Customary International Law
How state practice and opinio juris create binding international law without a treaty.
Law Without a Treaty
Not all international law is written down in treaties. Customary international law (CIL) arises from the general and consistent practice of states when they follow that practice out of a sense of legal obligation, known by the Latin term opinio juris. Article 38 of the ICJ Statute lists custom as one of the primary sources of international law, alongside treaties and general principles.
The formation of custom requires two elements. First, there must be widespread and representative state practice: states must actually behave in a certain way, consistently and over time. Second, that practice must be motivated by a belief that it is legally required, not merely done out of courtesy or convenience. When a state grants diplomatic immunity to foreign ambassadors, it does so because it believes international law requires it, not because it chooses to be polite. That belief, combined with consistent practice across many states, creates a binding rule.