Sources of international law (treaty, custom, jus cogens)

The canonical enumeration: Article 38(1) ICJ Statute

The authoritative catalogue of the sources of international law is Article 38(1) of the Statute of the International Court of Justice (1945), which reproduces Article 38 of the Statute of the Permanent Court of International Justice (1920). It directs the Court to apply, in deciding disputes:

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

A candidate must internalise the hierarchy implicit in this text. Sub-paragraphs (a)-(c) are the formal (primary) sources that create binding obligations; sub-paragraph (d) lists subsidiary means of identifying the law, not law-creating sources. Article 38(2) separately empowers the Court to decide ex aequo et bono (on grounds of equity) only if the parties consent — never invoked in a contentious ICJ judgment to date.

Treaties: the law of express consent

A treaty is, in the words of Article 2(1)(a) of the Vienna Convention on the Law of Treaties (VCLT, 1969, in force 1980), an international agreement concluded between States in written form and governed by international law. The governing principle is pacta sunt servanda (VCLT Article 26): every treaty in force binds the parties and must be performed in good faith. Article 27 forbids a State from invoking its internal law to justify non-performance.

Treaty-making proceeds through negotiation, adoption, authentication, and expression of consent to be bound — by signature, ratification, acceptance, approval or accession (VCLT Articles 11-15). A signatory that has not yet ratified is nonetheless obliged not to defeat the object and purpose of a treaty (Article 18). Reservations are permitted unless prohibited or incompatible with the object and purpose (Article 19), the test articulated in the ICJ's Reservations to the Genocide Convention advisory opinion (1951).

Custom: practice plus opinio juris

Customary international law requires two elements, settled in the ICJ's North Sea Continental Shelf cases (1969): (i) a general and consistent State practice, and (ii) opinio juris sive necessitatis — the belief that the practice is legally obligatory. Practice need not be universal but must be 'extensive and virtually uniform'. A State that consistently objects during the formation of a rule may escape it under the persistent objector doctrine, recognised obiter in the Anglo-Norwegian Fisheries case (1951). Custom binds all States regardless of consent, distinguishing it sharply from treaty law.

Jus cogens: peremptory norms that override consent

Above ordinary treaty and customary law sit peremptory norms (jus cogens), defined in VCLT Article 53 as norms 'accepted and recognized by the international community of States as a whole' from which no derogation is permitted and which can be modified only by a subsequent norm of the same character. A treaty conflicting with an existing peremptory norm is void (Article 53); a treaty conflicting with an emergent one becomes void and terminates (Article 64). The ILC's 2022 Draft Conclusions on Peremptory Norms offer an illustrative list: the prohibitions of aggression, genocide, slavery, racial discrimination and apartheid, crimes against humanity, torture, and the right to self-determination. The ICJ affirmed the prohibition of genocide as jus cogens in Armed Activities on the Territory of the Congo (2006) and torture in Belgium v. Senegal (2012).

Closely related are obligations erga omnes — owed to the international community as a whole — identified by the ICJ in the Barcelona Traction case (1970). All States have a legal interest in their protection. Jus cogens and erga omnes overlap but are not identical: the former concerns a norm's hierarchical superiority, the latter the universality of standing to invoke it.

Why this matters for the exam

This topic is the load-bearing foundation of every international-law question across the shared layer. For UPSC GS-2, sources underpin questions on India's treaty obligations, the doctrine of incorporation (Article 51(c) of the Constitution directs respect for international law and treaty obligations), and judicial use of custom in Vishaka v. State of Rajasthan (1997) and Vellore Citizens (1996). For the FSOT, expect multiple-choice items distinguishing treaty from custom and identifying VCLT default rules. CSS (Pakistan) and BCS (Bangladesh) International Law papers routinely pose the standing essay: 'Discuss the sources of international law under Article 38 of the ICJ Statute', or 'Is there a hierarchy of sources?'.

The high-yield retention set: memorise the four sub-paragraphs of Article 38(1) verbatim; the two elements of custom from North Sea Continental Shelf (1969); the VCLT articles 26 (pacta sunt servanda), 27, 53 and 64; and a stock list of jus cogens norms. The recurring PYQ angle is the distinction between formal sources (treaty, custom, general principles) and subsidiary means (judicial decisions, writings) — examiners penalise candidates who call Article 38(1)(d) a 'source'. Equally testable: there is no formal hierarchy among (a), (b) and (c), but lex specialis and lex posterior maxims resolve conflicts, and jus cogens trumps everything.