State responsibility is a fundamental principle of international law, arising out of the nature of the international legal system and the doctrines of state sovereignty and equality of states. It provides that whenever one state commits an internationally unlawful act against another state, international responsibility is established between the two. A breach of an international obligation gives rise to a requirement for reparation.

Accordingly, the focus is upon principles concerned with second-order issues, in other words the procedural and other consequences flowing from a breach of a substantive rule of international law. This has led to a number of issues concerning the relationship between the rules of state responsibility and those relating to other areas of international law. The question as to the relationship between the rules of state responsibility and those relating to the law of treaties arose, for example, in the Rainbow Warrior Arbitration between France and New Zealand in 1990. The arbitration followed the incident in 1985 in which French agents destroyed the vessel Rainbow Warrior in harbour in New Zealand. The UN Secretary-General was asked to mediate and his ruling in 1986 provided inter alia for French payment to New Zealand and for the transference of two French agents to a French base in the Pacific, where they were to stay for three years and not to leave without the mutual consent of both states.