Definition
Public international law is the body of rules, norms, and principles that governs the legal relationships between sovereign states and, increasingly, between states and other international actors such as international organizations, non-governmental entities, and individuals. It is distinguished from private international law (conflict of laws), which governs disputes between private parties where more than one national legal system is implicated.
Public international law encompasses:
1. The rights and duties of states in their relations with one another, including rules on the acquisition and loss of territory, recognition, state succession, and diplomatic relations.
2. The law of treaties — the formation, interpretation, validity, and termination of binding international agreements between states and international organizations.
3. International humanitarian law and the laws of war, governing the conduct of armed conflict and the protection of civilian populations.
4. International human rights law, establishing obligations states owe to individuals within their jurisdiction.
5. The law of international organizations, including the legal frameworks of bodies such as the United Nations, the International Court of Justice, and regional organizations.
6. Rules governing jurisdiction, immunity, and the resolution of disputes between states.
The principal sources of public international law are: treaties (both bilateral and multilateral), customary international law (state practice accepted as legally binding), general principles of law recognized by civilized nations, and subsidiary sources such as judicial decisions and the writings of leading scholars.
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Common Confusion
PUBLIC INTERNATIONAL LAW vs. PRIVATE INTERNATIONAL LAW: The distinction is foundational and frequently mishandled in non-specialist research. Private international law — also called conflict of laws or choice of law — governs which nation's domestic law applies to a dispute between private parties that crosses borders. Public international law governs relations between states and other public entities. A contract dispute between a French company and a Brazilian company implicates private international law; a treaty dispute between France and Brazil implicates public international law. The two fields can intersect — as when a state-owned enterprise contracts with a foreign private party — but they are analytically separate bodies of law with different sources, institutions, and methodologies.
PUBLIC INTERNATIONAL LAW vs. TRANSNATIONAL LAW: Transnational law is a broader, less formal category covering all law that regulates conduct or events transcending national frontiers, including both public and private international law as well as domestic law applied to cross-border situations. Some modern scholars use it to capture regulatory regimes that do not fit neatly into either classical category. Researchers should note that older sources will not use the term "transnational law" in this sense; it gained currency from Philip Jessup's 1956 lectures.
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Core Elements
The binding character of public international law rests on several interlocking foundations that researchers should understand structurally:
CONSENT AND SOVEREIGNTY: Unlike domestic law, public international law lacks a central legislature or enforcement authority. Obligations generally bind states only through their consent, expressed through treaty ratification or participation in the customary practice that crystallizes into binding norms. This consent-based architecture shapes every interpretive question about the scope of a state's obligations.
CUSTOM: Customary international law requires two elements — consistent state practice (what states actually do) and opinio juris (the belief that the practice is legally required, not merely courteous or political). Distinguishing custom from comity is one of the persistent interpretive challenges in this field.
JUS COGENS: A subset of norms — peremptory norms from which no derogation is permitted — stands above ordinary treaty obligations. These include prohibitions on genocide, slavery, and torture. No treaty may validly override a jus cogens norm.
ERGA OMNES OBLIGATIONS: Certain obligations are owed not to a specific counterparty state but to the international community as a whole. All states have a legal interest in their observance. This concept affects who has standing to raise a claim before international tribunals.
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Why It Matters in Research
The Law Mind corpus reflects a long historical arc during which the content, scope, and very legitimacy of public international law changed dramatically. Researchers must account for several specific hazards:
TERMINOLOGY SHIFT: Older sources — including Bouvier and many nineteenth-century treatises — use "law of nations" where modern sources say "public international law." The terms are functionally equivalent for most pre-twentieth-century purposes, but the shift to "public international law" tracks a substantive narrowing and then re-expansion of the field's scope. When searching historical materials, both terms are essential.
THE COLONIAL-ERA BASELINE: A large portion of the classical literature on the law of nations was written during the era of European colonial expansion. Doctrines of terra nullius, the standard of civilization, and the right of conquest appear in historical sources as settled law. Modern public international law has repudiated these doctrines, but they will appear without caveat in nineteenth and early twentieth-century dictionaries and treatises. Researchers working on indigenous land rights, decolonization, or the legal status of non-European states in earlier periods must read these sources with awareness of this context.
INDIVIDUALS AS SUBJECTS: Classical public international law treated only states as legal subjects — individuals had no direct rights or duties under it. The post-1945 human rights revolution fundamentally altered this. Sources predating the UN Charter will not reflect modern international human rights law. Bouvier's dictionary, for example, provides no meaningful treatment of this dimension because it did not exist in its modern form when Bouvier was writing.
STATE IMMUNITY EVOLUTION: The doctrine of sovereign immunity — whether and when a state may be sued in a foreign court — has shifted substantially from absolute to restrictive immunity over the twentieth century. Historical sources will describe the absolute doctrine as settled; researchers must check the date of any source carefully.
CORPUS CONNECTIONS: Public international law intersects with constitutional law (especially the domestic incorporation of treaties), admiralty and maritime law (a historically foundational area of the law of nations), the law of war, diplomatic law, and international trade law. Researchers investigating any of these topics will regularly need to move between domestic legal materials and international sources.
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Historical Dictionary Support
Bouvier's Law Dictionary provides minimal direct treatment of public international law as a unified category. The fragment preserved in the Law Mind corpus connects "public" as an adjective to matters of public easement, public property, and special privileges — structural usages in domestic law that do not engage the international dimension. This absence is itself informative: Bouvier organized his dictionary around domestic American legal categories, and the law of nations, while referenced in other entries, was not systematically codified as "public international law" in the mid-nineteenth-century American legal dictionary tradition.
The great treatise writers of the formative period — Vattel, Grotius, Pufendorf, and later Wheaton — are the primary historical authorities on the substantive content of the field. Wheaton's Elements of International Law (1836) was the leading American authority through much of the nineteenth century and is referenced in later editions of Bouvier. Researchers relying on Bouvier alone for international law content will find the dictionary inadequate; the treatise literature is indispensable.
Historical dictionaries uniformly reflect the "law of nations" framing. The shift to "public international law" as standard terminology is largely a twentieth-century development, solidified by the establishment of the Permanent Court of International Justice (1920) and the International Court of Justice (1945) and their associated statute and rules.
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Jurisdictional Note
By definition, public international law is not the law of any single jurisdiction. However, the relationship between international obligations and domestic law varies sharply across national legal systems. Monist systems (such as the Netherlands) incorporate international law directly into domestic law without separate legislative action. Dualist systems (such as the United Kingdom and the United States) require domestic legislative implementation before a treaty has internal legal effect. This distinction is critical for researchers tracing how international obligations have been applied — or resisted — in domestic courts.
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