This chapter considers the UK’s assertion of sovereignty over the Chagos Archipelago in the wake of the Advisory Opinion of the International Court of Justice (ICJ) in Legal Consequences of the Separation of the Chagos Archipelago from Mauritius in 1965. It will be argued in this chapter that the UK’s assertion to sovereignty is tenuous as it rests on weak foundations which are increasingly undermined by the colonial context in which the then colony of Mauritius agreed to the detachment of the Chagos Archipelago. The UK’s sovereignty over the Chagos Archipelago is contested by Mauritius (and by the majority of countries that have given their support to the campaign to force the return of the territory). The Republic of Mauritius first challenged the UK’s sovereignty at the United Nations General Assembly (UNGA) in 1980, some twelve years after gaining its independence.1 The Court’s Advisory Opinion is a landmark in terms of how sovereignty over the Chagos Archipelago must now be approached.