What Is International Law?

International law has recognised sources, set out most clearly in the statute of the International Court of Justice. The two main ones are treaties, which are written agreements between states, and customary international law, which arises from consistent state practice followed out of a sense of legal obligation.

General principles recognised across legal systems, judicial decisions and scholarly writing serve as further sources. Notably there is no legislature. International law is made by states agreeing to it rather than being imposed on them.

The field has expanded enormously over the last century:

• •The law of treaties, governing how agreements are made and interpreted
• •The law of armed conflict, limiting conduct in war
• •Human rights law, covering how states treat individuals
• •The law of the sea, governing territorial waters and shipping
• •Trade law, administered largely through the World Trade Organization
• •International criminal law, prosecuting individuals for the gravest crimes

This is the honest weak point. There is no police force and no compulsory jurisdiction in most cases, since the International Court of Justice generally requires states to consent to being sued.

Enforcement therefore relies on other mechanisms: reciprocity, since a state that breaks a rule invites others to do the same to it, reputation and the ability to make future agreements, economic sanctions, and domestic courts giving effect to international obligations. These are weaker than domestic enforcement but they are not nothing.

Despite the enforcement gap, compliance rates with most international law are actually high. Trade rules, postal and aviation agreements, diplomatic immunity and maritime boundaries are followed routinely, without drama.

The reason is largely self interest. Most international law solves coordination problems that states genuinely want solved, and a state that becomes unreliable finds it harder to make agreements it needs. Compliance is visible in the boring areas and violations are visible in the dramatic ones, which distorts the general impression.

The failures are real and concentrated where power is involved. Powerful states can violate rules with limited consequence, the Security Council veto blocks action against permanent members and their allies, and international criminal prosecutions have disproportionately targeted weaker states.

Critics reasonably ask whether something unenforceable against the powerful deserves the name law at all. The usual response from international lawyers is that it functions as law most of the time in most areas, and that a system working imperfectly in the hardest cases is different from a system that does not exist.

For most of its history international law governed states alone, and individuals had no standing in it whatsoever. That changed substantially after the Second World War.

The Nuremberg trials established that individuals could be held personally responsible for crimes under international law, rejecting the defence that they were following state orders. Human rights treaties then gave individuals rights they could invoke, and some regional systems allow people to bring cases directly. The International Criminal Court, established in 2002, prosecutes individuals rather than states, though several major powers have not joined it.

The law of the sea shows how international law divides up space in practice. Under the United Nations Convention on the Law of the Sea, agreed in 1982, a coastal state has sovereignty over a territorial sea extending up to 12 nautical miles from its coast, though foreign ships keep a right of innocent passage. Beyond that, up to 200 nautical miles, lies an exclusive economic zone where the coastal state controls fishing, oil, gas and other resources, while other states keep freedoms such as navigation and laying cables.

The convention also created ways to settle disputes, including a specialist tribunal and arbitration. In 2016 an arbitral tribunal ruled in a case brought by the Philippines against China over the South China Sea, finding that China's broad historical claims to resources within its so-called nine-dash line had no legal basis under the convention. China rejected the ruling and did not take part in the proceedings. The case illustrates both sides of the system: clear rules and a formal judgement, but limited means to enforce the outcome against a powerful state.

Modern international law has roots in seventeenth-century Europe. The Dutch jurist Hugo Grotius argued in Mare Liberum, published in 1609, that the open sea could not be owned by any nation, an idea that still underlies freedom of navigation. His larger work On the Law of War and Peace, published in 1625, set out rules for when war was justified and how it should be fought. The Peace of Westphalia in 1648, which ended the Thirty Years War, is often taken as a symbol of a system of sovereign states treating each other as legal equals, though historians debate how new that really was.

One basic principle is pacta sunt servanda, Latin for 'agreements must be kept'. The Vienna Convention on the Law of Treaties of 1969 states it formally and sets out how treaties are made, interpreted and ended. The same convention recognises a small category of peremptory norms, known as jus cogens, that no treaty can override, and a treaty conflicting with one of them is void. The prohibitions on genocide, slavery, torture and aggressive war are widely accepted examples. Two states cannot lawfully agree between themselves to set them aside.

Customary law can sound vague, so it helps to watch one take shape. In 1945 President Truman proclaimed that the United States had jurisdiction over the natural resources of its continental shelf, the shallow seabed extending out from its coasts, largely because of interest in offshore oil. No treaty allowed this. Yet other coastal states quickly made similar claims, and few objected, since most of them stood to gain. Within little more than a decade the idea had been written into a 1958 Geneva convention, and courts came to treat the basic principle as customary law.

The episode shows the two ingredients lawyers look for. One is state practice: what governments actually do, claim and accept, shown in legislation, official statements and the way they treat each other. The other, called opinio juris, is evidence that states act this way because they believe the law requires or permits it, not merely out of habit or courtesy. A state that objects clearly and consistently while a custom is forming may avoid being bound by it, under the persistent objector rule, though that escape route does not apply to the most fundamental norms.

Whether a treaty can be used in a country's own courts depends on how that country links international and national law, and systems broadly follow one of two approaches. In dualist systems, such as the United Kingdom, a treaty signed by the government creates obligations between states but changes nothing at home until Parliament passes a law putting it into effect, so a British court cannot normally enforce an unincorporated treaty. In monist systems, such as the Netherlands, ratified treaties become part of the domestic legal order directly, and Dutch courts can even set aside national legislation that conflicts with treaty provisions binding on everyone.

Many countries mix the two, and the United States shows how complicated that can be. Its Constitution calls treaties part of the supreme law of the land, yet American courts distinguish between treaties that apply on their own and those that need Congress to act first. In 2004 the International Court of Justice ruled that the United States had failed to tell dozens of Mexican nationals sentenced to death of their right to contact their consulate, and ordered their cases reviewed. In Medellín v Texas in 2008, the Supreme Court held that the ruling was not directly enforceable in American courts without legislation, and Texas went ahead with the execution.

International law is built from treaties and customary practice rather than legislation, it lacks centralised enforcement and relies on reciprocity and self interest, and it works well in routine areas while struggling exactly where powerful states are involved.