Skeptics like to ask whether international law is law at all. There is no world legislature, no global police force and no court with automatic power over every state. Yet foreign ministries employ large teams of lawyers, and states rarely admit to breaking the law even when they plainly do. That alone suggests the rules matter.
Where international law comes from
The Dutch jurist Hugo Grotius is often called the father of international law; his On the Law of War and Peace (1625) argued that rules bind states even in war. Today the standard list of sources is Article 38 of the Statute of the International Court of Justice. The main sources of international law are treaties, customary law and general principles of law, with court decisions and the writings of leading scholars as secondary aids. Treaties bind the states that join them, under the old principle pacta sunt servanda, agreements must be kept. Customary international law forms when states follow a practice widely and do so out of a sense of legal obligation, which lawyers call opinio juris. The immunity of diplomats, for instance, was custom for centuries before it was written into a treaty in 1961.
Why states comply
Without a police force, compliance rests on other things. The American legal scholar Louis Henkin observed in 1968 that almost all states keep almost all of their international obligations almost all of the time. States comply mainly because of reciprocity, reputation and the convenience of shared rules: a state that mistreats foreign diplomats or ignores a trade ruling can expect the same treatment in return. Realists answer that compliance is easy when the rules ask for what states wanted anyway, and that the hard cases, such as the use of force by great powers, are exactly where the law is weakest.
The United Nations and the use of force
The UN Charter, signed in San Francisco in June 1945, was written to prevent another world war. Article 2(4) of the UN Charter forbids states to threaten or use force against the territorial integrity or political independence of any state. The Charter allows only two exceptions to this ban: self-defense against an armed attack, under Article 51, and force authorized by the Security Council. The Council has 15 members, five of them permanent: China, France, Russia, the United Kingdom and the United States, each with a veto. The veto was the price of keeping the great powers inside the organization, unlike the League of Nations, which the United States never joined. Critics note that it also means the Council cannot act against any permanent member or its close friends.
The International Court of Justice in The Hague settles legal disputes between states. The International Court of Justice can hear a dispute only when the states involved have accepted its jurisdiction, whether by a special agreement, a treaty clause or a general declaration.
The individual in international law
For most of history international law concerned states, and what a government did to its own people was its own affair. That changed after 1945. The Nuremberg trials held individuals, not only states, criminally responsible for aggression and atrocities, and the Universal Declaration of Human Rights of 1948 set common standards for how every person should be treated.
Courts and campaigners have since pushed further. In 1998 the former Chilean dictator Augusto Pinochet was arrested in London on a Spanish warrant. In 1999 Britain's highest court ruled that Pinochet had no immunity from extradition for torture, because the Convention against Torture made it an international crime. He was eventually sent home on health grounds, but the case showed that a former head of state could be held answerable abroad.
Activists also learned to work across borders. Margaret Keck and Kathryn Sikkink described what they called the boomerang pattern. In the boomerang pattern, activists blocked at home enlist foreign allies who pressure their government from outside. In 2005 the UN World Summit endorsed the responsibility to protect, the idea that sovereignty includes a duty to protect one's own population, though its use in Libya in 2011 proved deeply controversial.
Worked example: Nicaragua v. United States
In the early 1980s the United States armed the contras, rebels fighting Nicaragua's leftist Sandinista government, and the CIA helped mine Nicaraguan harbors. Nicaragua took the United States to the International Court of Justice. The United States argued that the Court lacked jurisdiction and that it was acting in collective self-defense of El Salvador, whose rebels Nicaragua was helping to arm.
In 1986 the Court ruled against the United States, finding that its support for the contras and the mining of the harbors violated the ban on force and on intervention, and that it owed reparations. The United States, which had already withdrawn its general acceptance of the Court's jurisdiction, refused to comply and vetoed a Security Council resolution calling for compliance. A realist sees proof that law bends to power. A liberal or constructivist notices something else: the United States spent years arguing in legal terms, the judgment shaped later law on self-defense, and Congress had already restricted aid to the contras amid public unease. Law did not stop a superpower, but it set the terms on which that superpower's conduct was judged.