Treaties and international law

Intuition [Beginner]

Domestic law is enforced by governments: police arrest lawbreakers, courts try them, and prisons hold them. International law has no global police force, no world court with compulsory jurisdiction, and no global executive to enforce its judgments. Yet international law exists, is widely followed, and matters enormously.

International law is the body of rules that governs the relations between states (and, increasingly, between states and other actors like international organizations and individuals). Its sources are treaties (written agreements between states), customary law (practices that states follow out of a sense of legal obligation), and general principles of law recognized by civilized nations.

A treaty is a written agreement between two or more states that is binding under international law. Treaties go by many names -- convention, protocol, covenant, pact, accord, agreement -- but they all have the same legal character: states voluntarily accept obligations that they are then legally bound to fulfill.

The puzzle of international law is: why do states obey it when there is no global enforcer? Several answers:

• Reputation: states that break their agreements are trusted less in future negotiations.
• Reciprocity: states follow rules because they want others to follow them too.
• Domestic enforcement: many treaties are implemented through domestic law, enforced by domestic courts.
• Institutional pressure: international organizations monitor compliance and publicize violations.
• Habit and legitimacy: most states most of the time follow most rules because doing so is normal and expected.

International law is not weak -- it governs trade, diplomacy, war, human rights, the environment, aviation, shipping, and much of daily life across borders. But it is imperfect: enforcement is uneven, powerful states sometimes ignore it, and it evolves slowly.

Visual [Beginner]

Sources of international law

Source	What it is	Example
Treaties	Written agreements between states, binding on parties	UN Charter (1945), Geneva Conventions (1949), Paris Agreement (2015)
Customary international law	General practice of states accepted as law	Diplomatic immunity, prohibition of genocide, principle of non-refoulement
General principles	Principles common to major legal systems	Good faith, proportionality, due process
Judicial decisions	Rulings by international courts (subsidiary source)	ICJ advisory opinions, ICTY/ICTR rulings
Scholarly writings	Treatises by qualified publicists (subsidiary source)	International Law Commission drafts

How a treaty is made

Step	What happens
1. Negotiation	States discuss and draft the text (in a conference, through an IO, or bilaterally)
2. Adoption	The text is adopted (often by vote at a conference or in an international organization)
3. Signature	Representatives sign the text, indicating intent to consider ratification
4. Ratification	Each state's domestic process (legislative approval, executive decision) to become legally bound
5. Entry into force	The treaty becomes binding when a specified number of states have ratified
6. Implementation	States incorporate treaty obligations into domestic law and practice
7. Monitoring	Treaty bodies, peer review, or international courts assess compliance

International courts and tribunals

Court	Jurisdiction	Binding?	Enforcement
International Court of Justice (ICJ)	Disputes between states	Yes (for parties to the case)	Security Council can recommend enforcement; limited in practice
International Criminal Court (ICC)	Individuals for genocide, war crimes, crimes against humanity	Yes	Depends on state cooperation; no independent police force
WTO Dispute Settlement	Trade disputes between WTO members	Yes	Retaliatory tariffs authorized if non-compliant
European Court of Human Rights	Individual complaints against ECHR member states	Yes	Committee of Ministers monitors compliance
Court of Justice of the EU	EU law disputes	Yes	Fines can be imposed on non-compliant member states
Inter-American Court of Human Rights	Human rights cases from OAS members	Yes (for parties)	Limited enforcement capacity

Worked example [Beginner]

Consider how the Paris Agreement on climate change (2015) works as a treaty:

1. Negotiation: 196 parties negotiated the agreement at the UN Climate Change Conference (COP21) in Paris. The negotiations were complex, involving differences between developed and developing countries, fossil-fuel-dependent economies and vulnerable island states, and countries with different views on how binding the commitments should be.

2. Adoption: The agreement was adopted by consensus on December 12, 2015.

3. Signature and ratification: States signed and then ratified according to their domestic procedures. The US (under Obama) ratified by executive agreement; the US (under Trump) withdrew in 2020; the US (under Biden) rejoined in 2021. This illustrates how domestic politics affects treaty participation.

4. Structure: The Paris Agreement is a "pledge and review" system. Each country submits a Nationally Determined Contribution (NDC) -- its own plan for reducing emissions. The NDCs are not legally binding in their specific content (no country is required to hit a specific emissions target), but the process of submitting NDCs, reporting progress, and participating in periodic review is binding.

5. Enforcement: There is no enforcement mechanism for failure to meet NDC targets. The agreement relies on transparency (countries must report their emissions), peer pressure (global scrutiny of NDCs), and the expectation that naming and shaming will encourage compliance. Critics argue this is too weak; supporters argue that a stricter treaty would not have been ratified by enough countries.

