International Law Beginner's Guide
Treaties, custom, jus cogens — the sources of law that bind states across borders.
Sources
ICJ Statute Article 38
Article 38(1) of the Statute of the International Court of Justice is the most authoritative listing of international law sources. Adopted as part of the UN Charter (1945), it codifies a framework that traces to the 1920 Statute of the Permanent Court of International Justice. The hierarchy isn't strict — sources interact, and modern practice recognizes additional sources (unilateral declarations, certain UN GA resolutions) that fit awkwardly within Article 38's four categories. The International Law Commission's work and the writings of publicists continue to refine the framework.
Key Points
- International conventions (treaties): written agreements between states, governed by the Vienna Convention on the Law of Treaties (VCLT, 1969).
- International custom: state practice + opinio juris (psychological element — belief that the practice is legally required).
- General principles of law recognized by civilized nations: principles common to major legal systems (good faith, estoppel, res judicata, proportionality).
- Judicial decisions and scholarly writings (subsidiary means under Article 38(1)(d) — not formal sources but persuasive evidence of what the law is).
- Equity (ex aequo et bono) under Article 38(2) — only if parties agree; never used in ICJ contentious cases.
- Soft law instruments (UN GA resolutions, declarations, codes of conduct) — not formally binding but evidence of opinio juris and often crystallize into custom.
- Jus cogens norms (peremptory norms) sit above ordinary treaty and custom — VCLT Articles 53 and 64.
Treaties
Treaties are written agreements between states, governed by the 1969 Vienna Convention on the Law of Treaties (VCLT) — itself widely treated as codifying customary international law on this topic (the ICJ has consistently held so in cases like Gabčíkovo-Nagymaros). The VCLT covers conclusion, entry into force, interpretation, modification, termination, and invalidity. The 1986 Vienna Convention covers treaties involving international organizations. Multilateral treaty practice has accelerated since 1945, with the UN Treaty Section serving as depository for 600+ multilateral treaties.
Negotiation + adoption
Delegates negotiate text in diplomatic conferences or through standing bodies (ILC, UN GA Sixth Committee). Adoption requires consent of negotiating states — typically by consensus, or 2/3 majority under VCLT Article 9(2) at conferences.
Signature
Authenticates text and expresses provisional commitment (VCLT Art 18: 'object and purpose' obligation not to defeat the treaty). Doesn't yet bind the state to the treaty's substantive obligations.
Ratification
Domestic process expressing definitive consent to be bound. US: Senate 2/3 advice and consent (Constitution Art II §2); UK: Crown prerogative with Parliament notification under CRAG 2010; Germany: Bundestag/Bundesrat approval. Binds the state once instruments are deposited with the depositary.
Accession
Joining a treaty without prior signature — typically available for multilateral treaties under accession clauses. Functionally equivalent to ratification.
Entry into force
Usually after a minimum number of ratifications specified in the treaty itself (Rome Statute of the ICC needed 60 ratifications; entered force July 1, 2002). Bilateral treaties typically enter force on a specified date or exchange of instruments.
Reservations
Unilateral statements modifying treaty effect for the reserving state. Permissible unless prohibited by the treaty or incompatible with object and purpose (VCLT Art 19). The Reservations to the Genocide Convention Advisory Opinion (1951) is foundational.
Termination and suspension
By consent (VCLT Art 54), material breach (Art 60), supervening impossibility (Art 61), fundamental change of circumstances/rebus sic stantibus (Art 62 — narrowly construed). Withdrawal often requires notice periods (e.g., US notice to withdraw from JCPOA, Paris Agreement).
Customary international law
Customary international law is practice that states follow out of a sense of legal obligation. Two elements: state practice (the objective element) + opinio juris (the subjective element — belief that the practice is legally required). Identification is contested — the ILC's 2018 Draft Conclusions on the Identification of Customary International Law (A/73/10) provide the authoritative modern framework. Custom binds all states unless they qualify as persistent objectors.
Key Points
- State practice: must be consistent, general, and representative — though uniformity isn't required (North Sea Continental Shelf, 1969).
- Opinio juris: states believe the practice is legally required, not just convenient or polite. Hardest element to prove.
- Persistent objector rule: a state that consistently objects from the outset can avoid being bound — applies only to ordinary custom, not jus cogens.
- Relationship with treaties: treaties can crystallize, codify, or generate custom (Art 38 VCLT). The same rule can exist in both treaty and customary form.
- Examples: prohibition on torture, diplomatic immunity (Vienna Convention on Diplomatic Relations 1961 codifies custom), freedom of navigation, immunity of state property from execution.
- Modern challenges: cyber operations, autonomous weapons, AI governance — customary rules forming in real time with disputed content.
- Regional custom: special custom binding specific regions (Asylum Case, Colombia v. Peru, 1950) — narrowly recognized.
