Treaties vs. customary law; how treaties are made

The Two Principal Sources

Article 38(1) of the Statute of the International Court of Justice enumerates the sources the Court applies: (a) international conventions (treaties), (b) international custom "as evidence of a general practice accepted as law," (c) general principles of law recognized by civilized nations, and (d) judicial decisions and the teachings of the most qualified publicists as subsidiary means. Treaties and custom are the two primary law-creating sources; the distinction between them is the spine of this lesson.

Treaties: Consent in Writing

A treaty is an agreement concluded between States in written form and governed by international law (Vienna Convention on the Law of Treaties, 1969, Article 2(1)(a)). Treaties bind only the parties that consent to them — pacta tertiis nec nocent nec prosunt, codified in VCLT Articles 34–38: a treaty creates neither obligations nor rights for a third State without its consent. The governing maxim is pacta sunt servanda (VCLT Article 26): every treaty in force is binding upon the parties and must be performed in good faith. A party may not invoke its internal law to justify failure to perform (VCLT Article 27).

Custom: Practice Plus Belief

Customary international law requires two elements. First, general and consistent State practice (the objective element). Second, opinio juris sive necessitatis — the belief that the practice is legally obligatory (the subjective element). The ICJ insisted on both in the North Sea Continental Shelf cases (Germany v. Denmark; Germany v. Netherlands, 1969), holding that even widespread practice does not crystallize into custom absent opinio juris, and that treaty rules can pass into custom if accompanied by such belief. The Asylum case (Colombia v. Peru, 1950) shows that practice must be constant and uniform. A State that objects clearly and consistently while a rule is forming — the persistent objector — may escape its application, as discussed in the Anglo-Norwegian Fisheries case (1951).

How the Two Interact

Treaty and custom are not hierarchically ranked; they interact dynamically. A treaty may codify existing custom (much of the VCLT itself, and the 1958 Geneva Conventions on the Law of the Sea), crystallize an emerging rule, or generate new custom through subsequent widespread adoption (the North Sea test). Conversely, custom may modify treaty obligations over time. When norms conflict, lex posterior derogat priori (the later rule prevails between the same parties) and lex specialis derogat generali (the specific governs the general) guide resolution. Standing above both are peremptory norms — jus cogens (VCLT Articles 53 and 64) — such as the prohibitions of genocide, slavery, torture and aggression, from which no derogation is permitted and which void any conflicting treaty.

The Treaty-Making Sequence

The VCLT 1969 sets out a clear procedural chain. Candidates must be able to recite the stages in order.

1. Negotiation and adoption. Representatives with full powers (VCLT Article 7) negotiate the text. Heads of State, Heads of Government and Foreign Ministers are presumed to represent their State without producing full powers (Article 7(2)). Adoption of the text normally requires the consent of all participating States, or a two-thirds majority at an international conference (Article 9).

2. Authentication. The text is established as definitive and not subject to further alteration, by signature, initialling or other agreed procedure (Article 10).

3. Consent to be bound. Article 11 lists the means: signature, exchange of instruments, ratification, acceptance, approval or accession. Signature alone may bind where the treaty so provides; otherwise ratification — the formal confirmation by the competent domestic organ — is required (Article 14). Even before ratification, a signatory must not defeat the object and purpose of the treaty (the interim obligation, Article 18). The United States signed the Rome Statute in 2000 and the Kyoto Protocol but ratified neither; in 2002 it formally "unsigned" the Rome Statute to escape Article 18.

4. Reservations. A reservation is a unilateral statement purporting to exclude or modify the legal effect of certain provisions (Article 2(1)(d)). Reservations are permitted unless prohibited by the treaty or incompatible with its object and purpose (Article 19) — the test laid down by the ICJ in Reservations to the Genocide Convention (Advisory Opinion, 1951).

5. Entry into force occurs as the treaty provides, or upon consent of all negotiating States (Article 24). Multilateral treaties typically specify a threshold: the VCLT itself entered into force on 27 January 1980 after the 35th ratification.

6. Registration. UN Charter Article 102 requires treaties to be registered with the Secretariat; an unregistered treaty cannot be invoked before any UN organ, including the ICJ.

Why This Matters for the Exam

This topic is core to UPSC GS-2 (international relations and institutions), the FSOT "world history and government" segment, and the CSS/BCS international-law papers. Examiners test it three ways. First, direct recall: the VCLT articles for ratification, reservations and entry into force, and the two-element test for custom. UPSC and CSS routinely ask candidates to distinguish pacta sunt servanda from jus cogens or to define opinio juris. Second, application: a fact pattern (a State signs but does not ratify; a reservation is challenged) on which you deploy Article 18 or the Genocide compatibility test. Third, analytical essays on whether treaties or custom better bind powerful States. High-yield retainables: VCLT 1969 entered into force 1980; Article 26 (pacta sunt servanda), Article 53 (jus cogens); the North Sea Continental Shelf two-element test; ICJ Statute Article 38. Cite the article numbers — examiners reward precision over paraphrase.