The UK government’s deal to transfer the strategically located Chagos Islands to Mauritius appears to be under threat after the US withheld a key legal requirement.According to a Financial Times report, this has forced the UK government to drop, at least for now, a parliamentary Bill that would ratify its 2025 agreement over the Indian Ocean archipelago — which includes a joint US-UK base on Diego Garcia island. While US President Donald Trump had initially backed the UK-Mauritius deal, first announced in October 2024 and signed in May 2025, he has since attacked the pact repeatedly amid tensions with Britain as well as the North Atlantic Treaty Organisation (NATO) over Iran. Here’s a look at how the UK gained control over the archipelago, why the plan to cede it is on hold and how it has become a symbol of a deepening rift in US-Europe relations.What is the Chagos archipelago?The Chagos archipelago, comprising 58 islands, lies roughly 500 km to the south of the Maldives archipelago in the Indian Ocean and around 2,000 km north-east of Mauritius.These islands were uninhabited until the late 18th century, when the French brought in slave labour from Africa and India to work in newly established coconut plantations. In 1814, France ceded the islands to the British.The British then attached Chagos to Mauritius, another one of its colonies in the Indian Ocean, for administrative purposes. This continued up to 1965, when Britain “detached” Chagos from Mauritius to create the British Indian Ocean Territory (BIOT).Story continues below this ad Diego Garcia and Chagos.Three years later, when Mauritius gained independence, Chagos remained with Britain in return for a £3-million grant. Mauritius has argued that it was illegally forced to give the islands away as part of the independence deal.Around this time, Britain invited the US to build a military base on Diego Garcia, one of the islands in the Chagos archipelago, forcibly removing thousands of people from their homes in the process. Diego Garcia is likely named after a Portuguese explorer.Roughly 1,500 to 2,000 Chagossians were displaced between 1968 and 1973 and resettled in Mauritius and the Seychelles.In 1976, a few other BIOT islands were ceded to the Seychelles after it gained independence from Britain.Story continues below this adWhat is the Diego Garcia base?Diego Garcia is the largest island in the Chagos Archipelago. After World War II, as several countries gained independence, US naval planners grew alarmed that America’s access to overseas bases was shrinking compared to its Cold War rivals, China and the Soviet Union. The US Navy felt it needed a permanent foothold in the ocean. Diego Garcia, in the central Indian Ocean, was the answer.It sits approximately 3,000 km from both the Bab-el-Mandeb Strait at the mouth of the Red Sea and the Malacca Strait near the South China Sea, and around 4,000 km from the Strait of Hormuz near the Persian Gulf. This strategic location puts the three critical maritime chokepoints within the reach of long-range bombers. The Chagos archipelago. Diego Garcia is towards the bottom right. Wikimedia CommonsToday, the joint US-UK base is one of only two critical US bomber bases in the Indo-Pacific region, the other being Andersen Air Force Base in Guam. The Diego Garcia base hosts bombers, nuclear submarines, and guided-missile destroyers. The base is also critical to US Space Force tracking infrastructure.Story continues below this adNumerous air operations were launched from Diego Garcia during the Persian Gulf War in 1990-91, US-led strikes on Afghanistan in 2001, and the initial phase of the Iraq War in 2003.After the Iranian Revolution in 1979 dramatically destabilised West Asia, Diego Garcia underwent the biggest expansion of any US military site since Vietnam. The harbour was deepened to accommodate aircraft carriers; a 12,000-foot runway was built for B-1, B-2, and B-52 bombers; and massive pre-positioned ships, each roughly 1000 m, were loaded with enough weapons and supplies for an entire Marine brigade. Other advanced weaponry is also housed on the island.Why did the UK decide to cede Chagos to Mauritius?Since the 1980s, Mauritius has claimed sovereignty over the islands and moved international courts and tribunals. In 2019, the International Court of Justice issued an advisory opinion that the UK must hand over the islands “as soon as possible”. So in 2022, the UK and Mauritius agreed to begin negotiations on the exercise of sovereignty over the Chagos archipelago. This also came amid fears that not acting would push Mauritius towards China, which has deepened its footprint in Africa. Story continues below this adIn October 2024, the UK government announced its plans to return the islands. Shortly after, in May 2025, UK Prime Minister Keir Starmer signed a formal agreement transferring sovereignty of the Chagos Islands to MauritiusAs part of the deal, Diego Garcia would be immediately leased back to the UK for the next 99 years for £101 million a year. The UK government said that the treaty will protect the future of the military base and that the US would pay the base’s running costs.The agreement also includes a £40m trust fund to support Chagossians. Under the deal, Mauritius will be allowed to resettle islands other than Diego Garcia.Why has Trump blocked the Chagos deal?In February 2025, Trump indicated he was prepared to back the Chagos plan. And after the deal was signed in May 2025, US Secretary of State Marco Rubio said Trump had “expressed his support”.Story continues below this adA year later, however, in January 2026, Trump posted on Truth Social that the treaty was among a “very long line of National Security reasons” that underpinned his efforts to acquire Greenland from Denmark. At the heart of his criticism was the Diego Garcia military base.He wrote: “Shockingly, our ‘brilliant’ NATO Ally, the United Kingdom, is currently planning to give away the Island of Diego Garcia, the site of a vital US Military Base, to Mauritius, and to do so FOR NO REASON WHATSOEVER.”Trump toned down his criticism in early February, after a call with Starmer, but was back on the attack on February 18. He said “leases are no good when it comes to countries”, and that the UK risked making a “big mistake” with the “strategically located” island.Also Read | Tehran’s longest-range missile attack yet: Why Diego Garcia mattersTrump, who has long been critical of NATO, has been even more dissatisfied with his European allies after they refused to launch a military intervention to reopen the Strait of Hormuz. Story continues below this adDiego Garcia, meanwhile, became a flashpoint in bilateral relations after the UK refused to allow the US to use it to launch initial strikes against Iran. Britain has since given the US approval to use the base to attack Iranian missile sites and other military capabilities targeting shipping in the Strait of Hormuz. Iran also targeted the island with two ballistic missiles. Though neither missile hit the target, Iran had demonstrated its intermediate-range ballistic missiles for the first time. Now, as US-UK relations continue to plunge, Washington has refused to formally exchange letters to amend a 1966 British-American treaty on the Chagos Archipelago — an essential step in the process to transfer the islands’ sovereignty — the Financial Times reported.According to the report, the legislation had run out of time to proceed to the statute book within the current parliamentary session, which will end later this month.This is not the only blow the deal has suffered in recent times. In late March, the Supreme Court of the Chagos Archipelago overturned the ban on Chagossians living on the outer islands. While the ruling focuses on the outer islands, it is unclear what the decision could mean for future claims on Diego Garcia.