International law operates as a complex web of treaties, customs, and principles governing the relationships between sovereign states and increasingly, between states and other international actors. Yet the term “international law” encompasses dramatically different legal regimes, each with distinct characteristics, enforcement mechanisms, and normative foundations. From the public international law governing diplomatic relations and armed conflict to the private international law resolving cross-border commercial disputes, these distinctions shape how nations interact, how businesses operate globally, and ultimately how international peace and security are maintained. Understanding these fundamental differences is essential for legal practitioners, policymakers, and anyone seeking to grasp how the international legal order functions in practice. The evolution of international law over the past century has created increasingly sophisticated frameworks, but has also introduced tensions between competing sources of authority and different approaches to implementation.
The diversity within international law reflects the decentralised nature of the international system itself. Unlike domestic legal systems with centralised legislatures, courts, and enforcement agencies, international law emerges from multiple sources—some binding, some persuasive—and relies heavily on state consent and cooperation for its effectiveness. This creates a landscape where customary practices developed over centuries coexist with recently negotiated multilateral treaties, where regional organisations develop supranational legal frameworks whilst other states maintain strict dualist approaches to treaty implementation. These variations are not merely academic distinctions; they have profound practical implications for how international disputes are resolved, how international crimes are prosecuted, and how global challenges from climate change to cybersecurity are addressed through legal cooperation.
Public international law versus private international law: foundational distinctions
The most fundamental division within international law separates public international law from private international law, two distinct legal spheres that address fundamentally different questions and involve different actors. Public international law governs the relationships between sovereign states, international organisations, and other subjects of international law in matters affecting international peace, security, and cooperation. It encompasses treaty law, diplomatic relations, the law of the sea, international environmental law, and international human rights law. By contrast, private international law—also known as conflict of laws—addresses which nation’s laws should apply when private parties from different jurisdictions enter into transactions or disputes that span multiple legal systems.
Subject matter jurisdiction: state sovereignty and Inter-State relations
Public international law primarily concerns itself with matters of state sovereignty, territorial integrity, and the relations between sovereign equals in the international system. The United Nations Charter, the Vienna Convention on Diplomatic Relations, and the Geneva Conventions exemplify this body of law, establishing rules for how states conduct diplomacy, engage in armed conflict, and cooperate on matters of mutual concern. These legal instruments reflect the principle that states are the primary subjects of international law, possessing rights and obligations that flow from their status as sovereign entities. The International Court of Justice hears contentious cases only between states, reinforcing this state-centric character of public international law.
Private international law operates in an entirely different sphere, addressing practical questions that arise when individuals or corporations engage in cross-border activities. When a United States software company contracts with a European firm, and a dispute arises over contract performance, private international law determines whether American or European contract law applies, which courts have jurisdiction to hear the dispute, and whether a judgment obtained in one country will be recognised and enforced in another. This body of law does not regulate the behaviour of states themselves but rather provides the framework for resolving conflicts between different national legal systems when private rights are at issue.
Legal personality and standing: states, international organisations, and private entities
A critical distinction between public and private international law concerns who possesses legal personality and standing to bring claims. In public international law, states remain the primary subjects with full international legal personality, meaning they can make treaties, join international organisations, bring claims before international tribunals, and be held responsible for breaches of international obligations. International organisations like the United Nations also possess derivative international legal personality to the extent necessary to fulfil their functions. Increasingly, individuals can bring claims before international human rights courts such as the European Court of Human Rights or the Inter-American Court of Human Rights, but this represents a limited exception to the state-centric model.
Private international law, conversely, focuses almost entirely on private actors—individuals and corporations—and their cross-border legal relationships. Multinational corporations navigating the complexities of operating across multiple jurisdictions must grapple with conflicting regulations on data protection, illustrated by the challenges posed when compliance with China’s Personal Information Protection Law conflicts with European Union GD
PR requirements, or when Illinois’ Biometric Information Privacy Act interacts with global biometric processing standards. In these situations, private international law provides the tools—choice of law rules, jurisdiction clauses, and recognition and enforcement mechanisms—that allow private parties to predict which court will hear their dispute and which national law will ultimately govern their rights and obligations.
Sources of law: treaties, customary norms, and conflict of laws rules
Public international law draws its authority from the classic sources listed in Article 38(1) of the Statute of the International Court of Justice: treaties, customary international law, general principles of law, and subsidiary sources such as judicial decisions and scholarly writings. Multilateral treaties like the UN Charter, the Paris Agreement, or the Rome Statute create explicit, written obligations between states that consent to be bound. Customary international law, by contrast, grows out of consistent state practice accompanied by a belief that such practice is legally required (opinio juris), such as the prohibition of genocide or torture.
