Origins of International Law

The need for rules of conduct between independent political entities developed along with government in ancient times. Early civilizations established rules governing the conduct of hostilities, the making and observance of treaties, and the treatment of foreign traders, travelers, and diplomats. These rules were often based on ritual and custom. The oldest known treaty, preserved in an inscription on a stone monument, is a peace treaty between two city-states of Sumer, dating from about 2500 bc. The empires of the ancient Middle East concluded a considerable number of treaties between 2000 and 1000 bc concerning topics still debated today, such as the extradition of fugitives and the creation of military alliances.

Later civilizations further developed tenets of international law. Jewish law as set forth in the Old Testament in the Book of Deuteronomy contains prescriptions for the mitigation of warfare, notably prohibitions against the killing of women and children. The Greek city-states had an elaborate treaty system governing many aspects of their mutual relations. In Asia the political units of ancient India and China, during certain periods, also developed and applied international law.

Beginning with the era of the Roman Republic (509 to 27 bc), the Romans made significant contributions to the evolution of international law. They developed the idea of a jus gentium, a body of laws designed to govern the treatment of aliens (noncitizens) subject to Roman rule and the relations between Roman citizens and aliens. They recognized in principle the duty of a nation to refrain from engaging in warfare without a just cause and originated the idea of a just war.

Modern international law began to develop with the rise of national states in Europe after the 15th century, when the basic ideas of national territory and jurisdiction were established. In 1625, building on the work of previous legal writers, the Dutch jurist Hugo Grotius published his celebrated treatise De Jure Belli ac Pacis (On the Law of War and Peace). Grotius argued that existing customs governing the relations between nations had the force of law and were binding unless contrary to natural justice or the law of nature (natural law), an immutable higher law governing all human conduct. Grotius’s influence on international affairs and the settlement of wars was great, and he is sometimes called the father of modern international law. His ideas became the cornerstone of the international system as established by the Peace of Westphalia (1648), a treaty that ended the Thirty Years’ War.

Other scholars and statesmen further described and developed the basic rules of international law, among them the Dutch jurist Cornelis van Bynkershoek and the Swiss diplomat Emmerich de Vattel. Vattel’s book, Le droit des gens (1758; Law of Nations), greatly influenced the framers of the Constitution of the United States with its ideas of natural law governing the behavior of states. Over time scholars gave increasing emphasis to the idea of state sovereignty, so that by the end of the 19th century the theoretical foundation of international law had shifted from natural law to a strictly consensual approach known as positivism. Positivism claims that each nation is bound only by the international rules that it freely accepts to limit its otherwise unlimited freedom of action. The clash between positivists and adherents of natural law continues today. Conflict is most pronounced over the issue of whether there are fundamental “higher norms” of international law, a principle called jus cogens, that sovereign states are obliged to respect.