The Trump administration's refusal to authorize the transfer of sovereignty over the Chagos Archipelago has frozen a bilateral treaty, left the joint U.S.-U.K. military base at Diego Garcia without a settled legal foundation, and placed Washington on a collision course with Mauritius, which has pledged to pursue every available diplomatic and legal avenue to reclaim the islands.
The underlying instrument is a 1966 exchange of notes between the United States and the United Kingdom that governs American use of Diego Garcia as a naval and air logistics hub on British-administered territory. Because the 2025 UK-Mauritius sovereignty agreement would require amendment of that 1966 treaty, formal letters must be exchanged between Washington and London, and the Trump administration has declined to provide them. The UK has accordingly been forced to pause its plan to transfer the Chagos Islands to Mauritius. The UK government ran out of time to pass the necessary implementing legislation, the Diego Garcia Military Base and British Indian Ocean Territory Bill, in the current parliamentary session, and there are no plans to introduce a new bill in the next parliament. For practical purposes, the deal is stalled indefinitely.
The sovereignty dispute traces directly to international law. In February 2019, the International Court of Justice issued an advisory opinion finding that the detachment of the Chagos Archipelago from Mauritius had violated the Chagossian people's right to self-determination, and that the United Kingdom was obliged to end its administration "as rapidly as possible." Separately, the International Tribunal for the Law of the Sea resolved a maritime border dispute between Mauritius and the Maldives in 2023, and its 2021 Special Chamber judgment recognized the "legal effect" of the 2019 ICJ advisory opinion. Those rulings provided the legal impetus for the UK-Mauritius treaty signed May 22, 2025, under which the UK agreed to cede sovereignty of the Chagos Archipelago to Mauritius but retain the right to operate the Diego Garcia military base. In exchange, the UK agreed to pay Mauritius an annual average of £101 million for 99 years in 2025-26 prices, totaling around £3.4 billion. The Trump administration initially signaled support for the arrangement. In February 2025, speaking alongside U.K. Prime Minister Keir Starmer at the White House, President Donald Trump stated that he was willing to support the agreement. He subsequently reversed that position. Trump later attacked the deal as an "act of great stupidity" in January 2026, then doubled down in February, directing the UK on social media to "DO NOT GIVE AWAY DIEGO GARCIA!" and citing a potential use of the base in operations against Iran.
With the treaty effectively blocked, Mauritius has signaled it will not stand down. Mauritian Foreign Minister Dhananjay Ramful, speaking at an Indian Ocean conference in Mauritius, declared that his government would regain control over the territory, vowing to "spare no effort to seize any diplomatic or legal avenue to complete the decolonisation process." British parliamentary analysts and legal experts assessed that Mauritius is likely to escalate the matter through international tribunals. A House of Lords committee warned that if the agreement was not ratified and a future government continued "resisting international pressure" to transfer sovereignty, Mauritius was "likely to resume its campaign through international courts with a view to obtaining a legally binding judgment on sovereignty against the UK." The specific forum in question is the International Tribunal for the Law of the Sea. According to POLITICO, Ben Judah, a former special adviser to then-U.K. Foreign Secretary David Lammy who participated in the treaty negotiations, stated that there was a "firm government assessment" by U.K. officials that Mauritius would pursue an ITLOS claim if the treaty failed [POLITICO]. Judah further argued to POLITICO that an adverse ITLOS ruling, extending Mauritius' maritime borders to encompass the Chagos Archipelago, would create a scenario in which the U.S. and U.K. presence at Diego Garcia could be characterized internationally as an unlawful occupation, potentially opening the door for Mauritius to invite third parties, including China, onto the islands without any legal recourse short of military force [POLITICO].
Republican members of Congress have taken note and are publicly backing the administration's hard line. Sen. Ted Cruz (R-Texas), Sen. John Kennedy (R-La.), Rep. Chuck Fleischmann (R-Tenn.), and Rep. Robert Aderholt (R-Ala.) have each warned Mauritius not to move unilaterally on a sovereignty claim, according to POLITICO. Cruz, in remarks to POLITICO, stated that any such move by Mauritius would draw a decisive response from the president and that Congress would legislate whatever tools are necessary to support that response [POLITICO]. Kennedy, in a statement to POLITICO, framed the concern in terms of Chinese strategic competition, arguing that Mauritius' control of the archipelago would constitute a gain for Beijing and flagging the potential for pressure on U.S. nuclear posture at Diego Garcia [POLITICO]. Analysts have cautioned against overstating Chinese influence in Port Louis. Defense analysts note that Mauritius maintains multidimensional ties with India, whose navy conducts operations in Mauritius' exclusive economic zone, and that India will ensure its strategic relationship with Mauritius remains paramount even as Mauritius maintains economic relations with China.
The procedural impasse turns on a technical but legally significant step. It is understood the UK has still not received a formal exchange of notes from Washington, a technical step but a legal necessity for the treaty to be enacted. The UK government has long acknowledged that the deal cannot proceed without U.S. support. Amendment of the 1966 U.S.-U.K. treaty requires a formal exchange of letters, and the U.S. has refused to provide its letter. The consequence, as one former U.K. Foreign Office permanent secretary told reporters, is that the agreement is effectively in the "deep freeze," with no near-term path to ratification. The UK retains formal sovereignty over the Chagos Islands, but several international law judgments declare that it must return the islands to Mauritius to complete the decolonization process. The longer the stalemate holds, the greater the gap between the legal posture asserted by Washington and London and the conclusions increasingly adopted by the rest of the international community, a gap that any ITLOS or ICJ proceeding would be designed to close on Mauritius' terms.
References:
[1] House of Commons Library. (2025, July 15). 2025 treaty on the British Indian Ocean Territory/Chagos Archipelago. https://commonslibrary.parliament.uk/research-briefings/cbp-10273/
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