What Are the Primary Sources of International Law and How Do They Interact in Practice?

Japanese Attorney at Law - Bengoshi L.L.

• 10 min read

International law, the framework governing relationships between sovereign states and other international actors, is a complex and dynamic field. For corporate legal professionals and international business people, a foundational understanding of where this law comes from—its sources—is crucial for navigating cross-border transactions, investments, and dispute resolution. Unlike domestic legal systems, which often have a clear hierarchy of laws stemming from a constitution and legislative bodies, international law derives from a more decentralized and multifaceted set of origins. This article explores the primary sources of international law, primarily through the lens of Article 38(1) of the Statute of the International Court of Justice (ICJ), and examines how these sources interact in practice.

Article 38(1) of the ICJ Statute: The Authoritative Starting Point

The most widely accepted enumeration of the sources of international law is found in Article 38(1) of the Statute of the ICJ, an integral part of the United Nations Charter. This provision directs the Court, whose function is to decide in accordance with international law such disputes as are submitted to it, to apply:

a. international conventions (treaties), whether general or particular, establishing rules expressly recognized by the contesting states;
b. international custom, as evidence of a general practice accepted as law;
c. the general principles of law recognized by civilized nations;
d. subject to the provisions of Article 59, judicial decisions and the teachings of the most highly qualified publicists of the various nations, as subsidiary means for the determination of rules of law.

While Article 38(1) is technically a directive to the ICJ, it is broadly considered to represent a definitive statement on the sources of international law. It's important to note that this list is not necessarily exhaustive, nor does it establish a rigid hierarchy among the primary sources (treaties, custom, and general principles), though "subsidiary means" are clearly subordinate.

1. International Treaties (Conventions)

Treaties are formal, written agreements between states (or between states and international organizations, or between international organizations themselves) that are governed by international law. They can be bilateral (between two states) or multilateral (between three or more states) and cover a vast array of subjects, from trade and investment to human rights and environmental protection.

Formation and Binding Force:
The process of treaty-making typically involves several stages:

  • Negotiation: States' representatives discuss and agree on the terms.
  • Adoption of the text: The text is finalized.
  • Authentication: The text is established as authentic and definitive, usually by signature or initialing.
  • Expression of consent to be bound: This is the crucial step that makes a treaty legally binding on a state. Common methods include:
    • Signature: For some treaties, signature alone signifies consent to be bound, especially in simpler, bilateral agreements or "executive agreements."
    • Ratification: This is a two-step process where, after signature, the state internally approves the treaty (often involving parliamentary approval, as is common in Japan for significant treaties ) and then formally confirms its consent to be bound to the other parties, usually by exchanging or depositing an instrument of ratification. The Vienna Convention on the Law of Treaties (VCLT) 1969 provides detailed rules on this.
    • Accession: This is the method by which a state that did not participate in the original negotiation and signature becomes a party to an existing treaty.
    • Acceptance or Approval: These terms are sometimes used in a manner similar to ratification or accession, often depending on a state's domestic constitutional requirements.

A treaty generally enters into force on a date specified within the treaty itself or as agreed by the negotiating states. Once in force and binding on a state, the principle of pacta sunt servanda (agreements must be kept) applies. This fundamental rule, enshrined in Article 26 of the VCLT, dictates that every treaty in force is binding upon the parties to it and must be performed by them in good faith. A state cannot invoke its domestic law as a justification for its failure to perform a treaty.

Reservations:
Multilateral treaties often deal with complex issues, and states may wish to become parties while excluding or modifying the legal effect of certain provisions in their application to them. This is done through reservations. The VCLT (Articles 19-23) provides a framework for reservations, generally permitting them unless prohibited by the treaty, if the treaty provides that only specified reservations may be made, or if the reservation is incompatible with the object and purpose of the treaty. The ICJ, in its advisory opinion on Reservations to the Convention on Genocide (Judgment of May 28, 1951), established the "object and purpose" test as a key criterion for the permissibility of reservations. The interplay of reservations and objections by other states creates a complex web of bilateral relations under a multilateral treaty.

Interpretation:
The VCLT (Articles 31-33) also codifies rules for treaty interpretation. Article 31(1) states that a "treaty shall be interpreted in good faith in accordance with the ordinary meaning to be given to the terms of the treaty in their context and in the light of its object and purpose." This emphasizes a textual approach complemented by systemic and teleological considerations. Subsequent agreements and practice, as well as relevant rules of international law applicable between the parties, are also to be taken into account (Article 31(3)). If these primary means leave the meaning ambiguous or lead to a manifestly absurd or unreasonable result, recourse may be had to supplementary means of interpretation, including the preparatory work of the treaty (travaux préparatoires) and the circumstances of its conclusion (Article 32).

