The ‘nationality of claims’ rule is, on the face of it, beguilingly clear and simple. It requires that a state instituting international legal proceedings against another state in respect of damage which that other state has caused to a private person must show that that person possesses its nationality.

The rule is well established in customary international law, and is of pivotal importance for the whole process whereby states may be held responsible for acts causing damage to private persons. Its underlying rationale was set out by the PCIJ in the Panevezys–Saldutiskis Railway case, where it said that ‘in taking up the case of one of its nationals … a State is in reality asserting its own right … This right is necessarily limited to intervention on behalf of its own nationals because, in the absence of a special agreement, it is the bond of nationality between the State and the individual which alone confers upon the State the right of diplomatic protection.’

The rule underlies every case in which a state institutes proceedings in the ICJ in respect of damage done to a private person at the hands of the respondent state. In most cases its requirements cause no real difficulty. In three, however, its application has been particularly significant – the Advisory Opinion on Reparations for Injuries Suffered in the Service of the United Nations, and the judgments in the Nottebohm case, and the case concerning the Barcelona Traction, Light and Power Company Limited.