The body of rules, treaties, customs, and principles that govern relations between states and, increasingly, between states and individuals — including the laws of war, human rights obligations, trade regimes, and environmental agreements — enforced not by a world government but through reciprocity, reputation, domestic courts, and international tribunals whose authority states themselves must accept.
International law is the only legal system in the world with no guaranteed enforcement mechanism. There is no world police force. There is no world court with compulsory jurisdiction over states that refuse it. When a state violates international law, the consequences depend entirely on what other states choose to do about it — which depends on their interests, their relationships, and the particular violation. And yet international law is not simply fiction. States comply with it far more often than they defy it. The question international law forces is why — and what happens when the answer breaks down.