History of Public International Law - Modern Customary International Law

Modern Customary International Law

An important development in modern international law is the concept of "consent." Before World War II, a nation would not have been considered to be bound by a rule unless it had formally agreed to be bound by it, or it was already customarily abiding by that rule. Now, however, merely consenting to an international practice is sufficient to be bound by it, without signing a treaty.

An evolution of the positivist approach of Grotius, the concept of consent is an element of customary international law. Customary international law is essentially what states actually do, plus the opinio juris of what states believe international law requires them to do.

Customary international law applies to every country, regardless of whether they have formally agreed to it. At the same time, all countries take part in forming customary international law by their practices and decisions. As new rules arise, countries accept, reject or modify them. When most countries are following a rule, everyone else will be held to it. Therefore, doing nothing is the same as consenting. Nations that did not take action may find themselves bound by an international law that is not to their advantage.

Customary international law can be overruled, however, by a treaty. For this reason, much customary international law has been agreed to formally by treaties between nations.