Private international law relies on a different mix of sources. While some aspects are harmonised by international conventions—such as the Hague Convention on Choice of Court Agreements or the New York Convention on the Recognition and Enforcement of Foreign Arbitral Awards—most conflict of laws rules are embedded in domestic legislation and case law. For example, the EU’s Rome I and Rome II Regulations codify which law applies to contractual and non-contractual obligations in civil and commercial matters, while common law jurisdictions rely heavily on judge-made principles. In practice, this means that when we talk about “sources of law” in private international law, we are usually looking at national conflict rules that operate as a gateway to deciding which substantive law will be applied to a cross-border dispute.
Enforcement mechanisms: ICJ adjudication versus national court jurisdiction
One of the starkest differences between public and private international law lies in how rules are enforced. In public international law, enforcement is decentralised and often political. The International Court of Justice (ICJ) can adjudicate disputes between states, but only where they have consented to its jurisdiction, whether through compromissory clauses in treaties, optional clause declarations, or special agreements. Even when the ICJ issues a binding judgment under Article 94 of the UN Charter, compliance ultimately depends on the losing state’s willingness to implement the decision or, in rare cases, on Security Council pressure.
Private international law, on the other hand, is primarily enforced through the ordinary machinery of national courts and arbitral tribunals. When a court in one state recognises and enforces a foreign judgment or arbitral award, private rights are vindicated through mechanisms such as asset seizure or injunctions, just as in purely domestic litigation. Instruments like the New York Convention, now ratified by more than 170 states, have made it easier to enforce international arbitration awards across borders, giving private parties a powerful enforcement tool that often works more reliably than inter-state adjudication. For businesses and individuals, this means that although public international law may set the overall framework, it is private international law that often determines whether a contract can be enforced or damages can actually be collected.
Treaty law versus customary international law: normative hierarchies
Beyond the public–private divide, another key distinction within international law lies between treaty law and customary international law. Both serve as primary sources of obligations for states, but they differ in how they are created, who they bind, and how flexible they are over time. Understanding these differences is crucial when we ask questions such as: can a state escape a particular obligation by withdrawing from a treaty, or does a parallel customary rule keep that obligation alive?
Vienna convention on the law of treaties: codification and binding force
The Vienna Convention on the Law of Treaties (VCLT) codifies many of the foundational rules governing treaties between states. Under Article 2, a treaty is defined as an international agreement in written form governed by international law, regardless of its particular label (convention, protocol, covenant, etc.). The central principle of treaty law, expressed in Article 26, is pacta sunt servanda: treaties in force are binding upon the parties and must be performed in good faith. This means that once a state has validly consented to a treaty and that treaty has entered into force, it cannot simply ignore its obligations without breaching international law.
Treaties offer clarity and speed. States can negotiate detailed obligations on new issues—like cybercrime, climate change, or digital trade—and specify when and how those obligations become binding. They can also include withdrawal or termination clauses, allowing states to exit under defined conditions. For that reason, treaty law has become the dominant method for creating new international obligations in many fields. However, treaties bind only their parties; a state that never ratifies the Paris Agreement, for example, is not bound by its specific emission targets, even though it may still be subject to broader customary rules on environmental protection.
Opinio juris and state practice: formation of customary norms
Customary international law develops differently. It is not written down in a single authoritative instrument but inferred from how states behave and what they say about their behaviour. Two elements are required: consistent, general state practice and opinio juris, the belief that such practice is carried out of a sense of legal obligation rather than mere habit or convenience. For instance, the widespread practice of granting diplomatic immunity, combined with states’ explicit statements that such immunity is required by law, helped crystallise diplomatic immunity as customary international law even before it was codified in the Vienna Convention on Diplomatic Relations.
This process can be slow and sometimes contested. How many states must act in a particular way, and for how long, before we can say a customary norm exists? What happens when powerful states object? These questions make customary law more flexible but also more uncertain than treaty law. Yet custom has decisive advantages: it binds all states (with limited exceptions) and can fill gaps where no treaty exists or where treaty coverage is patchy. In fields like the law of the sea, humanitarian law, and the law of state responsibility, customary norms work alongside treaties, sometimes reinforcing them and sometimes going further than the written text.
