Understanding Where International Law Actually Comes From
Most people think international law comes from some neat codex. It doesn't. The closest thing to a definitive list is Article 38 of the Statute of the International Court of Justice, which was written in 1945 by people who hadn't seen the last forty years of state practice and probably didn't think ahead about a lot of what would come after. Even so, it remains the starting point anyone has to deal with. Article 38(1) lists four formal sources. Treaties are the cleanest. A treaty is a written agreement between states (or between states and international organizations) governed by international law. When a state ratifies something, it's generally bound by it. Vienna Convention on the Law of Treaties, 1969, articles 26 and 27 basically say you can't use your own domestic law to get out of a treaty obligation. I've seen governments try this in dispute after dispute. It never works. International custom is the messier one. It requires two elements: state practice that is widespread and representative, plus opinio juris, which means states follow the practice because they believe they are legally obliged to. The ICJ's North Sea Continental Shelf cases (1969) are the go-to reference here. The court said that mere consistency of practice isn't enough. You need to show that states felt they had a legal duty. I spent weeks going through diplomatic correspondence and voting records in a North Sea boundary dispute to separate actual legal conviction from just political convenience. The difference matters. It changed the outcome of my analysis entirely.
The Other Sources Nobody Talks About Enough
Judicial decisions and the writings of publicists are listed in Article 38(1)(d) as subsidiary means for the determination of rules of law. That wording is deliberate. They aren't sources in their own right. They help you figure out what the sources actually say. But in practice, ICJ and ICTY decisions carry enormous weight. Scholars like Malcolm Shaw and Rosalyn Higgins get cited constantly because they're good at connecting scattered practice into coherent arguments. General principles of law recognized by civilized nations is the third formal source. This one gets dismissed too quickly. It fills gaps when treaties and custom run out. Things like res judicata, good faith, equity — principles that all major legal systems share. I encountered a situation where neither a treaty nor established custom covered a specific procedural question before an arbitral tribunal. We fell back on general principles of due process, drawing from civil law and common law traditions simultaneously. It worked because the tribunal needed an answer and the alternative was a vacuum.
Soft Law and What It Actually Does
UN General Assembly resolutions, declarations, model laws, guidelines — none of these are formally binding. The Labelle case at the ITLOS Seabed Disputes Chamber is interesting because it suggested that some soft law instruments can contribute to the formation of customary international law over time. The Assembly resolution on the law of the sea (Resolution 2749, 1970) declaring the deep seabed as the common heritage of mankind is a textbook example. It wasn't binding when adopted. It became binding through the 1982 Convention on the Law of the Sea, which incorporated it. That sequence matters. Soft law also shapes behavior in ways that formal instruments don't always achieve. States comply with soft law because it's easier to adopt and harder to politically cost to violate. The Paris Agreement's nationally determined contributions are a modern illustration. Not legally enforceable in any traditional sense, but politically consequential. I've advised clients who treated soft law commitments as de facto obligations because the reputational cost of ignoring them exceeded any short-term gain from non-compliance.
Hard Cases and Where the System Fractures
The biggest problem with identifying sources isn't that they're unclear on paper. It's that states disagree about whether a practice has crystallized into custom, or whether a treaty provision reflects customary law. The ICJ addressed this in the Nuclear Weapons advisory opinion (1996), stating that certain treaty provisions may also reflect customary international law independently. But it didn't specify which ones. That ambiguity persists. I worked on a case involving humanitarian intervention where the UK argued that a unilateral use of force was justified under a developing customary norm. The opposing side pointed to consistent State Department statements opposing such a doctrine. The counter argument relied on actual military deployments and NATO decision-making records. Neither side was wrong about the facts. They disagreed about what the facts proved. This is the real difficulty in applying Article 38. It assumes you can identify opinio juris objectively. You can't, not reliably.
What Beginners Get Wrong
People routinely confuse hierarchy with sequence. Article 38 doesn't establish a strict hierarchy. Treaties don't automatically trump custom. Lex specialis and lex posterior handle conflicts between treaties. Custom can modify treaty obligations between the same states if it reflects a subsequent practice accepted as law. Articles 31(3)(b) and 41 of the Vienna Convention cover this. I've seen junior lawyers miss this and argue from the wrong source entirely, wasting days in briefing work that should have taken hours. Another trap is assuming regional practice counts as universal custom. It doesn't. The Asylum case (ICJ, 1950) made this clear. Regional or bilateral customs exist but they don't bind third states. If you're advising a non-regional client on customs that originated in Inter-American practice, you need to demonstrate that the practice has attained universal recognition. That's a high bar.
Practical Takeaway
Start with treaties. Check the state's ratifications and any reservations. Move to custom only when treaties are silent or ambiguous. Use judicial decisions and scholarship to interpret both. Watch for soft law that may be hardening into custom. Be skeptical of any claim that something is customary law without pointing to specific state practice and opinio juris. The system is uneven. It lacks central enforcement. It works because states generally want it to, not because they're forced to comply. That's the reality you're working with.
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