Public international law is the body of legal rules governing relationships between states and other recognised participants in the international legal system. It covers subjects such as treaties, diplomatic relations, international organisations, state responsibility, human rights, the use of force and international environmental obligations. For governments, international organisations and businesses operating across borders, understanding this area of law can provide important context for how international rules are created and applied.
Unlike domestic law, which operates within a particular country’s legal system, public international law operates across national boundaries. Its rules can arise from treaties, customary international law, general principles of law and other recognised international sources. The practical effect of a particular rule depends on its source, the states concerned and the relevant international legal framework.
What Is Public International Law?
Public international law governs legal relationships involving states and other entities recognised by international law. It addresses issues that cannot always be dealt with effectively through the domestic law of one country alone.
Examples include agreements between governments, diplomatic relations, international disputes, territorial questions, international criminal responsibility and cooperation through international organisations.
The term “public” helps distinguish this area from private international law. Private international law generally concerns questions such as which country’s courts should hear a cross-border private dispute or which country’s law should apply. Public international law instead focuses primarily on the legal relationships and obligations arising at the international level.
Sources of Public International Law
International law does not have a single global parliament that creates every international rule. Instead, international legal rules arise from several recognised sources.
Article 38 of the Statute of the International Court of Justice identifies international conventions, international custom, general principles of law and, subject to limitations, judicial decisions and the teachings of highly qualified publicists as subsidiary means for determining rules of law. ([icj-cij.org](https://www.icj-cij.org/statute))
These sources are important because they help determine whether an international legal obligation exists and how that obligation should be interpreted.
Treaties and International Agreements
Treaties are one of the most visible sources of public international law. States can enter into international agreements to establish rights and obligations covering particular subjects.
Treaties can address areas including trade, human rights, environmental protection, international security, taxation and diplomatic relations. The obligations created by a treaty depend on its wording, the parties involved and the applicable international rules governing treaties.
The Vienna Convention on the Law of Treaties provides a major framework for understanding treaties between states. It addresses matters such as treaty formation, interpretation, observance, amendment and termination.
A treaty can therefore become an important part of the legal framework governing relations between states, but its precise effect must always be considered in light of the relevant treaty and the parties to it.
Customary International Law
Not all international legal rules are contained in written treaties. Customary international law develops through general state practice accepted as law.
For a customary rule to exist, international legal analysis generally considers both the practice of states and whether states regard that practice as legally required. This can make customary international law more difficult to identify than a rule contained in a single written treaty.
Custom can nevertheless play a significant role in international law, including in circumstances where no applicable treaty governs a particular issue.
Who Is Bound by Public International Law?
States are the primary subjects of public international law. International organisations can also possess international legal personality and have rights and obligations under international law within the scope of their functions.
Individuals can also have rights and responsibilities under certain areas of international law. Human rights treaties, international criminal law and other international legal regimes demonstrate that international law is not exclusively concerned with relationships between governments.
The precise legal status of an entity depends on the relevant rules. International legal personality is therefore a question that must be considered within the particular legal context.
Public International Law and State Responsibility
State responsibility is an important principle of public international law. Where a state commits an internationally wrongful act attributable to it and the conduct breaches an international obligation, international responsibility may arise.
The International Law Commission’s Articles on Responsibility of States for Internationally Wrongful Acts provide an important reference point for analysing these issues. The framework addresses matters including attribution, breach, circumstances precluding wrongfulness and the legal consequences of internationally wrongful acts.
State responsibility can therefore become relevant where a government is alleged to have violated an international obligation arising from a treaty or customary international law.
International Courts and Dispute Resolution
International disputes can be resolved through several mechanisms. The appropriate process depends on the consent of the states involved and the legal instrument governing the dispute.
The International Court of Justice is the principal judicial organ of the United Nations. It deals with disputes between states and also provides advisory opinions on legal questions referred to it by authorised United Nations organs and agencies.
International arbitration is another mechanism used in some disputes. Treaties and commercial agreements can contain provisions specifying how disputes should be resolved and which tribunal or arbitral mechanism may be used.
Public International Law and the United Nations
The United Nations plays a significant role in the development and application of international law. Its Charter establishes a framework governing relations between member states and contains fundamental principles concerning matters such as sovereign equality, peaceful settlement of disputes and the prohibition on the use of force, subject to the Charter’s recognised exceptions.
United Nations bodies also contribute to the development of international legal standards through treaties, resolutions, institutional practice and international cooperation.
However, not every UN resolution has the same legal effect. The legal consequences of a particular resolution depend on the UN Charter, the body adopting it and the nature of the resolution itself.
Sovereignty and Public International Law
State sovereignty is a foundational concept in international law. States generally possess authority over their territory and domestic affairs, while international law establishes rules governing how states interact with one another.
Sovereignty does not mean that states operate entirely outside international legal obligations. States can undertake obligations through treaties and can also be bound by applicable customary international law.
Questions involving territorial sovereignty can nevertheless be highly complex, particularly where states have competing claims or where historical agreements and international legal principles overlap.
Human Rights and Public International Law
International human rights law is an important area of public international law. States have adopted numerous international and regional instruments establishing rights and obligations concerning individuals.
