International law

• 1. Introduction

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  International law is the body of rules, norms, and standards that states and other actors feel obligated to follow in their relations with one another. Unlike domestic law, it has no police force, no universal court with binding authority over all states, and no single legislature to write its rules. A 1947 survey found that 75% of Americans wanted an international police force to maintain world peace, but only 13% wanted it to exceed the size of the US military. That paradox captures the central tension that has defined international law for centuries: a world that desperately wants order but is unwilling to surrender the sovereignty that makes enforcement so difficult. How did this system come to exist? Who is bound by it, and how? What happens when powerful states simply ignore it? And what does it actually accomplish in a world where states remain the final arbiters of their own conduct?

• 2. Origins in Antiquity

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  A peace treaty between the Mesopotamian city-states of Lagash and Umma, dated to approximately 3100 BCE, is among the earliest recorded examples of states agreeing to rules that govern their relations. Centuries later, in 1279 BCE, the Egyptian pharaoh Ramesses II and the Hittite king Hattusili III concluded a formal agreement, one of the earliest known bilateral treaties between major powers. The Roman Empire gave these practices a theoretical foundation in jus gentium, a legal framework governing relations between foreigners and Roman citizens that drew on Greek ideas of natural law. The Romans conceived of it as universal. Notably, though, Roman jus gentium applied to individuals rather than to political units like states, which marks an important difference from the system that eventually emerged in the modern world. China during the Spring and Autumn period, beginning in the eighth century BCE, also developed parallel traditions. As numerous states found themselves in recurring conflict, rules for diplomacy and treaty-making emerged, including ideas about just grounds for war, the rights of neutral parties, and how states could be consolidated or partitioned. On the Indian subcontinent, similar processes produced rules of neutrality, treaty law, and the establishment of both temporary and permanent embassies. These parallel developments across different civilizations suggest that whenever states must coexist, they tend to develop shared rules, regardless of culture or geography.

• 3. Medieval and Islamic Roots

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  Muhammad al-Shaybani, writing in the eighth century, published Al-Siyar Al-Kabir, which became a foundational reference for siyar, the subset of Sharia law governing foreign relations. His framework divided the world into three zones: the dar al-Islam, where Islamic law prevailed; the dar al-sulh, non-Islamic territories that had concluded an armistice with a Muslim government; and the dar al-harb, contested lands. The guidelines al-Shaybani set out for military conduct, including rules for distinguishing between civilians and combatants and for caring for the sick and wounded, foreshadowed what would later become international humanitarian law. In Christian Europe, following the fall of the Western Roman Empire in the fifth century CE, political power fractured across the Church, mercantile city-states, and competing kingdoms. The need for stable, predictable relations produced early legal instruments like canon law governing ecclesiastical institutions and the lex mercatoria, or merchant law, governing trade. The Rolls of Oleron regulated shipping in northwestern Europe, and the Laws of Wisby emerged among the commercial Hanseatic League of the Baltic region. By the Middle Ages, the central preoccupation of European international law was the concept of "just war," and philosophers including Maimonides (1135-1204) and Thomas Aquinas (1225-1274) worked to combine natural law with religious principles to define when war could be legitimate. These efforts created the intellectual infrastructure that later thinkers would transform into something recognizable as modern international law.

• 4. The Founding Scholars

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  Francisco de Vitoria (1486-1546) confronted the treatment of indigenous peoples by Spain and invoked the law of nations as the basis for their inherent dignity and rights, articulating an early version of the principle of sovereign equality between peoples. For this work he has been called the originator of modern international law. Alberico Gentili (1552-1608) took a secular approach, and his book Law of War 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. His 1625 work De Jure Belli ac Pacis laid out a system of natural law principles that he argued bound all nations regardless of local custom. Grotius inspired two competing schools that would shape the discipline for generations. The naturalist camp included German jurist Samuel von Pufendorf (1632-1694), whose 1672 work Of the Law of Nature and Nations expanded on Grotius and argued that natural law regulated only external acts of states. Pufendorf challenged the Hobbesian view that the natural condition of the world was war, arguing instead that it was peaceful but fragile. He was also among the earliest scholars to argue that international law should apply to all peoples, not just European Christian nations. The opposing positivist camp, represented by Richard Zouche (1590-1661) and Cornelis van Bynkershoek (1673-1743), held that international law should derive from actual state practice rather than from Christian or Greco-Roman philosophical sources. As the empiricist approach to philosophy gained ground in Europe, positivism became the dominant school, shifting the field's focus from the laws of war toward the law of the sea and commercial treaties.

