Park an embassy car in front of a fire hydrant in almost any capital in the world and the ticket will quietly go nowhere. The diplomat inside cannot be arrested or sued in the country where they serve, under a treaty (the 1961 Vienna Convention on Diplomatic Relations) that nearly every state has joined. No police force makes anyone keep it. States keep it because each of them wants its own diplomats treated the same way abroad.
That is the whole puzzle of this subject in one parking space. Inside a country, law has a legislature that writes it, courts that apply it and police who enforce it. Between countries there is no world parliament, no court every state must obey, and no world police. The international lawyer Louis Henkin put the surprising result well in 1979: almost all nations observe almost all principles of international law and almost all of their obligations almost all of the time. The headlines are about the exceptions. This piece is about why the rule is so often the rule.
Every legal system has to answer one question first: what counts as law? Statute and precedent answer it at home. Between states, the answer is written in Article 38 of the Statute of the International Court of Justice (1945), which lists what the Court applies: treaties, custom, and general principles of law recognised by civilised nations (an old phrase, now read as principles found in the world's legal systems, such as good faith). Court decisions and scholars' writings are only "subsidiary means" of finding the law.
A treaty binds the states that join it, because of a rule so basic it is the first rule of the subject: pacta sunt servanda, agreements must be kept (Article 26 of the 1969 Vienna Convention on the Law of Treaties). A treaty usually says when it comes into force, often after a number of ratifications. Custom is harder to see. It is a practice states follow because they believe the law requires it. Two ingredients: what states actually do (state practice) and why they do it (opinio juris, the sense of legal obligation). The International Court asked for practice that is "extensive and virtually uniform", including among the states most affected, in the North Sea Continental Shelf cases (1969). Custom matters because it binds even states that never signed anything. A state that objects from the start and keeps objecting (a persistent objector) can escape a new custom, but not the few peremptory norms (jus cogens), such as the bans on genocide and slavery, which no treaty can override (Article 53 of the Vienna Convention).
A teaching toy: score = practice × (1 if the most-affected states join, else 0.6) × (1 if from a sense of legal duty, else 0.25). Real tribunals weigh evidence, not scores. At 70 or above: likely custom; 40 to 70: emerging or doubtful; below 40: not custom.
Why does "courtesy or convenience" matter? Because ships of every flag stay out of each other's way at sea, but that alone is a habit; custom needs the belief that doing otherwise would be a breach. Now the treaty side: a treaty is not law for anyone until enough states say yes.
Thresholds: Vienna Convention 35 ratifications (Art. 84); Rome Statute 60 (Art. 126); Law of the Sea 60, then a 12-month wait (Art. 308); Paris Agreement 55 parties covering at least 55% of emissions (Art. 21). Real entry into force also counts days after the last deposit.
Each of the three families of states treats the result differently at home. This is the first big comparison in the subject: when a treaty binds the country, does it also bind its courts and citizens automatically?
| Family | How treaties enter domestic law |
|---|---|
| United States (common law) | A treaty the Senate approves by two-thirds is "supreme law of the land" (Constitution, Art. VI), but courts treat many as non-self-executing: they need a statute before a private person can invoke them. The US has not joined the Rome Statute and has signed but not ratified the American Convention on Human Rights. |
| Latin America (civil law) | Often generous. Argentina's 1994 constitution gives listed human rights treaties constitutional rank (Art. 75 no. 22); Colombia's courts speak of a bloque de constitucionalidad (Art. 93); Brazil's 2004 amendment gives human rights treaties approved by supermajority the rank of constitutional amendments; Mexico's 2011 reform puts treaty rights alongside the constitution's own (Art. 1). |
| Continental Europe | France's 1958 constitution puts ratified treaties above statutes (Art. 55); Germany ranks general rules of international law above statutes (Art. 25) while treaties generally have statute rank. The UK is dualist: a treaty needs a statute, which is why it passed the Human Rights Act 1998. |
The biggest treaty of all is the UN Charter, signed at San Francisco on 26 June 1945 and in force from 24 October 1945. Its central rule is Article 2(4): states must refrain from the threat or use of force against another state's territorial integrity or political independence. It had two exceptions: self-defence against an armed attack (Article 51), and action authorised by the Security Council under Chapter VII.
