International law reflects the establishment and subsequent modification of a world system founded almost exclusively on the notion that independent sovereign states are the only relevant actors in the international system. The essential structure of international law was mapped out during the European Renaissance, though its origins lay deep in history and can be traced to cooperative agreements between peoples in the ancient Middle East. Among the earliest of these agreements were a treaty between the rulers of Lagash and Umma (in the area of Mesopotamia) in approximately 2100 bce and an agreement between the Egyptian pharaoh Ramses II and Hattusilis III, the king of the Hittites, concluded in 1258 bce. A number of pacts were subsequently negotiated by various Middle Eastern empires. The long and rich cultural traditions of ancient Israel, the Indian subcontinent, and China were also vital in the development of international law. In addition, basic notions of governance, of political relations, and of the interaction of independent units provided by ancient Greek political philosophy and the relations between the Greek city-states constituted important sources for the evolution of the international legal system.

Many of the concepts that today underpin the international legal order were established during the Roman Empire. The jus gentium (Latin: “law of nations”), for example, was invented by the Romans to govern the status of foreigners and the relations between foreigners and Roman citizens. In accord with the Greek concept of natural law, which they adopted, the Romans conceived of the jus gentium as having universal application. In the Middle Ages, the concept of natural law, infused with religious principles through the writings of the Jewish philosopher Moses Maimonides (1135–1204) and the theologian St. Thomas Aquinas (1224/25–1274), became the intellectual foundation of the new discipline of the law of nations, regarded as that part of natural law that applied to the relations between sovereign states.

After the collapse of the western Roman Empire in the 5th century ce, Europe suffered from frequent warring for nearly 500 years. Eventually, a group of nation-states emerged, and a number of supranational sets of rules were developed to govern interstate relations, including canon law, the law merchant (which governed trade), and various codes of maritime law—e.g., the 12th-century Rolls of Oléron, named for an island off the west coast of France, and the Laws of Wisby (Visby), the seat of the Hanseatic League until 1361. In the 15th century the arrival of Greek scholars in Europe from the collapsing Byzantine Empire and the introduction of the printing press spurred the development of scientific, humanistic, and individualist thought, while the expansion of ocean navigation by European explorers spread European norms throughout the world and broadened the intellectual and geographic horizons of western Europe. The subsequent consolidation of European states with increasing wealth and ambitions, coupled with the growth in trade, necessitated the establishment of a set of rules to regulate their relations. In the 16th century the concept of sovereignty provided a basis for the entrenchment of power in the person of the king and was later transformed into a principle of collective sovereignty as the divine right of kings gave way constitutionally to parliamentary or representative forms of government. Sovereignty also acquired an external meaning, referring to independence within a system of competing nation-states.

Early writers who dealt with questions of governance and relations between nations included the Italian lawyers Bartolo da Sassoferrato (1313/14–1357), regarded as the founder of the modern study of private international law, and Baldo degli Ubaldi (1327–1400), a famed teacher, papal adviser, and authority on Roman and feudal law. The essence of the new approach, however, can be more directly traced to the philosophers of the Spanish Golden Age of the 16th and 17th centuries. Both Francisco de Vitoria (1486–1546), who was particularly concerned with the treatment of the indigenous peoples of South America by the conquering Spanish forces, and Francisco Suárez (1548–1617) emphasized that international law was founded upon the law of nature. In 1598 Italian jurist Alberico Gentili (1552–1608), considered the originator of the secular school of thought in international law, published De jure belli libri tres (1598; Three Books on the Law of War), which contained a comprehensive discussion of the laws of war and treaties. Gentili’s work initiated a transformation of the law of nature from a theological concept to a concept of secular philosophy founded on reason. The Dutch jurist Hugo Grotius (1583–1645) has influenced the development of the field to an extent unequaled by any other theorist, though his reputation as the father of international law has perhaps been exaggerated. Grotius excised theology from international law and organized it into a comprehensive system, especially in De Jure Belli ac Pacis (1625; On the Law of War and Peace). Grotius emphasized the freedom of the high seas, a notion that rapidly gained acceptance among the northern European powers that were embarking upon extensive missions of exploration and colonization around the world.

