The Mists of Time: Intertemporality and Self-determination’s Territorial Integrity rule in the ICJ’s Chagos Advisory Opinion

The UK Government’s critique of the International Court of Justice’s Chagos Advisory Opinion has focused upon the Court’s approach to the norms of international law applicable at the time of the British Indian Ocean Territory’s (BIOT) creation, which involved the excision of territories from two of its then-colonies, Mauritius and Seychelles. The decision turned on whether norms of self-determination relating to the territorial integrity of colonised territories had crystalised as customary international law before the BIOT was created in November 1965, a proposition the UK Government continues to publicly reject. This is a dispute about temporality and the development of customary international law. This article interrogates the UK Government’s claims by reviewing the holdings of the UK National Archives, which detail how ministers, legal advisers and officials understood the norms of self-determination applicable to the BIOT. They demonstrate that the UK Government was acutely aware of the implications of these rules for the new colony’s creation and sought to distract UN organs from the relevant legal questions. They therefore provide a window into how certain voices have long been prioritised over others in the processes by which customary international law develops.