2. Customary International Law

Customary international law results from a general and consistent practice of states followed by them from a sense of legal obligation. It is a particularly important source because its rules bind all states (except for persistent objectors, a concept discussed below), regardless of whether they have affirmatively consented to them in a treaty.

The Two Elements:
The formation of a customary rule requires two elements:

  • State Practice (usus or diuturnitas): This is the objective element, referring to the consistent and widespread conduct of states. State practice can be evidenced by a variety of materials, including diplomatic correspondence, policy statements, press releases, opinions of official legal advisers, national laws, decisions of national and international courts, treaty practice (where treaties are indicative of custom), and the practice of international organizations. The practice need not be of ancient vintage; even a short period can suffice if the practice is extensive and virtually uniform, particularly if it involves states whose interests are specially affected (as stated by the ICJ in the North Sea Continental Shelf Cases, Judgment of February 20, 1969). Both actions and omissions (abstentions from acting) can constitute state practice.
  • Opinio Juris sive Necessitatis (acceptance as law): This is the subjective, psychological element. States must engage in the practice (or abstain from acting) out of a belief that they are legally obligated or entitled to do so, not merely out of courtesy, convenience, or tradition. The ICJ in the Lotus case (Judgment of September 7, 1927) emphasized that abstention from exercising jurisdiction, for example, could only lead to a customary rule if it was based on a consciousness of a duty to abstain. Proving opinio juris can be challenging but can be inferred from the nature of the practice, public statements by states, votes on UN General Assembly resolutions concerning legal questions, and other evidence indicating a state's legal conviction.

The Persistent Objector:
A state that has consistently and openly objected to a developing rule of customary international law from the outset of its formation may not be bound by that rule once it crystallizes. This is known as the "persistent objector" rule. However, the threshold for establishing persistent objector status is high, requiring clear and sustained objection. Moreover, this rule generally does not apply to peremptory norms of international law (jus cogens).

Interaction with Treaties:
Treaties and custom are not mutually exclusive sources; they constantly interact.

  • Codification: A treaty may codify existing customary international law (e.g., much of the VCLT). In such cases, the treaty provisions will bind non-parties as custom, and parties as both treaty and custom.
  • Crystallization: A treaty-making process, or the widespread adoption of a treaty, can crystallize an emerging rule of custom—meaning it solidifies a rule that was in the process of formation. The ICJ in the North Sea Continental Shelf Cases discussed this possibility.
  • Generation of New Custom: A treaty, particularly a widely ratified multilateral one that enunciates new legal principles, may form the basis for the development of new customary international law if its provisions are subsequently followed by extensive and uniform state practice accompanied by opinio juris, even by non-parties.

3. General Principles of Law Recognized by Civilized Nations

Article 38(1)(c) refers to "the general principles of law recognized by civilized nations." The term "civilized nations," a relic of a past era, is now understood to mean general principles of law recognized in the principal legal systems of the world, including both common law and civil law traditions, as well as other major legal systems.

The primary purpose of this source is to avoid a non liquet (a situation where a court cannot decide a case due to a lack of applicable law) by allowing the ICJ to draw upon principles common to domestic legal orders when treaties or custom do not provide a clear rule. These are not principles of "international law" in the first instance, but rather principles of legal logic or procedure that are so fundamental that they are found in most, if not all, developed national legal systems.

Examples of general principles of law that have been applied in international law include:

  • The principle of good faith (invoked in the Nuclear Tests Cases, Judgments of December 20, 1974).
  • The principle of res judicata (a matter once judged cannot be re-litigated between the same parties).
  • The principle of estoppel (a party may be prevented from going back on a previous representation or conduct if another party has relied on it to their detriment).
  • The obligation to make reparation for a breach of an engagement (articulated in the Chorzów Factory Case, Judgment of September 13, 1928).
  • Principles related to the administration of justice, such as the equality of parties before a tribunal and the inadmissibility of evidence obtained by illegal means.

The process of identifying these principles involves a comparative law exercise, though the ICJ has not always explicitly undertaken such detailed surveys, sometimes relying on the shared understanding of its judges representing the world's principal legal systems.

