, among the earliest extant examples of an international agreement
Antiquity With origins tracing back to
antiquity, states have a long history of negotiating interstate agreements. An initial framework was conceptualised by the Ancient Romans and this idea of
ius gentium has been used by various academics to establish the modern concept of international law. Among the earliest recorded examples are
peace treaties between the
Mesopotamian city-states of
Lagash and
Umma (approximately 3100 BCE), and
an agreement between the
Egyptian pharaoh,
Ramesses II, and the
Hittite king,
Ḫattušili III, concluded in 1279 BCE. Interstate pacts and agreements were negotiated and agreed upon by
polities across the world, from the eastern
Mediterranean to
East Asia. In
Ancient Greece, many
early peace treaties were negotiated between its
city-states and, occasionally, with neighbouring states. The
Roman Empire established an early conceptual framework for international law,
jus gentium, which governed the status of foreigners living in Rome and relations between foreigners and
Roman citizens. Adopting the Greek concept of
natural law, the Romans conceived of
jus gentium as being universal. However, in contrast to modern international law, the Roman law of nations applied to relations with and between foreign individuals rather than among political units such as states. Beginning with the
Spring and Autumn period of the eighth century BCE,
China was divided into numerous states that were often at war with each other. Rules for diplomacy and treaty-making emerged, including notions regarding
just grounds for war, the rights of neutral parties, and the consolidation and partition of states; these concepts were sometimes applied to relations with
barbarians along China's western periphery beyond the
Central Plains. The subsequent
Warring States period saw the development of two major schools of thought,
Confucianism and
Legalism, both of which held that the domestic and international legal spheres were closely interlinked, and sought to establish competing normative principles to guide foreign relations. Similarly, the
Indian subcontinent was divided into various states, which over time developed rules of neutrality,
treaty law, and international conduct, and established both temporary and permanent
embassies.
Middle Ages Following the
fall of the Western Roman Empire in the fifth century CE,
Europe fragmented into numerous often-warring states for much of the next five centuries. Political power was dispersed across a range of entities, including the
Church,
mercantile city-states, and kingdoms, most of which had overlapping and ever-changing jurisdictions. As in China and India, these divisions prompted the development of rules aimed at providing stable and predictable relations. Early examples include
canon law, which governed
ecclesiastical institutions and clergy throughout Europe; the
lex mercatoria ("merchant law"), which concerned trade and commerce; and various codes of
maritime law, such as the
Rolls of Oléron — aimed at regulating shipping in North-western Europe — and the later
Laws of Wisby, enacted among the commercial
Hanseatic League of northern Europe and the
Baltic region. In the
Islamic world,
Muhammad al-Shaybani published
Al-Siyar Al-Kabīr in the eighth century, which served as a fundamental reference work for
siyar, a subset of
Sharia law, which governed foreign relations. This was based on the division of the world into three categories: the
dar al-Islam, where Islamic law prevailed; the
dar al-sulh, non-Islamic realms that concluded an armistice with a Muslim government; and the
dar al-harb, non-Islamic lands which were contested through
jihad.
Islamic legal principles concerning
military conduct served as precursors to modern
international humanitarian law and institutionalised limitations on military conduct, including guidelines for commencing war, distinguishing between civilians and combatants and caring for the sick and wounded. During the European
Middle Ages, international law was concerned primarily with the purpose and legitimacy of war, seeking to determine what constituted
"just war". The Greco-Roman concept of natural law was combined with religious principles by Jewish philosopher
Maimonides (1135–1204) and Christian theologian
Thomas Aquinas (1225–1274) to create the new discipline of the "law of nations", which unlike its eponymous Roman predecessor, applied natural law to relations between states. In Islam, a similar framework was developed wherein the law of nations was derived, in part, from the principles and rules set forth in treaties with non-Muslims.
Renaissance and emergence of modern international law The 15th century witnessed a confluence of factors that contributed to an accelerated development of international law. Italian jurist
Bartolus de Saxoferrato (1313–1357) was considered the founder of
private international law. Another Italian jurist,
Baldus de Ubaldis (1327–1400), provided commentaries and compilations of Roman, ecclesiastical, and
feudal law, creating an organized source of law that could be referenced by different nations.
Francisco de Vitoria (1486–1546), who was concerned with the treatment of
indigenous peoples by Spain, invoked the law of nations as a basis for their innate dignity and rights, articulating an early version of sovereign equality between peoples. For his extensive and seminal work on the topic, he has been called the originator of modern international law.