The scholars who followed Grotius can be grouped into two schools, the naturalists and the positivists. The former camp included the German jurist Samuel von Pufendorf (1632–94), who stressed the supremacy of the law of nature. In contrast, positivist writers, such as Richard Zouche (1590–1661) in England and Cornelis van Bynkershoek (1673–1743) in the Netherlands, emphasized the actual practice of contemporary states over concepts derived from biblical sources, Greek thought, or Roman law. These new writings also focused greater attention on the law of peace and the conduct of interstate relations than on the law of war, as the focus of international law shifted away from the conditions necessary to justify the resort to force in order to deal with increasingly sophisticated interstate relations in areas such as the law of the sea and commercial treaties. The positivist school made use of the new scientific method and was in that respect consistent with the empiricist and inductive approach to philosophy that was then gaining acceptance in Europe. Elements of both positivism and natural law appear in the works of the German philosopher Christian Wolff (1679–1754) and the Swiss jurist Emerich de Vattel (1714–67), both of whom attempted to develop an approach that avoided the extremes of each school. During the 18th century, the naturalist school was gradually eclipsed by the positivist tradition, though, at the same time, the concept of natural rights—which played a prominent role in the American and French revolutions—was becoming a vital element in international politics. In international law, however, the concept of natural rights had only marginal significance until the 20th century.

Positivism’s influence peaked during the expansionist and industrial 19th century, when the notion of state sovereignty was buttressed by the ideas of exclusive domestic jurisdiction and nonintervention in the affairs of other states—ideas that had been spread throughout the world by the European imperial powers. In the 20th century, however, positivism’s dominance in international law was undermined by the impact of two world wars, the resulting growth of international organizations—e.g., the League of Nations, founded in 1919, and the UN, founded in 1945—and the increasing importance of human rights. Having become geographically international through the colonial expansion of the European powers, international law became truly international in the first decades after World War II, when decolonization resulted in the establishment of scores of newly independent states. The varying political and economic interests and needs of these states, along with their diverse cultural backgrounds, infused the hitherto European-dominated principles and practices of international law with new influences.

The development of international law—both its rules and its institutions—is inevitably shaped by international political events. From the end of World War II until the 1990s, most events that threatened international peace and security were connected to the Cold War between the Soviet Union and its allies and the U.S.-led Western alliance. The UN Security Council was unable to function as intended, because resolutions proposed by one side were likely to be vetoed by the other. The bipolar system of alliances prompted the development of regional organizations—e.g., the Warsaw Pact organized by the Soviet Union and the North Atlantic Treaty Organization (NATO) established by the United States—and encouraged the proliferation of conflicts on the peripheries of the two blocs, including in Korea, Vietnam, and Berlin. Furthermore, the development of norms for protecting human rights proceeded unevenly, slowed by sharp ideological divisions.

The Cold War also gave rise to the coalescence of a group of nonaligned and often newly decolonized states, the so-called “Third World,” whose support was eagerly sought by both the United States and the Soviet Union. The developing world’s increased prominence focused attention upon the interests of those states, particularly as they related to decolonization, racial discrimination, and economic aid. It also fostered greater universalism in international politics and international law. The ICJ’s statute, for example, declared that the organization of the court must reflect the main forms of civilization and the principal legal systems of the world. Similarly, an informal agreement among members of the UN requires that nonpermanent seats on the Security Council be apportioned to ensure equitable regional representation; 5 of the 10 seats have regularly gone to Africa or Asia, two to Latin America, and the remainder to Europe or other states. Other UN organs are structured in a similar fashion.

The collapse of the Soviet Union and the end of the Cold War in the early 1990s increased political cooperation between the United States and Russia and their allies across the Northern Hemisphere, but tensions also increased between states of the north and those of the south, especially on issues such as trade, human rights, and the law of the sea. Technology and globalization—the rapidly escalating growth in the international movement in goods, services, currency, information, and persons—also became significant forces, spurring international cooperation and somewhat reducing the ideological barriers that divided the world, though globalization also led to increasing trade tensions between allies such as the United States and the European Union (EU).

Since the 1980s, globalization has increased the number and sphere of influence of international and regional organizations and required the expansion of international law to cover the rights and obligations of these actors. Because of its complexity and the sheer number of actors it affects, new international law is now frequently created through processes that require near-universal consensus. In the area of the environment, for example, bilateral negotiations have been supplemented—and in some cases replaced—by multilateral ones, transmuting the process of individual state consent into community acceptance. Various environmental agreements and the Law of the Sea treaty (1982) have been negotiated through this consensus-building process. International law as a system is complex. Although in principle it is “horizontal,” in the sense of being founded upon the concept of the equality of states—one of the basic principles of international law—in reality some states continue to be more important than others in creating and maintaining international law.