Jus cogens
Jus cogens norms are peremptory norms of general international law from which no derogation is permitted — even by treaty. They are the highest tier of international law, recognized by Articles 53 and 64 of the VCLT. The International Law Commission's 2022 Draft Conclusions on Peremptory Norms (A/77/10, ch. IV) compiled the existing canon: prohibition of aggression, genocide, crimes against humanity, slavery, racial discrimination/apartheid, torture, and the right of self-determination. The list is illustrative, not exhaustive.
Key Points
- Accepted jus cogens: prohibition of aggression, genocide, slavery, torture, apartheid, crimes against humanity, racial discrimination, and right of self-determination.
- Treaties conflicting with jus cogens are void ab initio (VCLT Art 53) or terminated by emergence of a new norm (Art 64).
- ILC's 2022 draft conclusions on jus cogens (A/77/10) formalized existing practice and provide the modern authoritative restatement.
- Erga omnes obligations (Barcelona Traction, 1970) are related but distinct — obligations owed to the international community as a whole; all states have legal interest in compliance.
- Practical impact: state immunity defenses fail against jus cogens claims in some jurisdictions (though Jurisdictional Immunities of the State, Germany v. Italy, 2012, narrowed this).
- Universal jurisdiction: many jus cogens violations (torture, genocide, war crimes) ground universal jurisdiction in domestic courts.
- Open questions: is the prohibition of the use of force jus cogens? (likely yes for aggression, contested for lesser uses). Is forced labor distinct from slavery? (modern slavery instruments suggest convergence.)
Structure
Subjects of international law
International law was built by and for states, but the modern system recognizes a wider range of subjects. The Reparations for Injuries Advisory Opinion (1949) established that the UN has international legal personality even though states are the primary subjects. International criminal law and human rights law have given individuals direct status in some contexts.
Key Points
- States: primary subjects — sovereign equality (UN Charter Art 2(1)). Recognition is declarative (constitutive theories largely rejected).
- International organizations: derived legal personality (Reparations Advisory Opinion, 1949) — capacity depends on the constituent treaty.
- Individuals: limited but growing — international criminal law (Nuremberg, ICC), human rights (individual petitions to ECtHR, treaty bodies).
- Non-state actors: armed groups, corporations, NGOs — treated ad hoc. Common Article 3 of the Geneva Conventions binds non-state armed groups in non-international armed conflicts.
- Peoples: right of self-determination (UN Charter Art 1(2), ICCPR/ICESCR Common Art 1) — operationalized for colonial peoples; contested for sub-state peoples.
- Indigenous peoples: UNDRIP (2007) — declaratory but increasingly cited as evidence of customary norms (FPIC — free, prior, and informed consent).
Statehood
The 1933 Montevideo Convention's four criteria remain the canonical statement: permanent population, defined territory, government, and capacity to enter relations with other states. Modern practice adds recognition (politically central even if not legally constitutive) and effectiveness. Difficult cases: Kosovo (declared independence 2008; ICJ Advisory Opinion 2010 found declaration did not violate international law), Palestine (recognized by 140+ states; UN observer state status since 2012), Taiwan (de facto statehood without UN recognition).
Monism vs dualism
How domestic legal systems treat international law. The distinction is academic — most systems are hybrid in practice — but it shapes how individuals can invoke international obligations in domestic courts.
Monism
International and domestic law are one system. International law directly applies in domestic courts once binding on the state. Netherlands (Constitution Art 94 — international law prevails over conflicting statutes), France.
Dualism
Separate systems. International law applies domestically only via domestic implementing legislation. UK, most Commonwealth countries. Treaties don't create rights enforceable in UK courts unless incorporated by statute.
Hybrid (US)
Self-executing vs non-self-executing treaties; Foster v. Neilson (1829, Chief Justice Marshall) is the foundation. Most US treaties require implementing legislation. Medellín v. Texas (2008) confirmed even ICJ judgments are not self-executing.
Customary international law in domestic law
UK: Trendtex (1977) — custom is part of common law, but statute prevails. US: The Paquete Habana (1900) — international law is 'part of our law,' subject to controlling executive/legislative acts. Germany: Basic Law Art 25 — general rules of international law have supra-statutory rank.
Applying the Law
Treaty interpretation
VCLT Articles 31-33 govern treaty interpretation. The general rule (Art 31): a treaty shall be interpreted in good faith in accordance with the ordinary meaning of its terms in their context and in light of its object and purpose. Supplementary means (Art 32 — preparatory work, circumstances of conclusion) only if the general rule produces ambiguous, obscure, manifestly absurd, or unreasonable results. The ICJ applies this framework consistently; investor-state tribunals (ICSID) have generated extensive jurisprudence on Art 31.
Key Points
- Article 31(1): good faith, ordinary meaning, context, object and purpose — applied together, not sequentially.
- Subsequent practice and agreements (Art 31(3)(b)): how parties have actually applied the treaty matters.
