Introduction and sources of international law

The essentials

International law (public international law) is the body of rules that govern relations between states and other international persons such as international organisations. It differs from private international law (conflict of laws), which decides which country's law applies to private disputes with a foreign element. Hugo Grotius is often called the father of international law. Because there is no world legislature, international law grows mainly through treaties and the practice of states.

Key points

• Article 38(1) of the Statute of the ICJ lists the sources the Court applies:
• International conventions (treaties) that establish rules expressly recognised by the states in dispute.
• International custom, as evidence of a general practice accepted as law.
• General principles of law recognised by civilised nations (for example, good faith, res judicata).
• Judicial decisions and teachings of the most highly qualified publicists as subsidiary means for determining rules of law.
• Custom needs two elements: consistent state practice and opinio juris (a belief that the practice is legally required).
• Treaties: governed by the Vienna Convention on the Law of Treaties, 1969; pacta sunt servanda – treaties must be performed in good faith.
• ICJ decisions bind only the parties and only in respect of that case (Article 59 of the Statute), but they strongly influence the law.
• Jus cogens: peremptory norms, such as the ban on genocide, from which no derogation is allowed.

Exam tips

• Remember that judicial decisions are a subsidiary source, not a primary one.
• State practice alone is not custom; opinio juris is also needed.

Common mistakes

• Thinking ICJ judgments bind all states as precedents.
• Confusing public international law with private international law.