As the name says, customary international law is not written. It arises from international practices that have become so widespread that states begin to recognize they have a legal obligation to continue them (opinio juris).

Current literature says that the non-usage of nuclear weapons has become a widespread international practice, but that the resistance of nuclear powers has prevented the formation of an “opinio juris” thus far. What is at stake is whether an international custom can be formed despite the opposition of certain states — as long as several other states acknowledge the custom.