What has the International Court of Justice said on this?

The UK should end its control of the Chagos Islands in the Indian Ocean as rapidly as possible. The islands are not lawfully separated from the former colony of Mauritius.

What’s the issue?

• Britain detached the Chagos Islands from Mauritius in 1965, three years before Mauritian independence. From 1967 to 1973, some 1,500 Chagos islanders were gradually forced to leave their homes so that the largest island, Diego Garcia, could be leased to the US for a strategic airbase. Today, Diego Garcia hosts a major US military base.
• In 2016, after several judicial challenges, Britain extended Diego Garcia’s lease until 2036 and declared that the expelled islanders would not be allowed to go back. In 2017, Mauritius successfully petitioned the United Nations to seek an ICJ advisory opinion on the legality of the separation.
• Mauritius claims it was forced to give up the islands – now a British overseas territory – in 1965 in exchange for independence, which it gained in 1968.

Arguments by Mauritius:

• Mauritius argues it was illegal for Britain to break up its territory. It claims sovereignty over the archipelago and demands the right to resettle former residents.
• The crux of the Mauritian claim is the right of self-determination. In its submission to the court, the Mauritian government claimed that the separation of the islands from Mauritius was in clear breach of UN resolution 1514, also known as the Colonial Declaration. Passed in 1960, it enshrined the right of self-determination for colonial peoples and specifically banned the breakup of colonies prior to independence. This was intended to keep borders stable, and to prevent colonial powers from simply absorbing colonial territory into their overseas territory so as to retain their sovereignty.
• Yet in spite of this resolution, a number of states (including France and the UK) kept possession of parts of their former colonies following the decolonisation process.