Jus cogens principles: peremptory norms and non-derogability
Within both treaty and customary law, some norms occupy a higher status as jus cogens, or peremptory norms of general international law. Article 53 of the VCLT defines a peremptory norm as one accepted and recognised by the international community of states as a whole as a norm from which no derogation is permitted and which can be modified only by a subsequent norm of the same character. Examples widely cited include the prohibitions on genocide, slavery, torture, and aggressive war.
Why does this matter in practice? Because any treaty that conflicts with a jus cogens norm is void, and no state can legitimize conduct such as torture or genocide by agreement. Even if two states were to sign a bilateral treaty permitting slavery, that instrument would have no legal effect. For businesses and non-state actors, the growing emphasis on jus cogens is reflected in expectations around human rights due diligence, supply chain responsibility, and corporate complicity in grave violations. In effect, these peremptory norms set a non-negotiable floor of conduct for the entire international community.
Persistent objector doctrine: exceptions to universal custom
Customary international law is generally binding on all states, but the “persistent objector” doctrine creates a narrow exception. Under this doctrine, a state that consistently and clearly objects to an emerging customary norm from the outset of its formation, and maintains that objection over time, may avoid being bound by that particular rule once it crystallises. The logic is that customary law is based on state consent expressed through practice and belief; a state that persistently withholds that consent should not be forced into the bargain.
In reality, the doctrine is difficult to invoke successfully. A state must show a continuous pattern of objection, both in its statements and its conduct, and it cannot escape peremptory norms like the prohibition of genocide or torture. Moreover, for many global issues—such as environmental protection of the high seas or basic rules of diplomatic law—practical interdependence makes persistent objection politically costly. Still, the doctrine illustrates how treaty law and customary law differ in terms of consent: while treaty obligations hinge on express acceptance (signature and ratification), customary obligations arise from the broader pattern of practice, leaving only limited room for unilateral opt-outs.
International humanitarian law versus international human rights law
Another crucial internal distinction in international law lies between international humanitarian law (IHL) and international human rights law (IHRL). Both seek to protect human beings from serious harm, but they operate in different contexts, follow different logics, and rely on distinct mechanisms of implementation. For policymakers, military planners, and human rights advocates, understanding where these regimes overlap—and where they diverge—is essential.
Geneva conventions and additional protocols: armed conflict regulation
International humanitarian law, sometimes called the law of armed conflict, governs conduct during armed conflicts, whether international (between states) or non-international (between a state and non-state armed groups, or between such groups themselves). Its core instruments are the four Geneva Conventions of 1949 and their Additional Protocols, which have near-universal ratification. These treaties protect wounded and sick combatants, prisoners of war, and civilians not directly participating in hostilities, and they impose obligations on parties to distinguish between combatants and civilians, to refrain from unnecessary suffering, and to respect the principle of proportionality.
IHL accepts that certain forms of violence are lawful when carried out in accordance with its rules. For example, targeting a lawful military objective may be permissible even if it carries a risk of incidental civilian harm, provided that harm is not excessive in relation to the concrete and direct military advantage anticipated. In this sense, IHL functions like a specialised rulebook for wartime, seeking to humanise conflict rather than to outlaw it entirely. Compliance is monitored by states, the International Committee of the Red Cross (ICRC), and, in cases of serious violations, international criminal tribunals and the International Criminal Court.
ICCPR and ECHR: protection standards during peacetime
International human rights law, by contrast, is designed primarily for peacetime, although it continues to apply during armed conflict. Key instruments include the International Covenant on Civil and Political Rights (ICCPR), the International Covenant on Economic, Social and Cultural Rights, and regional treaties such as the European Convention on Human Rights (ECHR), the American Convention on Human Rights, and the African Charter on Human and Peoples’ Rights. These instruments set out minimum standards for the treatment of individuals by states, including rights to life, liberty, fair trial, privacy, and freedom of expression.
Unlike IHL, which regulates both state and non-state parties to a conflict, IHRL obligations traditionally run from states to individuals under their jurisdiction. Enforcement mechanisms are also different: regional human rights courts like the European Court of Human Rights can issue binding judgments against states, while UN treaty bodies such as the Human Rights Committee issue views and recommendations that, while not strictly binding, carry significant moral and political weight. For businesses and organisations, these human rights obligations increasingly shape expectations around corporate social responsibility and ESG (environmental, social, governance) standards.
