International Law: Definition, Nature, Theories and Development

International law is the body of legal rules and principles governing relations among States and, increasingly, international organisations, individuals and other entities recognised by international law. It regulates matters ranging from treaties and diplomatic relations to human rights, armed conflict, international trade, environmental protection and international criminal responsibility.

Key Takeaways

• International law regulates relations at the international level. Traditionally, it primarily governed relations between sovereign States, but modern international law also recognises international organisations and, in certain contexts, individuals as holders of international rights and obligations.
• The principal sources of international law are treaties, international custom and general principles of law. Article 38 of the Statute of the International Court of Justice also recognises judicial decisions and the teachings of highly qualified publicists as subsidiary means for determining rules of law.
• International law is decentralised. Unlike municipal law, there is no single world legislature, executive or police authority exercising general jurisdiction over all States.
• International law is nevertheless recognised as law. States regularly enter treaties, follow customary rules, invoke international legal rights, accept international obligations and use courts, tribunals and other mechanisms for dispute settlement.
• Several theories explain why international law is binding. Important approaches include natural law theory, positivism, consent theory, the theory of auto-limitation and Kelsen’s normative theory.
• Modern international law developed gradually over centuries. Its history includes ancient practices between political communities, natural law thought, the emergence of sovereign States, nineteenth-century positivism, the Hague Conferences, the League of Nations and the post-1945 United Nations system.
• International law continues to evolve. Human rights, international criminal law, environmental law, law of the sea, international economic law and regulation of global problems have considerably expanded its scope.

What Is International Law?

International law is a system of legal rules, principles and standards governing conduct in the international community.

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Traditionally, international law was known as the law of nations because its principal concern was the conduct of sovereign States in their relations with one another. Classical definitions consequently focused almost entirely on States.

Oppenheim described international law in terms of customary and conventional rules regarded as binding upon States in their relations with each other. This definition reflects the traditional State-centred character of international law.

Modern international law is considerably broader. States remain its principal subjects, but the international legal system also deals with international organisations, individuals and other entities in particular circumstances. The subjects of international law therefore extend beyond States alone. Individuals, for instance, possess rights under international human rights law and may incur individual criminal responsibility under international criminal law.

International law therefore cannot now be understood merely as a set of rules governing diplomatic relations between States.

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It covers subjects including:

• international peace and security;
• treaties and diplomatic relations;
• international human rights;
• international humanitarian law;
• international criminal law;
• law of the sea;
• environmental protection;
• international trade and investment;
• international organisations;
• State responsibility;
• nationality and statelessness; and
• peaceful settlement of international disputes.

What Is the Nature of International Law?

The nature of international law refers to its legal character, structure, operation and binding force.

International law differs substantially from municipal law. Within a State, there is ordinarily a legislature capable of making laws, an executive that implements them and courts having established jurisdiction to interpret and apply them.

International law has no equivalent centralised governmental structure exercising comprehensive authority over every State.

This makes international law a largely decentralised legal system.

International Law Is Primarily a Law Between Sovereign States

Sovereignty remains one of the foundations of the international legal order. States possess legal equality and exercise authority within their territories, subject to their international obligations.

Article 2(1) of the UN Charter expressly provides that the United Nations is based on the principle of the sovereign equality of its members. The Charter also requires members to fulfil their Charter obligations in good faith and settle international disputes peacefully.

International law therefore generally operates horizontally among sovereign entities rather than through a superior world government exercising legislative power over subordinate States.

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International Law Is Decentralised

There is no universal legislature capable of passing ordinary legislation binding all States in the manner in which Parliament enacts legislation within India.

International organisations can adopt resolutions and States can conclude multilateral conventions, but their legal effect depends upon the applicable rules of international law and the constitutive instrument concerned.

The United Nations was not given general legislative power to enact binding international rules by majority vote. Instead, the General Assembly was given functions relating to study, recommendation, progressive development and codification of international law.

Similarly, the jurisdiction of international courts is not identical to the compulsory jurisdiction ordinarily exercised by domestic courts. The International Court of Justice can exercise contentious jurisdiction only where an accepted legal basis for jurisdiction exists.

International Law Creates Legal Rights and Obligations

The decentralised character of international law does not mean that its rules are merely political or moral expectations.

Treaties create binding legal obligations for their parties. Article 26 of the Vienna Convention on the Law of Treaties embodies the principle of pacta sunt servanda, under which every treaty in force is binding upon its parties and must be performed in good faith.

