Public International Law: Rules Between States

Public international law is the body of rules that governs the relationships between sovereign states and, increasingly, international organizations and individuals. Unlike domestic law, which operates within a single country under a legislature and police power, international law functions in a decentralized community of equals. No world parliament enacts it, and no world police force enforces it, yet states observe its commands most of the time because doing so serves their interests and preserves the predictability on which diplomacy, trade, and travel depend.

The classical vision of the field emerged in early modern Europe, when writers such as Hugo Grotius argued that even sovereigns were bound by a law of nations derived from reason and custom. The Peace of Westphalia in 1648 is conventionally treated as the founding moment of the modern state system, in which each state exercises supreme authority within its own territory and accepts legal limits only by consent or long-settled practice. That tension between sovereignty and obligation remains the central drama of the subject.

Modern international law touches matters as varied as the delimitation of maritime boundaries, the treatment of foreign investment, the conduct of armed conflict, the protection of the atmosphere, and the prosecution of piracy. Its rules range from grand principles, such as the prohibition on the aggressive use of force, to highly technical regimes governing postage, aviation, and the allocation of satellite orbits.

Sources of Obligation

Lawyers conventionally locate the sources of international law in a famous list drawn from the statute of the International Court of Justice. Treaties come first: written agreements in which states undertake defined commitments, from bilateral extradition pacts to universal conventions on the law of the sea. Custom comes second, formed when states act in a consistent way out of a belief that the law requires it, a conviction known by the Latin phrase opinio juris. The list is completed by general principles of law common to major legal systems, and by the subsidiary means of judicial decisions and scholarly writing.

Some norms occupy a higher rank. Peremptory rules, often called jus cogens, forbid conduct such as genocide, slavery, torture, and wars of aggression. A treaty that conflicts with such a norm is void, and no state may contract out of them. This hierarchy marks a quiet revolution: the system recognizes that certain values belong to the international community as a whole rather than to any particular bargain.

Subjects and Enforcement

States remain the primary subjects of international law, but they no longer stand alone. International organizations such as the United Nations possess legal personality and may conclude agreements of their own. Individuals may bring claims before human rights bodies, and international criminal tribunals hold persons rather than governments responsible for atrocities. Multinational enterprises, though not full subjects, are deeply affected by rules on investment and trade.

Enforcement remains the system's acknowledged weakness and its peculiar strength. Because there is no central sheriff, compliance rests on self-interest, mutual dependence, and the desire of states to be regarded as law-abiding. Countermeasures, sanctions, arbitration, and adjudication supply more formal tools, while domestic courts frequently apply international rules once legislatures incorporate them. The result is a legal order that is real but fragile, maintained less by coercion than by the accumulated habit of cooperation.

This overview is educational and not legal advice.