Temporal application: lex specialis derogat legi generali principle
A recurring question is how IHL and IHRL interact during armed conflict. Do human rights “stop at the battlefield”? The answer, developed through case law and doctrine, is that both regimes apply concurrently, but IHL often serves as the lex specialis—the more specific law that takes precedence over the more general. This principle of lex specialis derogat legi generali helps resolve conflicts where the two bodies of law appear to set different standards for the same situation.
For example, the right to life under human rights law generally prohibits arbitrary deprivation of life, with the default assumption being that lethal force is exceptional and must follow strict law enforcement standards. In armed conflict, however, IHL’s rules on targeting allow for killing enemy combatants as long as principles of distinction and proportionality are respected. Human rights bodies, such as the International Court of Justice in the Nuclear Weapons advisory opinion, have recognised that in such contexts, the assessment of whether a deprivation of life is “arbitrary” must be informed by applicable IHL rules. For practitioners, this means that analysing an incident in a conflict zone requires a dual lens: human rights law for the overarching framework and IHL for the operational details.
Derogation provisions: article 15 ECHR and article 4 ICCPR distinctions
International human rights law does allow some flexibility in times of emergency through derogation clauses. Article 4 of the ICCPR permits states to derogate from certain obligations “in time of public emergency which threatens the life of the nation” and is officially proclaimed, provided measures are strictly required by the exigencies of the situation and are not discriminatory. Some rights, however, such as the right to life (with narrow exceptions), the prohibition of torture, and the prohibition of slavery, are non-derogable.
Article 15 of the ECHR provides a similar but not identical mechanism for Council of Europe member states. It allows derogations “in time of war or other public emergency threatening the life of the nation” but requires notification to the Secretary General and retains a closed list of non-derogable rights, including the right to life (except in respect of lawful acts of war), the prohibition of torture and inhuman or degrading treatment, and the prohibition of slavery. The precise drafting differences matter: for instance, the ECHR explicitly links lawful acts of war to its right-to-life provision, underscoring the interplay with IHL. For governments, getting derogation procedures right is not a mere formality; human rights courts have scrutinised whether emergencies were genuine and whether derogation measures were proportionate, striking down overreach in several cases.
Monism versus dualism: domestic implementation of international obligations
International law does not operate in a vacuum; it must be received and applied by domestic legal systems if it is to have real-world impact. This is where the classic debate between monism and dualism comes in. Monist systems treat international law and national law as part of a single legal order. Once a state ratifies a treaty, its provisions may become directly applicable in domestic courts without further legislation, particularly if they are “self-executing.” Countries such as the Netherlands and, to a significant extent, Germany follow this approach, often giving international law priority over conflicting domestic statutes.
Dualist systems, by contrast, view international and domestic law as separate spheres. In these countries, treaties create obligations on the international plane but do not automatically change domestic law. Instead, parliaments must enact implementing legislation before individuals can rely on treaty rights in national courts. The United Kingdom and India are prominent examples: both require domestic legislation to give effect to most treaty commitments, and courts have repeatedly affirmed that unincorporated treaties cannot override national statutes. The United States follows a hybrid model, distinguishing between self-executing treaties, which may be directly enforceable, and non-self-executing treaties, which require implementing legislation.
Why does this matter for you as a practitioner or policymaker? Because the same international norm can have very different practical effects depending on the domestic system. A human rights treaty might allow an individual in a monist state to bring a claim directly in national courts, while an individual in a dualist state must wait for parliament to implement the treaty or rely on courts creatively interpreting existing law in line with international standards. For businesses operating across borders, understanding whether a jurisdiction is monist or dualist can help predict litigation risk, compliance obligations, and potential avenues for challenging state measures inconsistent with international commitments.
Hard law versus soft law instruments: legal bindingness and compliance
Not all international norms are created equal in terms of their binding legal force. International “hard law” refers to rules that are legally binding on states or other actors and typically precise in their content, such as treaties or certain UN Security Council resolutions adopted under Chapter VII. By contrast, “soft law” encompasses non-binding instruments—declarations, guidelines, codes of conduct, or framework agreements—that articulate standards of behaviour without formal legal obligation. Examples include the UN Guiding Principles on Business and Human Rights, many climate pledges, and numerous resolutions of the UN General Assembly.