Article 27 further provides, subject to its stated qualification, that a party cannot rely upon its internal law as justification for failing to perform a treaty.

Legal obligations may also arise independently of treaties through customary international law and other recognised sources.

What Are the Sources of International Law?

Article 38(1) of the Statute of the International Court of Justice is commonly treated as the principal starting point for identifying the sources of international law and the recognised means for determining international law.

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It directs the Court to apply:

1. International conventions establishing rules expressly recognised by the States concerned;
2. International custom, as evidence of a general practice accepted as law;
3. General principles of law;
4. Judicial decisions and teachings of highly qualified publicists, subject to Article 59, as subsidiary means for determining rules of law.

Treaties

A treaty is an international agreement governed by international law through which States or other entities possessing treaty-making capacity undertake legal obligations.

The law of treaties in international law governs matters such as formation, interpretation, reservations, amendment and termination of treaties.

Treaties may be bilateral or multilateral and may be called conventions, covenants, protocols, charters or agreements.

Their binding force is closely connected with the principle of pacta sunt servanda.

International Custom

Customary international law develops from:

• a general practice; and
• acceptance of that practice as law, commonly called opinio juris.

Mere repetition of conduct is therefore insufficient. The practice must also carry the required legal conviction.

General Principles of Law

General principles help prevent gaps in the international legal system and provide principles capable of assisting courts and tribunals where appropriate.

Article 38 expressly recognises general principles of law as a category to be applied by the ICJ.

Judicial Decisions and Scholarly Writings

Article 38 treats judicial decisions and the teachings of highly qualified publicists as subsidiary means for the determination of rules of law, rather than placing them in precisely the same category as treaties or customary international law.

Further, Article 59 of the ICJ Statute provides that a decision of the Court has binding force only between the parties and in respect of that particular case. International law therefore does not operate through a rigid doctrine of precedent identical to the doctrine of stare decisis followed in certain domestic legal systems.

Is International Law Really Law?

Whether international law qualifies as “true law” has been one of the classic debates in jurisprudence.

The controversy developed largely because international law lacks some institutions normally associated with domestic legal systems.

Arguments Against International Law Being True Law

John Austin approached law through the command theory of jurisprudence. Under this conception, positive law consists essentially of commands issued by a determinate sovereign and backed by sanctions.

Since States do not recognise a common political superior exercising such sovereign authority over them, Austin did not place international law in the same category as positive municipal law. His broader understanding of sovereign authority can be studied through Austin’s Theory of Sovereignty.

Rules governing international relations were associated instead with positive morality.

Thomas Erskine Holland similarly emphasised the absence of an international sovereign and effective central sanctions. His description of international law as the “vanishing point of jurisprudence” became a well-known expression of this sceptical position.

The main objections traditionally raised are:

• absence of a world sovereign;
• absence of a universal legislature;
• limited compulsory jurisdiction of international courts;
• decentralised enforcement; and
• dependence upon State conduct and cooperation.

Why International Law Is Regarded as Law

The modern understanding does not generally make the existence of a single sovereign the exclusive test of law.

International law contains rules recognised by States as legally binding. States conclude and perform treaties, make legal claims based on international rules, protest alleged violations, invoke State responsibility and participate in international adjudicatory mechanisms.

Further, international law provides recognised consequences for wrongful conduct. Enforcement may operate through domestic courts, international adjudication, countermeasures subject to international law, institutional measures, diplomatic processes and, in appropriate situations, collective action under international organisations.

The existence of a decentralised enforcement structure therefore distinguishes international law from municipal law, but does not by itself convert international legal obligations into mere morality.

What Are the Fundamental Principles of International Law?

Several principles form part of the structure of contemporary international relations.

Sovereign Equality

States are legally sovereign and equal notwithstanding differences in geographical size, population, economic strength or military power.

The principle is expressly recognised in Article 2(1) of the UN Charter.

Non-Intervention

International law recognises limits on intervention in matters falling within the domestic affairs of States. The precise operation of the principle depends upon the applicable legal context.

Article 2(7) of the UN Charter also restricts UN intervention in matters essentially within domestic jurisdiction, while expressly preserving the application of enforcement measures under Chapter VII.

Peaceful Settlement of Disputes

Article 2(3) of the UN Charter requires members to settle international disputes through peaceful means so that international peace, security and justice are not endangered.

