Customary international law

• 1. Introduction

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  Customary international law governs the conduct of nations through rules that no state ever formally signed, yet virtually every state is expected to follow. How does a rule become binding on countries that never voted for it, never ratified it, and may not even have a written law about it at home? That question sits at the heart of one of the oldest and most contested corners of international legal theory.

  The International Court of Justice, the United Nations, and legal scholars across the world treat custom alongside treaties and general principles as among the primary sources of international law. Yet customary rules are typically less definite in their formulation, which leaves them subject to real doubt in practice. What counts as a rule, who decides, and what happens when a state simply refuses? Those are the questions this documentary will work through.

• 2. How Custom Becomes Law

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  Two requirements must both be satisfied before a rule qualifies as customary international law. First, there must be a state practice that appears to be sufficiently widespread, representative, and consistent, showing that a significant number of states have used and relied on the rule and that a significant number of states have not rejected it. Second, states must be motivated by a belief that they are legally compelled to accept the rule, a condition known as opinio juris, meaning "opinion of law".

  The International Court of Justice confirmed both elements in the Legality of the Threat or Use of Nuclear Weapons case. In an earlier ruling, the North Sea Continental Shelf case, the Court had laid out the psychological dimension with particular care. It held that acts must not only amount to a settled practice but must be carried out in a way that shows a belief this practice is rendered obligatory by the existence of a rule of law requiring it. The Court was emphatic that states must feel they are conforming to a legal obligation, not merely acting out of courtesy, convenience, or tradition. That same standard was later confirmed in Nicaragua v. United States of America.

  In 1950, the International Law Commission listed treaties, decisions of national and international courts, national legislation, opinions of national legal advisors, diplomatic correspondence, and the practice of international organizations as forms of evidence of customary international law. In 2018, the Commission adopted its Conclusions on Identification of Customary International Law with commentaries, and the United Nations General Assembly welcomed those Conclusions and encouraged their widest possible dissemination.

• 3. Jus Cogens: The Highest Tier

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  Some customary rules sit at a level so fundamental that no treaty and no conflicting law can override them. These are called peremptory norms, or jus cogens, a Latin phrase meaning "compelling law". The international community accepts them as non-derogable: no exceptions are permitted, and any rule that conflicts with a jus cogens norm is considered null and void.

  Historically, the strongest impetus for developing this concept came from the abuses carried out by the Nazis during World War II. Legal thinkers reacted to those atrocities by arguing that certain obligations had to stand above even the written agreements of sovereign states. The examples the source identifies include piracy, slavery, torture, genocide, war of aggression, and crimes against humanity. A state violates customary international law if it permits or engages in any of these acts.

  Jus cogens and ordinary customary international law are not interchangeable categories. All jus cogens norms qualify as customary international law through their adoption by states, but only a small subset of customary rules rise to the peremptory level. States can depart from ordinary customary law by signing treaties or enacting domestic legislation that conflicts with it. Jus cogens allows no such departure, which is precisely what distinguishes the two tiers.

• 4. Codification and the Laws of War

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  The laws of war, known in legal terminology as jus in bello, offer the clearest historical example of customary rules eventually being written down. Those rules existed as customary law long before they were codified in the Hague Conventions of 1899 and 1907 and then in the Geneva Conventions.

  Codification did not retire the role of custom in armed conflict. The Hague and Geneva Conventions do not claim to cover every legal question that can arise during war. Article 1(2) of Additional Protocol I fills the gap directly: customary international law governs legal matters concerning armed conflict that no other agreement addresses. In 1993, the United Nations Security Council formally adopted the Geneva Conventions as customary international law, meaning states that had never ratified those conventions were still expected to observe their provisions in good faith.

  The Vienna Convention on the Law of Treaties is a parallel case. It is widely described as codifying customary international law concerning treaties. Where a treaty codifies custom, or where a treaty has itself ripened into custom over time, it binds even states that never signed it, provided those states are not persistent objectors.

• 5. Silence, Consent, and Objection

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  Ordinary treaties require consent: a sovereign nation must sign and ratify in order to be bound. Customary international law works differently. A rule can become pervasive enough internationally that countries need not consent to be bound by it. All that is required is that a state has not objected to the rule.

  The "silence implies consent" principle carries a logical complication that legal scholars have noted. In a dispute with a nation that has not itself affirmed that principle, invoking it involves circular reasoning. The argument runs: customary international law is binding because silence implies consent, and silence implies consent because that very fact is one aspect of customary international law. The circularity matters because it shows the limits of the principle in adversarial settings.

  A state that has consistently and persistently objected to a customary rule may escape being bound by it, unless the rule has risen to the level of jus cogens. At that point, the persistent objector doctrine offers no refuge. The Right of Passage Over Indian Territory case, decided by the International Court of Justice and involving Portugal and India, illustrates the bilateral end of the spectrum. The Court found no reason why a long-continued practice between two states, accepted by them as regulating their relations, should not form the basis of mutual rights and obligations between those two states.

• 6. Scope and Regional Variation

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  Customary international law applies equally to all states as a general principle, but its reach is not uniform. Recognition can range from a bilateral arrangement between two countries all the way to worldwide multilateral acceptance. Regional customs can become binding customary law within their own regions without extending to nations outside them.

  Beyond armed conflict and treaty law, the source identifies immunity of visiting foreign heads of state and the principle of non-refoulement as other examples accepted or claimed as customary international law. Non-refoulement holds that a state may not return a person to a territory where they face serious harm. These examples illustrate how custom operates across domains as varied as diplomatic protocol and the treatment of people seeking refuge.

  The Article 38(1)(b) definition in the Statute of the International Court of Justice, incorporated into the United Nations Charter by Article 92, frames custom as "evidence of a general practice accepted as law". That framing has a deliberate direction to it: general practice demonstrates custom, not the reverse. To prove a customary rule exists, one must show a general practice that conforms to the rule and is accepted as law, a standard that keeps the analysis grounded in what states actually do rather than what scholars argue they should do.