Court’s own past case law, rests, for what concerns opinio juris, on an analysis of the motives underpinning or fuelling State practice. Indeed in Nicaragua, which the Court used as the authority to buttress its inference in the present case, the Court stressed how opinio juris should be deduced from the evidence “with all due caution”. Indeed, the Court would normally exercise its function carefully and without giving short shrift to any question, especially when they are the pivot for a decision.


It may well be that the conclusion the Court arrived at, that States cannot lawfully claim entitlements to an extended continental shelf that extends to within 200 nautical miles from the baselines of another State, is a sound policy decision. That, in turn, may make it a good legal decision in terms of sensing the realities in which such decision would fall. Indeed, Colombia observed in its pleadings (p. 16) how “Nicaragua’s claim has worldwide implications of the most disturbing kind”. This consideration may well have been lurking in the back of the Court’s “mind”. However, the unsatisfactory reasoning on the question of opinio juris makes the judgement at the same time a poor legal decision, especially considering all the questions it raises, including the potential implications of having created, with too much nonchalance, a “third constraint line to the entitlement to a continental shelf beyond 200 nm, as Woker observes, and having upset the “religiously” consolidated doctrine of the single continental shelf. However, it is perhaps not the first time that the Court, affected by “methodological indeterminacy”, pulls “fantastical opinio juris” out of its hat, and leave the audience in awe.

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Aljosa Noga says

August 3, 2023

Thank you for a great post! I was wondering when someone would comment on the Case in this regard. A great commentary.

Has the ICJ ever stated that State practice and opinio juris are separate criteria? I think this Case lends further support to those who have argued that opinio juris is the only relevant criteria for international law. For instance, Bin Cheng argued opinio juris is really the only relevant criteria because States create customary law, and that State practice, while an important element to consider, primarily acts as evidence of opinio juris. Practice can in this sense be utilized to supplement the lack of opinio juris when matters are, for instance, politically sensitive (such as in relation to immunity). Other research, such as interdisciplinary research in international law, including on rationality and international law, also many times support such a position due to how close such a perception of customary law lies with the interests of States who wield the power to make customary law, among other reasons. State practice is in this sense not uninteresting, but is not a separate criterion for customary law.

Whether or not the case is consistent with the ICJ's past practice seems, to me, at least be a matter of how we would interpret that case law from the past, as there does not seem to be any case which completely contrasts the court's ruling here.

Why are we so keen to demand two criteria for customary law--is it because we would not want to grant or admit such law-making powers to sovereign States and put more onus on scholarly writings? Or is it because we genuinely believe that the creation of customary international law is so constrained by strict and narrow criteria, which are many times quite difficult to meet and which remove to a greater extent the sovereign interests from the making of international law?

I do not know which is the more accurate position. A two-criteria approach seems more favorable for communitarian interests, but is also seems more of a ferenda-position when looking at contemporary and past customary law. For all the weaknesses that international law possesses, this just seems to be another such weakness.

Kind regards