Francisco Suárez (1548–1617), from the
School of Salamanca founded by Vitoria, emphasized that international law was founded upon natural law and human positive law.
Alberico Gentili (1552–1608) took a secular view to international law, authoring various books on issues in international law, notably
Law of War, which provided comprehensive commentary on the laws of war and treaties.Dutch jurist
Hugo Grotius (1583–1645) is widely regarded as the father of international law, being one of the first scholars to articulate an international order that consists of a "society of states" governed not by force or
warfare but by actual laws, mutual agreements, and customs. Grotius secularised international law; his 1625 work,
De Jure Belli ac Pacis, laid down a system of
principles of natural law that bind all nations regardless of local custom or law. He inspired two nascent schools of international law, the naturalists and the positivists. In the former camp was German jurist
Samuel von Pufendorf (1632–1694), who stressed the supremacy of the law of nature over states. His 1672 work,
Of the Law of Nature and Nations, expanded on the theories of Grotius and grounded natural law to
reason and the secular world, asserting that it regulated only external acts of states. Pufendorf challenged the
Hobbesian notion that the state of nature was one of war and conflict, arguing that the natural state of the world is actually peaceful but weak and uncertain without adherence to the law of nations. The actions of a state consist of nothing more than the sum of the individuals within that state, thereby requiring the state to apply a fundamental law of reason, which is the basis of natural law. He was among the earliest scholars to expand international law beyond European Christian nations, advocating for its application and recognition among all peoples on the basis of shared humanity. In contrast,
positivist writers, such as
Richard Zouche (1590–1661) in England and
Cornelis van Bynkershoek (1673–1743) in the Netherlands, argued that international law should derive from the actual practice of states rather than Christian or Greco-Roman sources. The study of international law shifted away from its core concern on the law of war and towards the domains such as the law of the sea and commercial treaties. The positivist school grew more popular as it reflected accepted views of state sovereignty and was consistent with the empiricist approach to philosophy that was then gaining acceptance in Europe.
Establishment of Westphalian system The developments of the 17th century culminated at the conclusion of the
Peace of Westphalia in 1648, which is considered the seminal event in international law. The resulting
Westphalian sovereignty is said to have established the current international legal order characterised by independent
nation states, which have equal sovereignty regardless of their size and power, defined primarily by non-interference in the domestic affairs of sovereign states, although historians have challenged this narrative. The idea of
nationalism further solidified the concept and formation of nation-states. Elements of the naturalist and positivist schools were synthesized, notably by German philosopher
Christian Wolff (1679–1754) and Swiss jurist
Emer de Vattel (1714–1767), both of whom sought a middle-ground approach. One of the first instruments of modern armed conflict law was the
Lieber Code of 1863, which governed the conduct of warfare during the
American Civil War, and is noted for codifying rules and articles of war adhered to by nations across the world, including the United Kingdom, Prussia, Serbia and Argentina. In the years that followed, numerous other treaties and bodies were created to regulate the conduct of states towards one another, including the
Permanent Court of Arbitration in 1899, and the
Hague and
Geneva Conventions, the first of which was passed in 1864.
20th and 21st century developments in 1979 Colonial expansion by European powers reached its peak in the late 19th century and its influence began to wane following the unprecedented bloodshed of
World War I, which spurred the creation of international organisations.
Right of conquest was generally recognized as international law before
World War II. The
League of Nations was founded to safeguard peace and security. International law began to incorporate notions such as
self-determination and
human rights. The
United Nations (UN) was established in 1945 to replace the League, with an aim of maintaining collective security. A more robust international legal order followed, buttressed by institutions such as the
International Court of Justice (ICJ) and the
UN Security Council (UNSC). The
International Law Commission (ILC) was established in 1947 to develop and codify international law. In the 1940s through the 1970s, the dissolution of the Soviet bloc and
decolonisation across the world resulted in the establishment of scores of newly independent states. As these former colonies became their own states, they adopted European views of international law. A flurry of institutions, ranging from the
International Monetary Fund (IMF) and the
International Bank for Reconstruction and Development (World Bank) to the
World Health Organization furthered the development of a multilateralist approach as states chose to compromise on sovereignty to benefit from international cooperation. Since the 1980s, there has been an increasing focus on the phenomenon of
globalisation and on protecting human rights on the global scale, particularly when minorities or
indigenous communities are involved, as concerns are raised that globalisation may be increasing inequality in the international legal system. == Sources of international law ==