At first glance, soft law may seem less important because it lacks enforceability in international courts. Yet it often plays a crucial role in norm development. Soft law can serve as a testing ground for new standards, enabling states to commit politically before they are ready to accept full legal obligations. Over time, repeated reference to soft law instruments and consistent state practice aligning with them can help crystallise customary international law or pave the way for formal treaties. For instance, non-binding environmental declarations in the 1970s and 1980s helped shape later binding agreements on ozone depletion and climate change.
From a practical standpoint, you will often see soft law exerting significant influence through reputational pressure, market expectations, and litigation strategies. Courts and arbitral tribunals sometimes cite soft law as evidence of emerging standards or as interpretive aids when construing treaty provisions. Investors and consumers increasingly expect companies to adhere to soft law standards on human rights and sustainability, even in the absence of binding regulation. In that sense, the line between hard law and soft law is less rigid than it appears: soft law can be a precursor to hard law, an interpretive tool, or a benchmark for assessing due diligence and corporate responsibility.
Universal jurisdiction versus complementarity principle in international criminal law
In international criminal law, two powerful but distinct ideas shape how atrocity crimes are prosecuted: universal jurisdiction and the complementarity principle. Both respond to the same concern—that some crimes are so serious that they affect the international community as a whole—but they allocate responsibility differently between national courts and international tribunals.
Rome statute article 17: ICC admissibility requirements
The International Criminal Court (ICC), created by the Rome Statute, operates on the basis of complementarity. Under Article 17, a case is inadmissible before the ICC if it is being genuinely investigated or prosecuted by a state with jurisdiction, or if the state has already genuinely tried the case. The Court steps in only when national authorities are “unwilling or unable” genuinely to carry out the investigation or prosecution—for example, where proceedings are a sham, shield powerful perpetrators, or are impossible due to collapse of the national justice system.
This complementarity principle reflects a core design choice: primary responsibility for prosecuting genocide, crimes against humanity, war crimes, and the crime of aggression remains with states. The ICC is a court of last resort, not a first-instance tribunal. For states and practitioners, this means that strengthening domestic capacity to investigate and prosecute international crimes is not just a matter of sovereignty but also a way to retain control over sensitive cases and avoid ICC scrutiny. It also incentivises reforms in criminal codes, military manuals, and judicial training to ensure that international crimes are properly incorporated and enforceable at home.
Pinochet case and eichmann trial: precedents in universal jurisdiction
Universal jurisdiction takes a different route. It allows (or, in some cases, obliges) states to prosecute certain grave crimes regardless of where they were committed or the nationality of the perpetrators or victims. Classic examples include piracy, war crimes, crimes against humanity, genocide, and torture. The rationale is that these offences are so serious that every state has an interest in repressing them, much as every state has an interest in suppressing piracy on the high seas.
Two landmark cases illustrate how universal jurisdiction works in practice. In the Eichmann trial (Israel v. Eichmann, 1961), Israel tried Adolf Eichmann for crimes against the Jewish people and humanity committed during the Holocaust, even though the crimes occurred before Israel’s establishment and outside its territory. The Israeli court invoked universal jurisdiction over crimes against humanity. Decades later, in the Pinochet case in the United Kingdom, Spanish judges sought the extradition of former Chilean dictator Augusto Pinochet for torture and other crimes. The House of Lords held that former heads of state could be subject to extradition for acts of torture committed after the UN Convention Against Torture entered into force, rejecting broad claims of immunity. These cases signalled that individuals accused of atrocity crimes cannot always rely on borders or high office to shield them from prosecution.
National prosecution obligations under geneva conventions
Universal jurisdiction is not merely permissive; in some areas, it is mandatory. The Geneva Conventions of 1949 require states parties to “search for persons alleged to have committed, or to have ordered to be committed, grave breaches” and either prosecute them before their own courts or hand them over to another state willing to prosecute—aut dedere aut judicare. Grave breaches include serious war crimes such as wilful killing, torture or inhuman treatment, and unlawful deportation of protected persons. Similar obligations appear in the Convention Against Torture and certain counter-terrorism treaties.
For national authorities, these obligations mean that international criminal law is not just an abstract concern for international tribunals. Domestic prosecutors may be required to open investigations when alleged perpetrators of grave breaches or torture are found on their territory, even if the crimes were committed abroad. This can create diplomatic sensitivities—especially where high-ranking foreign officials are involved—but it also offers a powerful tool against impunity. For practitioners advising states or international organisations, understanding the interplay between universal jurisdiction and complementarity is key to designing accountability strategies that are both legally sound and politically feasible.