The Paris Agreement illustrates a central feature of international law: it often reflects what states are willing to agree to, not what the problem demands. A treaty that perfectly solved climate change but that no major emitter would ratify would be legally elegant but practically useless. The Paris Agreement sacrifices binding specificity for broad participation.

Check your understanding [Beginner]

Formal definition [Intermediate+]

International law is the set of rules, norms, and principles that govern the conduct of states and other international actors in their relations with one another. Its formal sources are enumerated in Article 38(1) of the Statute of the International Court of Justice:

1. International conventions (treaties)
2. International custom (state practice + opinio juris)
3. General principles of law recognized by civilized nations
4. Judicial decisions and scholarly writings (as subsidiary means)

Treaty law. The Vienna Convention on the Law of Treaties (1969) is the "treaty on treaties," governing how treaties are made, interpreted, amended, and terminated. Key principles:

• Pacta sunt servanda: agreements must be kept. A state cannot invoke its domestic law as justification for failing to perform a treaty obligation (Article 27).
• Consent: treaties bind only those states that have consented to be bound (Article 11). There is no majority-rule legislation at the international level.
• Reservations: states may modify their treaty obligations through reservations, unless the reservation is prohibited by the treaty or incompatible with its object and purpose (Article 19).
• Interpretation: treaties are interpreted in good faith according to their ordinary meaning in context, in light of their object and purpose (Article 31).
• Invalidity: treaties can be invalidated by corruption, coercion, error, or conflict with a peremptory norm (jus cogens) of international law (Articles 46-53).

Jus cogens (peremptory norms) are rules from which no derogation is permitted. No treaty can violate them, and they bind all states regardless of consent. Accepted jus cogens norms include the prohibitions on genocide, slavery, torture, wars of aggression, and crimes against humanity. The concept is controversial because it imposes limits on state consent, which is the normal basis of international obligation.

Treaty reservations. A reservation is a unilateral statement by a state, made when signing or ratifying a treaty, that modifies or excludes certain provisions. Reservations allow states with different legal systems, cultural values, or policy preferences to join the same treaty while declining specific obligations. The practice promotes universality (more states join) but at the cost of uniformity (different states are bound by different obligations under the same treaty). Some treaties (like the European Convention on Human Rights) severely restrict reservations; others (like the Convention on the Elimination of All Forms of Discrimination Against Women, CEDAW) have attracted extensive reservations.

Key concepts [Intermediate+]

Monism vs. dualism in domestic legal systems. How does international law interact with domestic law? Two theories:

Theory	Description	Effect	Examples
Monism	International and domestic law are one system; international law is automatically part of domestic law	Treaties take effect domestically without implementing legislation	France, Netherlands, Switzerland (varies in practice)
Dualism	International and domestic law are separate systems; international law must be "transformed" into domestic law through legislation	Treaties require implementing legislation before they create domestic rights and obligations	UK, Canada, Australia, India

In practice, most states fall somewhere between pure monism and pure dualism. The US follows a mixed approach: self-executing treaties (those that create directly enforceable rights) take effect without legislation; non-self-executing treaties require implementing legislation. Even in monist systems, courts may be reluctant to give direct effect to treaty provisions that conflict with domestic law.

The International Court of Justice (ICJ). The ICJ is the principal judicial organ of the UN. It hears two types of cases:

1. Contentious cases: legal disputes between states that have accepted its jurisdiction (either by special agreement, treaty clause, or optional clause declaration). Judgments are binding on the parties.
2. Advisory opinions: legal advice requested by UN organs and specialized agencies. Advisory opinions are non-binding but carry legal and moral authority.

The ICJ's jurisdiction is not compulsory. States must consent to its jurisdiction, and many states have made reservations that exclude certain categories of disputes. This means the ICJ can only resolve disputes when both parties agree to let it.

The International Criminal Court (ICC). Established by the Rome Statute (1998, entered into force 2002), the ICC has jurisdiction over individuals (not states) accused of:

• Genocide
• Crimes against humanity
• War crimes
• The crime of aggression (since 2018, with restrictions)

The ICC's jurisdiction is limited: it can only act when the crime was committed on the territory of a member state or by a national of a member state, or when the UN Security Council refers a situation. Major states (US, China, Russia, India) are not members, limiting the ICC's reach. The principle of complementarity means the ICC acts only when national courts are unwilling or unable to prosecute -- it is a court of last resort, not a replacement for domestic justice.