4. Subsidiary Means: Judicial Decisions and Teachings of Publicists

Article 38(1)(d) identifies judicial decisions and the teachings of the most highly qualified publicists as "subsidiary means for the determination of rules of law." This means they are not sources of law in themselves but can be used to help identify and interpret rules found in treaties, custom, or general principles of law.

Judicial Decisions:
This includes decisions of international courts and tribunals, such as the ICJ, the International Tribunal for the Law of the Sea (ITLOS), ad hoc arbitral tribunals, and even national courts when they deal with questions of international law. Under Article 59 of the ICJ Statute, the Court's decisions have no binding force except between the parties and in respect of that particular case; there is no formal doctrine of stare decisis (binding precedent) in international law.
However, in practice, ICJ judgments and advisory opinions are highly influential and are frequently cited by the Court itself (for consistency and legal stability), by other international tribunals, by states, and by scholars as evidence of the state of the law. They play a significant role in clarifying, interpreting, and even developing international law. Decisions of national courts can also provide evidence of state practice and opinio juris for the purpose of identifying customary rules.

Teachings of the Most Highly Qualified Publicists:
The writings of respected international law scholars can also serve as a subsidiary means for determining rules of law. Historically, in the formative stages of international law, the works of jurists like Grotius, Vattel, and Suarez were highly influential in shaping the law. Today, while individual writers may have less direct impact, the collective work of learned societies (e.g., the International Law Commission (ILC) in its codification efforts, the Institut de Droit International, the International Law Association) and the writings of eminent scholars can provide valuable analysis, evidence of custom, and insights into the interpretation and development of international law. The ICJ itself has occasionally referred to scholarly works, although less frequently than to judicial decisions.

Interaction and Hierarchy of Sources

While Article 38(1) lists the sources, it does not explicitly establish a strict hierarchy among treaties, custom, and general principles. In practice, however, certain principles of interaction and precedence are recognized:

  • Jus Cogens: Peremptory norms of general international law (jus cogens) are norms accepted and recognized by the international community of states as a whole as norms from which no derogation is permitted and which can be modified only by a subsequent norm of general international law having the same character (VCLT, Article 53). Examples include the prohibition of aggression, genocide, slavery, and torture. A treaty is void if, at the time of its conclusion, it conflicts with a peremptory norm. Similarly, if a new peremptory norm emerges, any existing treaty which is in conflict with that norm becomes void and terminates (VCLT, Article 64). Jus cogens norms thus represent a superior category of international law.
  • Lex Specialis Derogat Legi Generali: A special rule prevails over a general rule. This means that if a matter is governed by a specific treaty provision between certain states, that provision will typically apply between them rather than a more general customary rule on the same subject, provided the customary rule is not jus cogens. However, the general rule continues to govern matters not covered by the special rule and relations with states not party to the special rule.
  • Lex Posterior Derogat Legi Priori: A later rule prevails over an earlier rule. If two treaties between the same parties conflict, the later treaty will generally prevail to the extent of the inconsistency (VCLT, Article 30(3)). Similarly, a new rule of customary international law can supersede an earlier treaty or customary rule between states that have accepted the new custom, again, unless the earlier rule is jus cogens. The PDF mentions that where a treaty is concluded after a custom, the treaty prevails unless the custom is jus cogens.
  • Relationship between Treaties and Custom: As discussed, these sources are in constant interplay. A treaty can codify, crystallize, or help generate custom. Conversely, subsequent custom can modify or even terminate a treaty between parties who participate in the new custom (though this is a complex and debated area, especially regarding the VCLT's rules on treaty termination). The ICJ in the Nicaragua v. USA case (Merits, Judgment of June 27, 1986) affirmed that customary law and treaty law can exist side by side, even if their content is similar.

Conclusion

The sources of international law—primarily treaties, customary international law, and general principles of law, with judicial decisions and scholarly teachings as subsidiary means—form a complex but coherent system. There is no single legislative body in international law; rather, law emerges from the consent of states (explicitly in treaties, implicitly in custom) and from foundational principles shared across legal systems. Understanding these sources and their dynamic interaction is fundamental for anyone operating in the international arena, as it provides the basis for identifying applicable rules, assessing legal obligations, and resolving international disputes. The system is constantly evolving, with new treaties being concluded and customary practices adapting to new global challenges, requiring ongoing attention from legal professionals and business leaders alike.