• 5. The Westphalian Turning Point

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  The Peace of Westphalia in 1648 is considered the seminal event in the history of international law. The settlements that concluded the Thirty Years' War gave rise to what scholars call Westphalian sovereignty: the idea that independent nation states possess equal sovereignty regardless of their size or power, and that the affairs of a sovereign state are its own. This principle of non-interference became the cornerstone of the international legal order, though historians have debated how much the actual treaties established these ideas as opposed to later scholars reading them into the text. Jeremy Bentham coined the modern term "international law" in his 1789 book Introduction to the Principles of Morals and Legislation, replacing the older term "law of nations." Lassa Oppenheim later defined international law as "a law between sovereign and equal states based on the common consent of these states," a definition that legal scholars largely adopted. The practical codification of rules governing armed conflict moved forward with the Lieber Code of 1863, which governed conduct during the American Civil War and was subsequently adopted by the United Kingdom, Prussia, Serbia, and Argentina. The Permanent Court of Arbitration was created in 1899, and the Hague and Geneva Conventions followed, the first Geneva Convention having been passed as early as 1864. These institutions gave the Westphalian framework concrete mechanisms for managing conflict and trade, even as the underlying enforcement problem remained unsolved.

• 6. The 20th Century Order

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  The International Law Commission was established in 1947 to develop and codify international law systematically. That same decade saw the adoption of the Universal Declaration of Human Rights by the UN General Assembly in 1948, which established non-binding international standards for work, housing, education, the right to a fair trial, and the prohibition of torture. Two further treaties, the International Covenant on Civil and Political Rights and the International Covenant on Economic, Social and Cultural Rights, were adopted by the UN in 1966; together with the UDHR they form the International Bill of Human Rights. The establishment of the International Criminal Court through the 1998 Rome Statute represented a significant development: it became the first permanent international court with authority to prosecute genocide, war crimes, crimes against humanity, and the crime of aggression. As of the time the source was written, 123 states were parties to the court. The court has been criticized by African countries including The Gambia and Kenya for what they described as "imperialist" prosecutions. The United States signed the Rome Statute in 2000 but declared in 2002 that it did not intend to become a party, citing concerns that the court threatened national sovereignty. Meanwhile the UN Security Council, which holds the power under Chapter VII of the UN Charter to authorize collective action against threats to peace, had prior to 1990 exercised that power only once, in the case of Korea in 1950. The gap between the scale of the legal machinery built in the 20th century and the persistent reluctance of states to be bound by it has remained the defining characteristic of the field.

• 7. Sources, Enforcement, and Legitimacy

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  Article 38(1) of the Statute of the International Court of Justice lists the authoritative sources of international law: international treaties, customary international law, general legal principles, and, as a subsidiary source, judicial decisions and the teachings of prominent scholars. Customary international law requires two elements: a consistent practice among states and the conviction that the practice is legally obligatory, a requirement referred to as opinio juris. The ICJ set out this standard in the North Sea Continental Shelf case, holding that the acts in question must be carried out in a way that shows states believe the practice is required by a rule of law. The 1969 adoption of the Vienna Convention on the Law of Treaties established the concept of jus cogens, or peremptory norms, rules so fundamental that no state may derogate from them. The most widely agreed example is the prohibition on the use of force. Scholars and critics have long contested the system's legitimacy and effectiveness. John Austin argued that without a sovereign power capable of enforcing it, international law was not really law at all but "positive morality" consisting of "opinions and sentiments more ethical than legal in nature." Martin Wight argued that international society is better described as anarchy. Hans Morgenthau likened its decentralized enforcement to the law that prevails in preliterate tribal societies. Others have argued from a constructivist perspective that states are socialized into compliance by internalizing norms and caring about their reputation, a dynamic captured in what rationalist scholars called the "Three Rs": Reciprocity, Reputation, and Retaliation. The debate over whether states obey international law because they are coerced, because they have internalized its norms, or because they calculate it serves their interests remains unresolved, and the answer matters enormously for understanding when the system succeeds and when it fails. The question of democratic legitimacy adds another layer: many of the foundational documents, including the UDHR, were drafted when large parts of the world were still under colonial rule, and the countries of the Global South have since led efforts to reshape norms on human rights from within the system they largely inherited.