Prohibition on the Threat or Use of Force

Article 2(4) requires UN members to refrain from the threat or use of force against the territorial integrity or political independence of any State or in any other manner inconsistent with the purposes of the United Nations.

Pacta Sunt Servanda

The principle means that agreements in force must be performed in good faith. It is fundamental to the law of treaties and appears expressly in Article 26 of the Vienna Convention on the Law of Treaties.

What Are the Main Theories of International Law?

Different theories attempt to explain the foundation, validity and binding force of international law.

Natural Law Theory

Natural law theory considers law to be connected with principles of reason, justice and universal standards that exist independently of the commands of individual States.

Early international law was strongly influenced by natural law thinking. Writers attempted to identify principles governing relations among political communities by applying reason, morality and ideas of justice.

Hugo Grotius occupies an important place in this development. His De Jure Belli ac Pacis, published in 1625, systematically examined war, peace and relations among political communities. His work contributed significantly to the emergence of international law as an organised field of study.

Natural law theory was particularly influential before the rise of legal positivism.

Positivist Theory

Positivism and the development of international law emphasise law that can be identified through recognised legal sources rather than universal moral reasoning.

In international law, positivist approaches traditionally placed considerable importance on:

• State practice;
• treaties;
• consent;
• custom; and
• the sovereign will of States.

Positivism became especially influential during the nineteenth century, when the sovereign State became central to international legal analysis.

Modern international legal positivism is considerably more sophisticated than the simple proposition that every international rule requires individual consent from every State.

Consent Theory

Consent theory explains international obligations largely through the consent of States.

Consent is particularly visible in treaty law. States agree to treaties and thereby undertake legal obligations in accordance with the applicable rules governing treaties.

However, consent alone does not provide a complete explanation of modern international law. Jus cogens, customary international law and other features of the contemporary legal system make the international order more complex than a collection of contractual obligations voluntarily selected by individual States.

Auto-Limitation Theory

The auto-limitation theory in international law sought to reconcile State sovereignty with binding international obligations.

Associated particularly with Georg Jellinek, the theory holds that a sovereign State is capable of restricting its own freedom through its own will. International obligations can therefore bind a State because the State voluntarily accepts limitations upon its sovereign freedom.

The theory developed within the broader positivist effort to explain how sovereign States could nevertheless be legally bound internationally.

A major criticism is that if the binding character of international law depends entirely on continued State will, it becomes difficult to explain why a State cannot simply withdraw from obligations whenever it chooses.

Kelsen’s Normative Theory

Hans Kelsen developed a normative approach through his Pure Theory of Law.

Kelsen understood law as a hierarchy of norms whose validity must ultimately be explained through a fundamental norm or Grundnorm. His theory sought to separate legal validity from morality, politics and sociological explanations.

Kelsen accorded international law a particularly significant place and defended its character as law in its own right. His broader theory also treated international and municipal law within a monistic conception of legal order.

The relationship between international law and municipal law is commonly analysed through monist and dualist theories.

The importance of Kelsen’s approach lies in shifting attention away from the idea that international law must depend entirely upon commands issued by a sovereign.

Public International Law and Private International Law

International legal studies commonly distinguish public international law from private international law.

Public International Law

Public international law principally deals with international legal relations involving States, international organisations and other recognised participants in the international legal system.

It includes subjects such as:

• treaty law;
• State responsibility;
• international organisations;
• human rights;
• humanitarian law;
• international criminal law;
• law of the sea; and
• international dispute settlement.

Private International Law

Private international law, also known as conflict of laws, deals primarily with private disputes containing a foreign element.

It addresses questions such as:

• which country’s courts have jurisdiction;
• which country’s law applies; and
• whether a foreign judgment should be recognised or enforced.

Private international law therefore differs in subject matter and operation from public international law.

How Did International Law Develop?

International law did not originate suddenly with the United Nations or at a single historical event. Its development has taken place gradually through centuries of interaction among political communities.

Ancient and Medieval Foundations

Rules relating to diplomacy, warfare, treaties, commerce and relations between political communities existed in different civilisations long before the modern State system.

The Roman concept of jus gentium, or law of peoples, later became important to discussions concerning rules extending beyond purely domestic law.

During the medieval period, natural law, religious thought, commerce and diplomatic practice influenced the regulation of relations across political communities.

Rise of Early Modern International Law

The sixteenth and seventeenth centuries were particularly important for the intellectual development of international law.

