International Customary Law: Jurisprudential Analysis and Significance

I. Introduction: The Decentralized Legislative Process

International Customary Law (CIL) represents the oldest and most original source of international law. Unlike domestic legal systems where legislation is centralized, CIL is a decentralized, bottom-up process of law-making.¹ It is formally defined in Article 38(1)(b) of the Statute of the International Court of Justice (ICJ) as:

“International custom, as evidence of a general practice accepted as law.”²

it is crucial to recognize that CIL is not merely “unwritten law”; it is the manifestation of the sociological consensus of States hardened into legal obligation.

II. The Constitutive Elements (The Dualist Theory)

The orthodox view, confirmed in the North Sea Continental Shelf Cases (1969), is that CIL requires two distinct elements.³ The absence of either renders a norm mere usage or comity, not law.

**1. State Practice (Consuetudo / The Objective Element)**

This refers to the material acts of States.⁴ For practice to generate law, it must meet three criteria:

Duration: While ancient custom required immemorial antiquity, modern jurisprudence accepts that a short duration is not a bar if the practice is overwhelming.
Consistency: The practice need not be perfect, but “virtually uniform” (North Sea Cases).⁵
Generality: It must be widespread and representative, particularly including States whose interests are “specially affected.”⁶

**2. Opinio Juris sive Necessitatis (The Subjective Element)**⁷

This is the psychological belief that the practice is rendered obligatory by the existence of a rule of law requiring it.⁸ It distinguishes a legal custom (e.g., diplomatic immunity) from a mere habit of courtesy (e.g., rolling out a red carpet for heads of state).

Jurisprudential Paradox: The “Chronological Paradox”

The Problem: How can a new rule of customary law be created? For the first States to adopt a practice, they must believe they are already bound by law.⁹ If no law exists yet, their belief is false.

The Solution: Jurists like Bin Cheng argue for “Instant Custom”—in new fields like space law, opinio juris (expressed via UN Resolutions) can precede practice. Others argue that the initial acts are “proposals” that ripen into law upon acceptance (acquiescence) by other States.¹⁰

III. Significance in International Law

CIL serves functions that treaty law cannot fulfill, ensuring the international legal order remains complete (completezza).

1. Universality and Binding Force

Treaties only bind signatories (pacta tertiis nec nocent nec prosunt). CIL, however, binds all States (universal validity), regardless of consent, unless a State qualifies as a “Persistent Objector” (see below).

Case Law: In the Nicaragua v. United States (1986), the US had a reservation preventing the ICJ from applying multilateral treaties (like the UN Charter).¹¹ The Court ruled it could still adjudicate based on customary international law, which exists parallel to treaties even if the content is identical.

2. Interaction with Treaties (The Generation of Norms)

As established in the North Sea Cases, treaties interact with custom in three ways:

Declaratory: The treaty codifies existing custom (e.g., Vienna Convention on the Law of Treaties).¹²
Crystallizing: The treaty negotiation process hardens an emerging practice into law.
Generating: A treaty provision attracts widespread participation and practice by non-parties, generating a new customary norm.¹³

**3. Jus Cogens (Peremptory Norms)**¹⁴

Most norms of jus cogens (norms from which no derogation is permitted, e.g., prohibition of genocide, torture) derive their validity from CIL rather than treaties, as they bind states that might not be party to specific human rights conventions.

IV. Significance in State Practice

1. The “Persistent Objector” Rule

This rule protects the “voluntarist” nature of international law.¹⁵ If a State persistently and openly objects to a new norm while it is forming, it is not bound by it.¹⁶

Example: The US and UK were persistent objectors to the expansion of the Territorial Sea beyond 3 nautical miles, until the 12-mile limit became overwhelmingly accepted, forcing them to capitulate.
Significance: It prevents the “tyranny of the majority” in international law but is politically difficult to sustain indefinitely.

2. “Modern Custom” and the Sliding Scale

Traditional custom prioritized physical practice (what States do). Modern state practice, especially in Human Rights and Environmental Law, prioritizes verbal practice (UN General Assembly Resolutions, diplomatic statements).

Frederic Kirgis’s “Sliding Scale” Theory: If the opinio juris (moral importance) is extremely high (e.g., prohibition of torture), less physical practice is required to prove the custom exists.

3. Filling the Vacuum (Cyber and Space Law)

In domains where treaty negotiations are stalled (e.g., the weaponization of outer space or cyber warfare), State practice (and silence/acquiescence) is actively forming the “rules of the road” before any written convention exists.

V. Case Law

Case	Principle Established
*North Sea Continental Shelf (1969)*	Established the strict “two-element” test. Ruled that the “equidistance principle” was not CIL because state practice was not sufficiently uniform.
*The Lotus Case (1927)*	Permissive Principle: Whatever is not explicitly prohibited is permitted. Also ruled that “abstention” (inaction) only creates custom if there is a conscious duty to abstain.
*Nicaragua v. United States (1986)*	Parallel Existence: Customary norms (e.g., non-use of force) survive even when codified in treaties (UN Charter). Clarified that practice need not be perfect, only general; breaches treated as violations confirm the rule, not destroy it.
*Right of Passage (Portugal v. India)*	Regional Custom: Custom does not have to be global; it can be bilateral or regional (binding only two states).