Exercises

Exercise 1. The US is not a party to the Rome Statute and does not recognize the ICC's jurisdiction over its citizens. Yet the ICC claims jurisdiction over alleged war crimes committed by US nationals on the territory of Afghanistan (an ICC member). How is this legal tension resolved?

Reveal

The ICC asserts jurisdiction based on the territorial principle: the alleged crimes occurred in Afghanistan, which is a member state. Under the Rome Statute, the ICC has jurisdiction over crimes committed on the territory of a member state, regardless of the nationality of the accused. The US rejects this claim, arguing that a state cannot be subject to a treaty it has not ratified and that the ICC's assertion of jurisdiction over non-members violates state sovereignty. The US imposed sanctions on ICC officials in 2020 (later lifted). The ICC proceeded with its investigation but faces practical limits: it cannot compel US cooperation, cannot arrest individuals without member state assistance, and cannot try anyone in absentia. The standoff illustrates the limits of international law when powerful non-members resist it.

Exercise 2. A treaty is adopted that contradicts an earlier treaty that some of the same states have ratified. Which treaty prevails?

Reveal

Under the Vienna Convention on the Law of Treaties (Article 30), the general rule is that when states are parties to both treaties, the later treaty prevails over the earlier one to the extent of the inconsistency ("lex posterior derogat legi priori"). But if a state is party to the earlier treaty but not the later one, the earlier treaty governs relations between that state and other parties to the earlier treaty. If the earlier treaty establishes a rule of jus cogens, the later treaty is void to the extent of the conflict. In practice, conflicts between treaties are resolved through interpretation (courts try to read treaties as consistent), negotiation (states modify or supplement the treaties), or, if all else fails, the principle that the later expression of state intent prevails.

Political theory [Master]

Is international law really law?

The legal philosopher John Austin (1832) defined law as the command of a sovereign backed by a sanction. By this standard, international law is not "real" law because there is no global sovereign to command and no reliable sanction for non-compliance. Hart (1961) offered a more sophisticated analysis: law exists when there is a "rule of recognition" -- a shared standard for identifying valid legal rules. International law arguably has such a standard (the sources enumerated in ICJ Statute Article 38), but it lacks secondary rules of change and adjudication that are well-developed in domestic systems.

Modern legal scholars generally reject Austin's narrow definition. International law is treated as law by the institutions that apply it (international courts, domestic courts, treaty bodies), by the states that follow it, and by the legal profession that advises on it. Its compliance rate, while imperfect, is comparable to many domestic legal systems. Henkin (1979) observed that "almost all nations observe almost all principles of international law and almost all of their obligations almost all of the time."

International law and power.

Realist scholars argue that international law reflects and reinforces the interests of powerful states. The structure of the UN Security Council (P5 veto) gives major powers legal authority to block action against themselves and their allies. The ICC has primarily prosecuted individuals from African states, leading to accusations of selective justice. Trade law rules (WTO) reflect the preferences of economically dominant states at the time they were negotiated. The corpus of international law was largely developed by European states during the colonial era and initially excluded non-European peoples from its protections.

Critical legal scholars (Koskenniemi, 2001) argue that international law's claims to neutrality and objectivity mask its political function: it provides a vocabulary for justifying the exercise of power while appearing to constrain it. The indeterminacy of legal rules -- their capacity to support multiple interpretations -- means that law can be invoked to support any position, and outcomes depend on which interpreter has authority.

Compliance theories. Why do states comply with international law? Several explanations compete:

• Enforcement theory (Downs, Rocke, and Barsoom, 1996): states comply when the costs of violation (sanctions, retaliation, loss of reputation) exceed the benefits. Compliance is rational, not virtuous.
• Managerial theory (Chayes and Chayes, 1995): most non-compliance results from ambiguity, capacity limitations, or unforeseen circumstances, not deliberate violation. The appropriate response is transparency, technical assistance, and dispute resolution, not punishment.
• Legitimacy theory (Franck, 1990): states comply because they perceive the rules as legitimate -- fairly made, clearly stated, and applied consistently. Legitimacy creates a "compliance pull" independent of enforcement.
• Norm internalization (Finnemore and Sikkink, 1998): international norms become internalized through socialization, persuasion, and institutionalization. Over time, states follow norms not because of external pressure but because they have come to accept them as appropriate.

Each theory captures part of the truth. States comply for a mixture of reasons that vary across issues, states, and circumstances.

The enforcement gap. The most significant weakness of international law is the gap between legal obligation and enforcement. When a state violates international law:

• There is no automatic enforcement mechanism.
• The injured state may retaliate (self-help), but this favors powerful states that can impose costs.
• International courts can issue binding judgments, but cannot compel compliance.
• The UN Security Council can authorize enforcement action, but only if no P5 member vetoes.
• Economic sanctions can be imposed, but their effectiveness is debated and their humanitarian costs can be severe.