The Universal Declaration of Human Rights provided an influential foundation for the development of the modern international human rights framework. Subsequent treaties have established legally binding obligations for states that become parties to them.
The precise rights protected and enforcement mechanisms available depend on the relevant treaty and jurisdiction. International human rights law can therefore operate alongside domestic constitutional and human rights protections rather than simply replacing them.
What Is International Environmental Law?
International environmental law is a major branch of public international law dealing with environmental issues that cross national boundaries or require international cooperation.
Environmental problems such as climate change, biodiversity loss, marine pollution and transboundary pollution cannot always be addressed effectively by one country acting alone. International agreements therefore establish frameworks for cooperation, reporting, environmental protection and, in some cases, financial or technical assistance.
Major international environmental agreements include the Paris Agreement on climate change and the Convention on Biological Diversity. The legal obligations created by these instruments depend on their specific provisions and the states that have become parties to them.
Public International Law and Climate Change
Climate change illustrates how public international law can address problems with global effects. The Paris Agreement establishes an international framework under the United Nations Framework Convention on Climate Change for strengthening the global response to climate change.
The agreement includes commitments concerning nationally determined contributions, transparency and international cooperation. It does not operate as a single global environmental code, and the legal nature of individual provisions differs according to their wording.
International environmental law can also overlap with other areas of law, including trade, investment, energy regulation and domestic environmental legislation.
Public International Law and International Trade
International economic activity can involve both public and private legal rules. States may enter trade agreements that establish obligations concerning tariffs, market access, services and other aspects of international commerce.
For businesses, these government-to-government arrangements can influence the legal environment in which commercial transactions take place. This creates an important connection between international trade law and public international law.
A private commercial contract between two companies is different from an international treaty between states, even if both concern the same underlying economic activity. Understanding this distinction is important when identifying the appropriate legal framework.
Public International Law and Business
Businesses are not normally the primary actors in traditional state-to-state public international law, but international legal rules can have significant commercial consequences.
Companies operating internationally may be affected by government sanctions, trade restrictions, investment treaties, export controls, environmental commitments and international regulatory cooperation. Their commercial activities can therefore be influenced by legal obligations that originate at the international level.
This is one reason public international law can intersect with corporate and commercial law. A business may need to comply with domestic legislation implementing international obligations while also managing its private contractual relationships.
International Law and Domestic Law
International law and domestic law are related but distinct legal systems. The way international obligations take effect within a country depends on that country’s constitutional and legal framework.
In the UK, the position can differ according to the type of international obligation and the relevant domestic legislation. Treaties generally do not automatically become directly enforceable domestic law simply because the UK government has entered into them.
The Constitutional Reform and Governance Act 2010 provides a statutory framework for treaty ratification, while the UK’s constitutional arrangements determine how international obligations interact with domestic legislation.
This distinction matters because an international obligation and a domestic legal rule implementing that obligation are not necessarily the same thing.
Public International Law and International Criminal Law
Public international law also intersects with international criminal law, although the two should not be treated as identical.
International criminal law concerns individual responsibility for certain internationally recognised crimes. The International Criminal Court, for example, has jurisdiction over individuals for crimes within the scope of the Rome Statute and subject to its jurisdictional requirements.
This differs from state responsibility, where the question is whether conduct attributable to a state breaches an international obligation.
The distinction between state responsibility and individual criminal responsibility is therefore important when analysing international legal disputes.
Why Does Public International Law Matter Today?
Modern challenges increasingly cross national borders. Climate change, international trade, migration, armed conflict, cyber activity, global health issues and environmental protection can all involve questions that cannot be resolved entirely through domestic law.
Public international law provides mechanisms through which states can establish shared obligations, cooperate through international organisations and resolve certain disputes peacefully.
At the same time, international law is shaped by state consent, treaty obligations, customary practice and institutional mechanisms. Its operation can therefore be more complex than the domestic legal system of a single country.
When Is Specialist Advice Necessary?
International legal issues can involve several jurisdictions and multiple sources of law. Specialist advice may be appropriate where a business or organisation is affected by an international treaty, sanctions regime, cross-border regulatory issue, environmental obligation or international dispute.
Governments and public bodies may also require specialist advice when negotiating treaties or assessing international obligations. Businesses may need advice where international rules affect their commercial contracts, investments, supply chains or regulatory responsibilities.
Because public international law is highly specialised, the relevant treaty, customary rule, domestic legislation and jurisdiction should be identified before drawing conclusions about a particular legal issue.
Understanding Public International Law
Public international law provides a framework for legal relationships between states and other participants in the international legal system. Its sources include treaties, customary international law and general principles, while international courts and tribunals can play important roles in interpreting and applying international rules.
The subject covers a wide range of issues, including state responsibility, human rights, international disputes, international organisations and international environmental law. It can also intersect with international trade, taxation, business regulation and domestic legislation.
For UK businesses, understanding these connections can be particularly useful when operating internationally. International obligations may influence domestic legislation and commercial conditions, while private contracts and corporate structures determine how businesses manage their own relationships.
Public international law is therefore best understood as a broad legal framework rather than a single set of rules. The applicable law depends on the issue, the states involved, the relevant treaties and the wider international legal context.