- Relevant rules of international law (Art 31(3)(c)): treaty interpretation isn't insulated from broader international law (the principle of systemic integration).
- Authentic texts in multiple languages (Art 33): all are equally authoritative; presumption of same meaning.
- Evolutive interpretation: living instruments adapt over time (ECHR jurisprudence — Tyrer v UK 1978; the WTO's reasoning in US-Shrimp 1998 on 'exhaustible natural resources').
State responsibility
When states breach international obligations, legal consequences follow. The ILC's 2001 Articles on Responsibility of States for Internationally Wrongful Acts (ARSIWA) are the authoritative source — formally annexed to A/RES/56/83 but treated by international tribunals as restating customary law. The 2001 Articles cover attribution, breach, circumstances precluding wrongfulness, and consequences (cessation, reparation, non-repetition).
Key Points
- Attribution: the act must be attributable to the state. Conduct of state organs (Art 4), persons exercising governmental authority (Art 5), conduct directed or controlled by the state (Art 8 — effective control test, Nicaragua 1986).
- Breach: international obligation must be in force for the state at the time of the conduct and the conduct must not conform with the obligation (Arts 12-13).
- Consequences: cessation, non-repetition, reparation (restitution, compensation, satisfaction — Arts 30-37).
- Defenses (circumstances precluding wrongfulness): consent (Art 20), self-defense (Art 21), countermeasures (Art 22, 49-54), force majeure (Art 23), distress (Art 24), necessity (Art 25 — narrow).
- Serious breach of peremptory norm (Arts 40-41): all states must cooperate to bring violation to an end, not recognize as lawful, not aid in maintaining situation.
- Erga omnes obligations: all states have legal interest in compliance (Barcelona Traction 1970, East Timor 1995).
- Diplomatic protection: state may espouse claim of national against another state (ARSIWA Art 44; ILC Draft Articles on Diplomatic Protection, 2006).
Use of force
Article 2(4) of the UN Charter prohibits the use of force against the territorial integrity or political independence of any state. Two recognized exceptions: self-defense (Article 51, individual or collective, against armed attack) and SC authorization under Chapter VII (Article 42). A contested third category — humanitarian intervention/R2P — has been invoked (Kosovo 1999, Libya 2011) but lacks settled legal status.
Key Points
- Armed attack threshold (Nicaragua 1986): only grave forms of force trigger Article 51; lesser uses produce countermeasures and lawful proportionate response.
- Anticipatory self-defense: Caroline criteria (1837) — necessity 'instant, overwhelming, leaving no choice of means and no moment for deliberation' — extended controversially after 9/11.
- Self-defense against non-state actors: contested. ICJ has been narrow (Wall Opinion 2004, DRC v. Uganda 2005); state practice has been more permissive post-9/11.
- Collective self-defense (Art 51, Nicaragua 1986): requires declaration of attack and request for assistance from the victim state.
- Russian invasion of Ukraine (2022): cited Article 51 (collective self-defense of Donetsk and Luhansk) — rejected by 141 GA states (ES-11/1).
FAQ
Why do states comply with international law?
Chayes & Chayes (1995, 'The New Sovereignty'): the managerial model — compliance is largely about capacity, not intent. States want to comply; gaps reflect lack of capacity or unclear obligations. Keohane and rational choice scholars: reputation, reciprocity, and institutions create incentives to comply. Constructivists: states comply because they internalize norms as constitutive of identity. Most compliance is quiet and routine; high-profile violations are the exception that draws attention.
How is international law enforced?
Diffusely. ICJ decisions are binding (Charter Art 94) but enforcement depends on SC support — where P5 veto applies. Trade retaliation (WTO Dispute Settlement Body, though Appellate Body paralysis since 2019 weakens this), sanctions (UN SC, EU, US OFAC, UK OFSI), domestic courts applying international norms, naming-and-shaming by treaty bodies and special rapporteurs, criminal prosecution under universal jurisdiction. Enforcement is weakest where it matters most — major powers face few external enforcement mechanisms.
What's the difference between primary and secondary rules?
Borrowed from H.L.A. Hart. Primary rules impose obligations on subjects (prohibition of genocide, freedom of navigation). Secondary rules govern how primary rules are created, applied, and changed (the law of treaties, state responsibility, recognition, attribution). ARSIWA is secondary law — it tells you what happens when a primary rule is breached. The VCLT is secondary law — it tells you how treaties (which contain primary rules) are made and applied.
What is the difference between jus cogens and erga omnes?
Jus cogens describes the normative status of a rule — it is peremptory and cannot be derogated from. Erga omnes describes the structure of the obligation — it is owed to the international community as a whole, so all states have legal interest in compliance (Barcelona Traction 1970). All jus cogens norms create erga omnes obligations; not all erga omnes obligations are jus cogens. The distinction matters for standing in international litigation (e.g., The Gambia v. Myanmar genocide case before the ICJ).
Continue learning
Explore related MUN guides to deepen your skills.