That second exception is the Charter's bargain, and it shows why the subject is half law and half politics. The Council has 15 members, five of them permanent: China, France, Russia (which took the Soviet seat), the United Kingdom and the United States. A resolution needs nine "yes" votes, and under Article 27(3) a negative vote by any permanent member blocks it: the veto. A permanent member that merely abstains does not block it, by long practice. The design is deliberate: in 1945 the founders thought a rule that the great powers could outvote would be a rule they would leave.
Rule (Charter Art. 27): at least 9 yes votes of 15, and no permanent member voting no. Abstentions count as neither.
Notice what the veto does not do: it does not make the underlying rule disappear. A blocked resolution leaves Article 2(4) in place; it only removes the Council as the body that can authorise enforcement. When the Council is stuck, the General Assembly can debate and condemn (its resolutions are recommendations, not law), and the International Court can give opinions. In March 2022 the Court, by 13 votes to 2, ordered Russia to suspend its military operations in Ukraine while the case was heard. The order was binding in law and was not followed in fact. Both halves of that sentence are part of the subject.
Two courts in the Netherlands are constantly confused. The International Court of Justice (ICJ, the "World Court", at the Peace Palace in The Hague) is the UN's main judicial organ. Fifteen judges, elected for nine-year terms, settle disputes between states, and give advisory opinions when UN bodies ask. The catch is consent: a state can be sued only if it has accepted the Court's jurisdiction, by treaty clause, by a special agreement, or by a standing declaration. Nicaragua v. United States (1986), where the Court found the US had breached customary law by supporting the Contras and mining Nicaraguan harbours, shows both sides: a state can be judged, and the judged state can ignore it. Judgments are meant to be enforced through the Security Council (Charter Art. 94), where the veto sits.
The International Criminal Court (ICC) is a different animal: a treaty court created by the Rome Statute (adopted 17 July 1998, in force 1 July 2002) that tries individuals for genocide, crimes against humanity, war crimes and aggression. It is a court of last resort: under complementarity, it acts only when national courts are unable or unwilling to genuinely investigate. The ICC says 125 states were parties in mid-2026; several states have filed notices of withdrawal that take effect in 2027. The US, China, Russia and India are not parties. Supporters say it ended the idea that rank protects; critics say its docket has been uneven and its cases dependent on state cooperation for arrests. Both statements describe real features: the Court has no police of its own.
Human rights law turns the traditional picture around. Treaties between states used to be about what states owed each other. After the horrors of the Second World War, the Universal Declaration of Human Rights (General Assembly, 10 December 1948; adopted 48 to 0 with 8 abstentions) said every person has rights against their own government. The Declaration is not a treaty. Two treaties made it binding in 1966 (in force 1976): the Covenants on Civil and Political Rights and on Economic, Social and Cultural Rights. The global system mostly works through committees that review reports. The sharper teeth are regional.
| System | Founding text | Who can bring a case | Court |
|---|---|---|---|
| Europe (Council of Europe) | European Convention on Human Rights, signed 1950, in force 1953 | Any individual, directly, after exhausting national remedies (since 1998) | European Court of Human Rights, Strasbourg |
| The Americas (OAS) | American Declaration (1948); American Convention (1969, in force 1978) | Individuals go to the Commission (Washington) first; the Commission or a state brings the case to the Court | Inter-American Court, San José, Costa Rica |
| Africa (AU) | African Charter on Human and Peoples' Rights (1981, in force 1986) | The Commission (Banjul); direct individual access to the Court only if the state made a special declaration | African Court, Arusha, Tanzania |
The European system is the most used: its judgments are routinely followed, partly because they are supervised by the Council of Europe's ministers, and partly because Europeans' own courts cite them. Russia was expelled from the Council of Europe in March 2022 and ceased to be a party to the Convention in September 2022. The Inter-American Court's landmark is Velásquez Rodríguez v. Honduras (1988): a student who disappeared at the hands of state agents. The Court held that a state must prevent, investigate and punish such violations, even when it did not itself commit a particular one. Later it developed "conventionality control": national judges should check domestic law against the Convention. The African Charter is distinctive in listing the rights of peoples and the duties of individuals alongside individual rights. Its Court's reach is thinner: of the few states that accepted direct individual petitions, several (including Rwanda, Tanzania, Benin and Côte d'Ivoire) have since withdrawn the declaration. The same pattern appears elsewhere: courts are strongest where governments choose to leave their doors open.
