International Law: What It Is and Who Applies It

An order with no single legislator

International law is the body of rules that governs relations between states and, for some matters, how a state treats people within its own borders. According to the United Nations, its domain covers a wide range of issues: human rights, disarmament, international crime, refugees, migration, the treatment of prisoners, the use of force, the conduct of war, as well as the environment, sustainable development, international waters, outer space, global communications and world trade.

Unlike the law of a single state, international law has no parliament above states to pass it and no single government to enforce it: it grows out of the consent of states themselves, expressed in different forms.

The sources: treaties and custom

Article 38(1) of the Statute of the International Court of Justice lists the sources the Court draws on to decide a dispute. The two main ones, with no real equivalent in a single state’s domestic law, are treaties and customary international law. A treaty is a written agreement between states. It binds only the states that sign it: the Vienna Convention on the Law of Treaties, at Article 34, says a treaty creates no obligations and no rights for a third state without that state’s consent. Custom, by contrast, is written in no single text: it is a general practice accepted as law, to use the ICJ Statute’s own phrase, and it forms when a pattern of behavior becomes so widespread and repeated among states that they come to treat it as binding.

A legal norm inside a single state comes from a defined process, with a parliament that passes it and an authority that enforces it. A rule of international law instead comes from repeated conduct or from an explicit agreement between states.

Public and private international law

The international law discussed so far, the law of treaties and custom between states, is more precisely called public international law. There is also a field called private international law, but it is a different subject: it belongs to the domestic law of each country and it decides which national law applies when a specific case involves people or property from different states: a contract, a marriage, an inheritance. An Italian judge who has to decide which law governs a contract between a company in Milan and one in Lyon will apply the rules of Italian private international law, not a treaty between Italy and France.

Who applies international law

The United Nations distinguishes three bodies with different tasks. The UN Charter, signed in 1945, sets out the major principles of international relations, from the sovereign equality of states to the ban on the use of force: this is the political level, the founding text the other bodies refer back to.

Practical example: when a state joins the United Nations, it automatically becomes a party to the Statute of the International Court of Justice as well. Article 93 of the UN Charter provides for this: every UN member is a party to the Statute of the Court without signing any separate treaty. A state that is not a UN member can still accede to the Statute, but only on conditions set case by case by the General Assembly, on the recommendation of the Security Council.

The second level is judicial. The International Court of Justice (ICJ), the UN Charter’s principal judicial organ of the United Nations, sits in The Hague and rules on disputes between states. Its contentious function, governed by Article 94, produces rulings that bind the parties in the case, who can turn to the Security Council if the other side does not comply. Its advisory function, set out in Article 96, is different: the General Assembly and the Security Council can ask the Court for legal opinions on questions of law, but those opinions are not binding.

The third level is technical: the International Law Commission, as its name suggests, has the job of developing international law progressively and codifying it, putting into systematic written form rules that often already existed as custom.

On the ground, a state brings its positions before these bodies through its own diplomacy: for Italy, that task falls to the Italian Ministry of Foreign Affairs and International Cooperation.

ICJ and ICC, two courts in The Hague

There is a fourth institution, distinct from the first three because it tries not states but individuals: the International Criminal Court (ICC). It was created by the Rome Statute, adopted on 17 July 1998 and in force since 1 July 2002 after gathering 60 ratifications; by May 2026 its states parties numbered 125. It sits in The Hague, like the ICJ, and tries individuals accused of war crimes, genocide, crimes against humanity and the crime of aggression: it is the home of international criminal law, a branch separate from public international law, which governs relations between states.

Confusion between the two courts is common because both sit in The Hague, even though a case can only go before one of them.

The ICJ, for its part, did not appear out of nowhere in 1945: it took over from the Permanent Court of International Justice, established in 1920 by the League of Nations, the organization that preceded the UN. Within a single state, a system of courts with different levels and jurisdictions exists too: the entry on the Italian legal system tells that story, useful for comparison with a system, like the international one, that has no single judicial apparatus sitting above states.

Among the matters international law covers is the protection of people’s rights, an area with its own reference text in the Universal Declaration of Human Rights.