VI. Critical Evaluation & Conclusion

Critique: CIL is often criticized for its vagueness (“indeterminacy”). Powerful States often shape “practice” (e.g., through military incursions) more than weaker States, leading to the critique that CIL is a tool of hegemony (TWAIL – Third World Approaches to International Law).

Conclusion:

Despite the proliferation of treaties, International Customary Law remains the “background radiation” of the international legal order.17 It is dynamic, filling gaps where treaties fail, and serves as the primary vehicle for the universalization of fundamental human rights. mastering CIL is mastering the art of arguing what the law is, in the absence of a written text.

The North Sea Continental Shelf cases (1969) are the definitive authority on how Customary International Law is formed.
The most legally significant and frequently cited point is the Court’s definition of the two-element test required for a rule to become customary law:
State Practice (objective element) and
Opinio Juris (subjective element).

the exact relevant quotations from the Judgment.
1. On Opinio Juris (The Subjective Element)
This is the most famous passage from the case. It establishes that for a practice to become law, States must follow it because they believe they are legally bound to do so, not just out of habit or convenience.
“Not only must the acts concerned amount to a settled practice, but they must also be such, or be carried out in such a way, as to be evidence of a belief that this practice is rendered obligatory by the existence of a rule of law requiring it. The need for such a belief, i.e., the existence of a subjective element, is implicit in the very notion of the opinio juris sive necessitatis. The States concerned must therefore feel that they are conforming to what amounts to a legal obligation. The frequency, or even habitual character of the acts is not in itself enough.”
— North Sea Continental Shelf, Judgment, I.C.J. Reports 1969, p. 44, para. 77.

2. On State Practice (The Objective Element)
The Court also clarified that for a treaty provision to generate customary law, the practice must be widespread, particularly among states most affected by the rule.¹²³⁴

“…an indispensable requirement would be that within the period in question, short though it might be, State practice, including that of States whose interests are specially affected, should have been both extensive and virtually u5niform in the sense of the provision invoked; — and should moreover have occurred in such a way as to 6show a general recognition that a rule of law or legal obligation is involved.”78
— North Sea Continental Shelf, Judgment, I.C.J. R9eports 1969, p. 43, para. 74.10
Summary of the Point¹¹
The Court ruled t¹²hat the “equidistance principle” (a method for drawing maritime boundaries) was not customary international law because, while some states used it, there was no evidence they did so because they felt legally compelled (opinio juris). They may have just used it because it was convenient. Therefore, Germany was not bound by it.

1. The Lotus Case (1927)
Relevant Point: The “Lotus Principle” (Permissive Nature of International Law). The Court established that international law is a system of freedom, not restriction. States are free to act in any way they choose unless there is a specific rule (treaty or custom) that explicitly prohibits that action. The burden of proof is on the party claiming a restriction exists.
Exact Quotation:
“International law governs relations between independent States. The rules of law binding upon States therefore emanate from their own free will as expressed in conventions or by usages generally accepted as expressing principles of law… Restrictions upon the independence of States cannot therefore be presumed.“
“…far from laying down a general prohibition to the effect that States may not extend the application of their laws and the jurisdiction of their courts to persons, property and acts outside their territory, it leaves them in this respect a wide measure of discretion which is only limited in certain cases by prohibitive rules.” — S.S. Lotus (France v. Turkey), 1927 P.C.I.J. (ser. A) No. 10, pp. 18–19.

2. Nicaragua v. United States (1986)
Relevant Point: Inconsistent Practice & Confirmation of Custom. The Court ruled that for a custom to exist, state practice does not need to be “perfect” or “rigorous.” Crucially, if a state breaks a rule but argues it is acting under an exception (like self-defense), that excuse actually confirms the existence of the rule rather than weakening it.
Exact Quotation:
“The Court does not consider that, for a rule to be established as customary, the corresponding practice must be in absolutely rigorous conformity with the rule. In order to deduce the existence of customary rules, the Court deems it sufficient that the conduct of States should, in general, be consistent with such rules…”
“If a State acts in a way prima facie incompatible with a recognized rule, but defends its conduct by appealing to exceptions or justifications contained within the rule itself, then whether or not the State’s conduct is in fact justifiable on that basis, the significance of that attitude is to confirm rather than to weaken the rule.” — Military and Paramilitary Activities in and against Nicaragua, Judgment, I.C.J. Reports 1986, p. 98, para. 186.

3. Right of Passage (Portugal v. India) (1960)
Relevant Point: Local or Regional Custom. The Court confirmed that customary international law is not always global. A “local custom” can exist between just two states if there is a long-standing, accepted practice between them. This local custom prevails over general international law in their specific relations.
Exact Quotation:
“It is difficult to see why the number of States between which a local custom may be established on the basis of long practice must necessarily be larger than two. The Court sees no reason why long continued practice between two States accepted by them as regulating their relations should not form the basis of mutual rights and obligations between the two States.” — Right of Passage over Indian Territory, Judgment, I.C.J. Reports 1960, p. 39.