Rights are also not the same everywhere. The Charters differ on which rights, how they are limited, and whose duties come with them. A fair summary is that they share a core (life, freedom from torture, fair trial) and differ at the edges.
If nobody enforces, why comply? Lawyers and political scientists give overlapping answers, and they do not all agree on the weights. Reciprocity: a state that breaks a treaty invites others to break it back. Reputation: a state known to cheat finds fewer partners next time, and nearly every state is in a long series of repeated dealings. Countermeasures and sanctions: injured states can respond in proportion; the Security Council can impose sanctions. Domestic courts and politics: in many countries judges apply international law, and a government answers to voters, press and its own officials. Habit and shared interest: air traffic, mail and borders work because everybody gains from the rule.
Realists such as Hans Morgenthau and, more sceptically, Jack Goldsmith and Eric Posner argue that states follow law when it matches their interest and that the rule's apparent force is mostly coincidence. Institutionalists and Henkin's tradition argue the rule itself changes states' calculation and what they can justify. The data cannot settle this, because the easy cases are chosen by the states themselves. What both sides agree on: the cases of defiance are costly and rare, and powerful states can resist more cheaply.
Expected net gain = gain − chance × (retaliation + reputation lost). In this toy half of retaliation runs through the Security Council, so a veto-holder escapes that half. Invented numbers, in arbitrary units: it shows how the factors interact, not what any real state would do.
Drag the chance of being noticed to 100% and the reputation value up: even a big gain stops paying. Drag it to 10% and the same rule becomes a suggestion. That is why transparency (inspections, reporting, a free press) does so much work in international law: it raises the chance of being noticed.
One more route fills part of the gap. Some crimes (torture, genocide, war crimes) are so serious that any state's courts may try them wherever they happened: universal jurisdiction. In October 1998 British police arrested the former Chilean ruler Augusto Pinochet in London on a Spanish judge's warrant; the House of Lords held that a former head of state could not claim immunity for torture, and he was eventually sent home on health grounds rather than tried abroad. A US court in Filártiga v. Peña-Irala (1980) heard a Paraguayan family's claim over a torture death. The offence was committed abroad by a foreigner.
It cuts both ways. Immunity protects serving heads of state and diplomats from foreign courts, to let governments function (the ICJ confirmed it in the Arrest Warrant case, 2002). Critics from Africa and the Global South have charged that universal jurisdiction and the ICC are used against weaker states while great powers escape; defenders answer that the answer to selective justice is more justice, and that national courts, including Argentina's under universal jurisdiction, have opened cases the home courts would not. The honest summary: international law is a real, imperfect, evolving legal order, strongest where states have chosen to bind themselves and weakest where power and law collide.
International law is law made by the states it binds: treaties by consent, custom by practice believed to be obligatory. The UN Charter bans force and gives enforcement to a Council with five vetoes; the ICJ judges states if they consent; the ICC tries individuals when national courts will not; regional courts give people direct rights where governments allow. With no police, it works through reciprocity, reputation and being watched, which is why it usually holds and sometimes fails where it matters most.
Try this: next time international law is in the news, write down three things before reading the story: which source (treaty or custom), which court or body, and whether that body's jurisdiction depends on the accused state's consent.
Sources. Statute of the ICJ, Art. 38; Vienna Convention on the Law of Treaties 1969 (Arts. 26, 53, 84); UN Charter (Arts. 2(4), 27, 51, 94); ICJ, North Sea Continental Shelf (1969), Nicaragua v. United States (1986), Arrest Warrant (2002), and the order of 16 March 2022 in Ukraine v. Russian Federation (13 to 2); Rome Statute (adopted 17 July 1998, in force 1 July 2002; 125 states parties per the ICC Assembly of States Parties Presidency, June 2026, with withdrawals by Burkina Faso, Mali and Niger taking effect in 2027); ECHR, American Convention and African Charter texts; Council of Europe resolution and Russia's cessation of membership (March and September 2022); Velásquez Rodríguez v. Honduras (Inter-American Court, 1988); African Court Art. 34(6) declarations and the withdrawals of Rwanda, Tanzania, Benin and Côte d'Ivoire (African Court and Pretoria Centre for Human Rights statements); Henkin, How Nations Behave (1979); Goldsmith & Posner, The Limits of International Law (2005). Constitutional article numbers are from the national texts and should be checked against current versions. The interactives' numbers are toy models computed in node; thresholds from the treaty articles.