In May 2019, the UN's highest court ruled that the 60 or so Chagos Islands — which include Diego Garcia — had been transferred to British authority illegally in 1965.
The islands were previously part of Mauritius, but Mauritius handed control of them to Britain for a cash fee two years before gaining independence.
The UN court ruled that it was illegal for the British to demand possession of the islands as a condition of independence.
Shortly afterwards, the British leased Diego Garcia to the US.
The UN has now served an eviction notice to the British. Although the ruling is not enforceable, it exerts moral pressure, and has placed the future of the base in doubt.