Writers such as Francisco de Vitoria, Francisco Suárez, Alberico Gentili and Hugo Grotius contributed to the systematic study of war, sovereignty and relations among States.

Grotius is frequently associated with the foundations of modern international legal scholarship, although international law cannot accurately be attributed to a single scholar or civilisation.

Peace of Westphalia, 1648

The Peace of Westphalia ended the Thirty Years’ War in Europe and is traditionally treated as an important stage in the development of the European State system.

It is often connected with ideas of territorial sovereignty and independent States.

However, it should not be treated as the moment when international law suddenly began. Historical scholarship shows that the modern international legal system resulted from a much longer process of legal thought, State practice and institutional development.

Nineteenth-Century Development

The nineteenth century witnessed major expansion of treaty relations and international cooperation.

Positivism became influential, placing greater emphasis on State consent and actual State practice.

International regulation expanded into matters such as diplomatic relations, navigation, communications, intellectual property and humanitarian rules.

The Congress of Vienna of 1814–1815 contributed to international regulation in areas including international rivers, diplomatic rank and the slave trade. The later nineteenth century also saw sustained treaty-making and the growth of international institutions.

Hague Peace Conferences

The Hague Peace Conferences of 1899 and 1907 were important developments in efforts to codify international law.

They produced important conventions concerning the laws of war and peaceful settlement of disputes and gave significant momentum to international codification.

League of Nations Period

After the First World War, the establishment of the League of Nations marked an important development towards institutionalised international cooperation.

The Permanent Court of International Justice was also established during this period.

Under League auspices, attempts were made to advance international codification. A Committee of Experts for the Progressive Codification of International Law was created following a League Assembly resolution in 1924, and a major codification conference was held in 1930.

The League ultimately failed to prevent another major world war, but many institutional and legal ideas developed during this period influenced the post-1945 international order.

Development After the Second World War

The establishment of the United Nations in 1945 transformed the institutional framework of international law.

The UN Charter established principles relating to sovereign equality, peaceful settlement of disputes, prohibition on the use of force and international cooperation. It also established the International Court of Justice as the principal judicial organ of the United Nations.

The post-war period also witnessed major developments in:

• international human rights law;
• international humanitarian law;
• international criminal responsibility;
• decolonisation and self-determination;
• law of the sea;
• international trade;
• refugee law;
• environmental law; and
• international institutional law.

International law consequently became far broader than the traditional law governing diplomatic relations between European States.

How Is International Law Codified and Developed Today?

Article 13(1)(a) of the UN Charter gives the General Assembly a mandate to encourage the progressive development of international law and its codification.

The International Law Commission was established by the General Assembly in 1947 to assist in carrying out this task.

The two expressions have related but distinct meanings.

Codification generally involves the more precise formulation and systematic statement of rules in fields where considerable State practice, precedent and doctrine already exist.

Progressive development concerns the preparation of legal rules in fields that are not sufficiently regulated or where the law has not yet developed adequately through State practice.

In practice, the two processes frequently overlap.

International law therefore continues to develop through treaties, customary law, judicial activity, international organisations, State practice, codification processes and the emergence of new areas requiring international regulation.

Conclusion

International law is a distinct legal system governing relations within the international community. Although it lacks the centralised legislature, executive and enforcement machinery commonly found in domestic legal systems, it contains recognised rules that create legal rights and obligations for States and other international actors.

Its legal foundation can be examined through different theories. Natural law emphasises reason and universal principles, positivism focuses upon identifiable legal sources and State practice, consent theory stresses State agreement, auto-limitation explains obligations through voluntary restrictions on sovereignty and Kelsen’s normative theory approaches international law as part of a structured system of legal norms.

The historical development of international law has been equally significant. From early rules governing relations among political communities, international law developed through natural law thought, the rise of sovereign States, increasing treaty practice, nineteenth-century codification, the Hague Conferences, the League of Nations and ultimately the United Nations system.

Modern international law now extends far beyond relations between States. Human rights, international criminal responsibility, environmental protection, international organisations, trade, humanitarian law and global cooperation have transformed its scope. Its decentralised nature continues to create questions concerning compliance and enforcement, but international law remains the principal legal framework through which international rights, obligations, cooperation and responsibility are organised.

Note: This article was originally written by Kritika Katoch [Student, Himachal Pradesh National Law University, Shimla] and published on 29 December 2020. It was subsequently updated by the LawBhoomi team on 18 September 2026.