This enforcement gap means that international law is strongest when it aligns with powerful states' interests and weakest when it contradicts them. This is not a flaw that can be easily corrected without creating a world government that states do not want.

Historical context [Master]

The origins of international law. The modern system of international law emerged from the European state system. Hugo Grotius (1583-1645), often called the "father of international law," synthesized earlier traditions (Roman law, canon law, scholastic thought) into a systematic framework for the law of nations. His De Jure Belli ac Pacis (1625) addressed when war is justified (jus ad bellum) and how it should be conducted (jus in bello). The Peace of Westphalia (1648) established the principle of territorial sovereignty that underlies the modern international system: each state has exclusive authority over its territory and is legally equal to all other states.

The 19th century: positivism and colonialism. 19th-century international law was dominated by legal positivism -- the view that law is created by the will of states, not derived from natural law or divine authority. This period saw the development of rules on diplomacy, neutrality, maritime law, and the laws of war. It also saw international law used to justify colonialism: European states classified non-European societies as "uncivilized" and therefore not entitled to the full protections of international law. The Berlin Conference (1884-85) divided Africa among European powers using legal instruments that excluded the affected populations.

The Hague and Geneva traditions. Two parallel traditions developed the law of armed conflict. The Hague Conventions (1899, 1907) addressed the methods and means of warfare (what weapons may be used, how combatants must behave). The Geneva Conventions (1864, 1906, 1929, 1949) addressed the protection of victims of war (wounded soldiers, prisoners of war, civilians). Together they form the modern law of armed conflict (international humanitarian law). The Additional Protocols of 1977 expanded protections for civilians and combatants in both international and non-international conflicts.

The Nuremberg and Tokyo trials. After World War II, the victorious Allies prosecuted Nazi and Japanese leaders for crimes against peace (waging aggressive war), war crimes, and crimes against humanity. The defense argued that these were ex post facto laws -- acts that were not criminal when committed. The tribunal rejected this: the principles underlying the charges (that aggressive war is illegal, that individuals can be held criminally responsible under international law) had crystallized into customary international law before the war. The Nuremberg Principles were later affirmed by the UN General Assembly and formed the basis for the International Criminal Court.

The post-war treaty explosion. Since 1945, the number of multilateral treaties has grown enormously: human rights treaties (UDHR, ICCPR, ICESCR, CERD, CEDAW, CAT, CRC, CRPD), environmental treaties (UNFCCC, CBD, Montreal Protocol, CITES), trade treaties (GATT/WTO), law of the sea (UNCLOS, 1982), and many others. This "treaty explosion" has greatly expanded the scope of international law but also raised concerns about fragmentation (conflicting treaty regimes), complexity (states face overlapping obligations), and compliance capacity (developing states struggle to implement their commitments).

The International Criminal Court and individual accountability. The ad hoc tribunals for the former Yugoslavia (ICTY, 1993) and Rwanda (ICTR, 1994) demonstrated that international criminal prosecution was feasible. The Rome Statute (1998) created the permanent ICC. The ICC has been both praised (for ending impunity for mass atrocities) and criticized (for selective prosecution, for targeting African states, for limited effectiveness). Its existence reflects a fundamental shift in international law: from a system governing only state-to-state relations to one that also holds individuals accountable for international crimes.

Bibliography [Master]

• Brownlie, Ian. 2019. Principles of Public International Law. 9th ed. Oxford University Press.
• Chayes, Abram, and Antonia Handler Chayes. 1995. The New Sovereignty. Harvard University Press.
• Downs, George, David Rocke, and Peter Barsoom. 1996. "Is the Good News about Compliance Good News about Cooperation?" International Organization 50(3): 379-406.
• Finnemore, Martha, and Kathryn Sikkink. 1998. "International Norm Dynamics and Political Change." International Organization 52(4): 887-917.
• Franck, Thomas. 1990. The Power of Legitimacy among Nations. Oxford University Press.
• Hart, H.L.A. 1961. The Concept of Law. Oxford University Press.
• Henkin, Louis. 1979. How Nations Behave. 2nd ed. Columbia University Press.
• Koskenniemi, Martti. 2001. The Gentle Civilizer of Nations. Cambridge University Press.
• Shaw, Malcolm. 2017. International Law. 8th ed. Cambridge University Press.
• Simma, Bruno, et al., eds. 2012. The Charter of the United Nations: A Commentary. 3rd ed. Oxford University Press.