Sources of International Law
The sources of international law are the formal repositories from which the rights and obligations of states are drawn. The classic enumeration is Article 38(1) of the Statute of the International Court of Justice 1945, which directs the Court to apply:
- International conventions (treaties) — general or particular.
- International custom, as evidence of a general practice accepted as law.
- General principles of law recognised by civilised nations.
- Judicial decisions and teachings of the most highly qualified publicists — as subsidiary means.
The foundational catalogue of international-law sources. Although addressed to the ICJ, it is universally accepted as the authoritative list of sources. Equity (ex aequo et bono) is admitted only by consent of the parties under Article 38(2).
Treaties
Treaties — also called conventions, covenants, protocols, agreements — are the principal source of modern international law. The Vienna Convention on the Law of Treaties 1969 (VCLT) governs their formation, interpretation and termination. Treaties bind only states parties (pacta tertiis nec nocent nec prosunt — VCLT Art. 34) and rest on pacta sunt servanda (VCLT Art. 26 — agreements must be kept in good faith).
A treaty may be:
- Law-making (UN Charter 1945, Vienna Conventions, Geneva Conventions 1949) — codifies general norms.
- Treaty-contracts — bilateral, regulating specific issues.
Custom
Customary international law is created by:
- State practice — actual conduct of states (diplomatic acts, legislation, decisions, military operations).
- Opinio juris sive necessitatis — belief that the practice is legally required.
The North Sea Continental Shelf Cases (Germany v. Denmark; Germany v. Netherlands) ICJ Reports 1969 confirmed both requirements. The Nicaragua Case (Nicaragua v. USA) ICJ Reports 1986 showed that custom can persist alongside identical treaty law and bind non-parties.
- A persistent objector is not bound by an emerging custom if it consistently dissents (Anglo-Norwegian Fisheries Case ICJ 1951).
- Instant custom: rapid crystallisation, e.g. outer-space law from 1957–1967.
- Local/regional custom — between particular states (Asylum Case (Colombia v. Peru) ICJ 1950).
- Jus cogens — peremptory norms (VCLT Art. 53) like prohibition of genocide, slavery, aggression — no derogation permitted.
- Erga omnes obligations — owed to the international community (Barcelona Traction Case ICJ 1970).
General principles of law
Principles common to major legal systems — equity, estoppel, good faith, res judicata, unjust enrichment. The Chorzów Factory Case (Germany v. Poland) PCIJ 1928 invoked the principle that breach entails reparation. The Corfu Channel Case (UK v. Albania) ICJ 1949 applied principles of circumstantial evidence and state responsibility.
Subsidiary means
- Judicial decisions of the ICJ, PCIJ, ITLOS, WTO Appellate Body, regional courts (ECHR, IACHR), arbitral tribunals. Article 59 ICJ Statute denies formal precedent, but de facto the Court follows its own jurisprudence.
- Teachings of publicists — Grotius (De Jure Belli ac Pacis 1625), Vattel (Le Droit des Gens 1758), Oppenheim, Brownlie, Shaw — historically more influential than today.
Soft law
A growing category not in Article 38: declarations, General Assembly resolutions, codes of conduct, guidelines. The Universal Declaration of Human Rights 1948, although not a treaty, attained quasi-customary force. The Stockholm Declaration 1972 and Rio Declaration 1992 crystallised into binding environmental norms.
Hierarchy of sources
Strictly, the sources in Article 38(1)(a)–(c) are of equal rank; (d) is subsidiary. In practice:
| Norm type | Examples | Effect |
|---|---|---|
| Jus cogens | Genocide, slavery, aggression, torture | Override all other norms |
| UN Charter | Article 103 | Prevails over other treaty obligations |
| Treaty | VCLT, Geneva Conventions | Binding on parties |
| Custom | Diplomatic immunity | Binding on all states (subject to persistent objector) |
| Soft law | GA resolutions, declarations | Evidence of practice, opinio juris |
For CSS, always begin a "sources" answer by quoting Article 38(1) of the ICJ Statute verbatim, then explain each source with one named case: treaty (Reparations for Injuries 1949), custom (Nicaragua 1986), general principles (Chorzów Factory 1928), judicial decisions (Asylum Case 1950), publicists (Grotius). Mention jus cogens (Art. 53 VCLT) for higher marks.
Codification and progressive development
The International Law Commission (ILC), created in 1947 under UN Charter Art. 13(1)(a), codifies and progressively develops international law. Its products include the VCLT 1969, Articles on State Responsibility 2001, Articles on Diplomatic Protection 2006 and Guide to Practice on Reservations 2011. Many ILC drafts are influential even before becoming treaties — the Articles on State Responsibility are universally cited though not yet a treaty.
Equity in international law
Article 38(2) permits the ICJ to decide ex aequo et bono — but only with party consent (never invoked). Equity within the law (infra legem) is more common: the Tunisia/Libya Continental Shelf Case (1982) and North Sea Continental Shelf Cases (1969) invoked equitable principles to delimit maritime boundaries.