There was little scope for an international law in the period of ancient and medieval empires, and its modern beginnings coincide, therefore, with the rise of national states after the Middle Ages. Rules of maritime intercourse and rules respecting diplomatic agents (see diplomatic service) soon came into existence. At the beginning of the 17th cent., the great multitude of small independent states, which were finding international lawlessness intolerable, prepared the way for the favorable reception given to the De jure belli ac pacis [concerning the law of war and peace] (1625) of Hugo Grotius, the first comprehensive formulation of international law. Though not formally accepted by any nation, his opinions and observations were afterward regularly consulted, and they often served as a basis for reaching agreement in international disputes. The most significant principle he enunciated was the notion of sovereignty and legal equality of all states. Other important writers on international law were Cornelius van Bynkershoek, Georg F. von Martens, Christian von Wolff, and Emerich Vattel.
The growth of international law came largely through treaties concluded among states accepted as members of the "family of nations," which first included the states of Western Europe, then the states of the New World, and, finally, the states of Asia and other parts of the world. The United States contributed much to the laws of neutrality and aided in securing recognition of the doctrine of freedom of the seas (see seas, freedom of the). The provisions of international law were ignored in the Napoleonic period, but the Congress of Vienna (see Vienna, Congress of) reestablished and added much, particularly in respect to international rivers and the classification and treatment of diplomatic agents. The Declaration of Paris (see Paris, Declaration of) abolished privateering, drew up rules of contraband, and stipulated rules of blockade. The Geneva Convention (1864) provided for more humane treatment of the wounded. The last quarter of the 19th cent. saw many international conventions concerning prisoners of war, communication, collision and salvage at sea, protection of migrating bird and sea life, and suppression of prostitution. Resort to arbitration of disputes became more frequent. The lawmaking conventions of the Hague Conferences represent the chief development of international law before World War I. The Declaration of London (see London, Declaration of) contained a convention of prize law, which, although not ratified, is usually followed. At the Pan-American Congresses, many lawmaking agreements affecting the Western Hemisphere have been signed.
In World War I, no strong nations remained on the sidelines to give effective backing to international law, and the concept of third party arbitration was again endangered; many of the standing provisions of international law were violated. New modes of warfare presented new problems in the laws of war, but attempts after the war to effect disarmament and to prohibit certain types of weapons (see war, laws of) failed, as the outbreak and course of World War II showed. The end of hostilities in 1945 saw the world again faced with grave international problems, including rectification of boundaries, care of refugees, and administration of the territory of the defeated enemy (see trusteeship, territorial). The inadequacy of the League of Nations and of such idealistic renunciations of war as the Kellogg-Briand Pact led to the formation of the United Nations as a body capable of compelling obedience to international law and maintaining peace. After World War II, a notable advance in international law was the definition and punishment of war crimes. Attempts at a general codification of international law, however, proceeded slowly under the International Law Commission established in 1947 by the United Nations.
The nuclear age and the space age have led to new developments in international law. The basis of space law was developed in the 1960s under United Nations auspices. Treaties have been signed mandating the internationalization of outer space (1967) and other celestial bodies (1979). The 1963 limited test ban treaty (see disarmament, nuclear) prohibited nuclear tests in the atmosphere, in outer space, and underwater. The nuclear nonproliferation treaty (1968) attempted to limit the spread of nuclear weapons. The agreements of the Strategic Arms Limitation Talks, signed by the United States and the USSR in 1972, limited defensive and offensive weapon systems. This was first of many international arms treaties signed between the two nations until the dissolution of the Soviet Union. Other treaties have covered the internationalization of Antarctica (1959), narcotic interdiction (1961), satellite communications (1963), and terrorism (1973). The Law of the Sea treaty (1982, in force from 1994) clarified the status of territorial waters and the exploitation of the seabed. Environmental issues have led to a number of international treaties, including agreements covering fisheries (1958), endangered species (1973), global warming and biodiversity (1992). Since the signing of the General Agreement on Tariffs and Trade (GATT) in 1947, there have been numerous international trade agreements. The European Union (prior to 1993, the European Community) has made moves toward the establishment of a regional legal system; in 1988 a Court of First Instance was established to serve as a court of original jurisdiction on certain economic matters. The establishment of the International Criminal Court (2002), with jurisdiction over war crimes, crimes against humanity, and related matters, marked a major step forward in international law despite the United States' repudiation of the treaty under President George W. Bush.