The transfer of sovereignty over the Chagos Islands from the United Kingdom to Mauritius highlights the enduring relevance of international law in sustaining a rules-based maritime order
The Chagos Islands, comprising over sixty islands in the south-western Indian Ocean, have emerged at the centre of a seeming churn in the Indian Ocean region. The islands, officially defined as the British Indian Ocean Territory by the United Kingdom, have long been the subject of dispute. Shortly before the end of the British colonial occupation of Mauritius, the Chagos Islands were detached by Britain to control access to the region. Seen as an important pivot for Britain’s security interests and defence preparedness, the Chagos Islands assumed centrality in shaping the presence of great powers in the Indian Ocean region. While Mauritius has long disputed the United Kingdom’s control over the Chagos Islands, in 2025, the two countries signed an agreement to facilitate the transfer of sovereignty over the islands to Mauritius. Given that part of the deal suggests that the UK and the US will continue to retain their military bases on Diego Garcia in the Chagos Islands, the return of sovereignty over the islands to Mauritius appears to advance and strengthen efforts to establish a rules-based and norm-governed order in the Indian Ocean.
The return of sovereignty over the islands to Mauritius appears to advance and strengthen efforts to establish a rules-based and norm-governed order in the Indian Ocean.
Much of the history of the Indian Ocean and its littoral countries is characterised by colonial rule by European powers. In the past, Britain had a sizable presence across the Indian Ocean through its colonial holdings. While the British presence in the region diminished significantly as a result of the decolonisation wave underway in the mid-twentieth century, the Indian Ocean has since remained an important geography for the United Kingdom to secure its trade routes and project power and influence across the region. Prior to Mauritius’ independence in 1968, the United Kingdom retained administrative control of and access to the Chagos Islands in exchange for a grant of GBP 3 million. Earlier, in 1965, the region was rechristened the British Indian Ocean Territory after having been detached from Mauritius. Britain sought to retain control of the Chagos Islands, given its security interests amid the unfolding geopolitical dynamics of the wider Cold War era.
Soon after, in 1971, the UK and the US built a joint military base on Diego Garcia, one of the constituent islands of the Chagos archipelago. This joint base served as a crucial node in strengthening Washington’s efforts to bolster its security presence in the Indian Ocean, a vital strategic theatre in the Cold War contest for geopolitical influence. The UK, too, placed considerable emphasis on the base, given its interest in securing the vital trade routes passing through the region. Historically, Diego Garcia has served as a critical launch pad for America’s military involvement in the Middle East. The base has been vital in launching bombing strikes against Houthi targets in Yemen in 2024 and 2025, initiating humanitarian aid deployments to Gaza, and targeting the Taliban and Al Qaeda in Afghanistan in 2001. The Diego Garcia base has also been mired in controversy since its inception. In the 1960s and 1970s, residents of the island were displaced to build the base, drawing sharp criticism from Mauritius.
In 2019, the ICJ issued an advisory opinion at the request of the United Nations General Assembly (UNGA), supporting Mauritius’s claim and finding that the decolonisation of Mauritius had not been lawfully completed. The Court further held that the UK was under an obligation to bring its administration of Chagos to an end “as rapidly as possible”.
Since the 1980s, Mauritius has contested the UK’s access to and control over the Chagos Islands. In 2019, the ICJ issued an advisory opinion at the request of the United Nations General Assembly (UNGA), supporting Mauritius’s claim and finding that the decolonisation of Mauritius had not been lawfully completed. The Court further held that the UK was under an obligation to bring its administration of Chagos to an end “as rapidly as possible”. While London remained keen to retain access to the islands given its multifaceted security interests in the region, negotiations concerning the transfer of sovereignty over the Chagos Islands between the UK and Mauritius began in 2022. In May 2025, the UK and Mauritius reached an agreement, and the treaty providing for the transfer of sovereignty was signed by British Prime Minister Keir Starmer and Navin Ramgoolam, the Prime Minister of Mauritius. The terms of the agreement indicate that the UK and the US will retain the joint military base in Diego Garcia in exchange for lease payment for at least ninety-nine years. According to estimates put out by the UK government, the total cost for leasing the base for a period of ninety-nine years would amount to GBP 3.4 billion. However, the United Kingdom’s Conservative Party, which has expressed opposition to the deal, suggests that, in real terms, the cost would amount to approximately GBP 35 billion over 99 years, the duration for which the Diego Garcia base would remain on lease to the UK and the US. While this agreement drew criticism from US President Donald Trump, given Washington’s stakes in the base at Diego Garcia, conversations between Starmer and Trump appear to have resulted in the latter agreeing to back the deal. Trump, however, has explicitly warned that militarily securing Washington’s interests would remain an option he would be willing to exercise in case the deal falls apart in the future.
The terms of the agreement indicate that the UK and the US will retain the joint military base in Diego Garcia in exchange for lease payment for at least ninety-nine years.
The saga of the Chagos Islands and the transfer of their sovereignty to Mauritius, in addition to undoing a colonial-era fallacy, signals a critical aspect of maritime governance. In an era when China continues to flout rulings of apex judicial institutions, as evident in Beijing’s continued belligerence in the South China Sea despite the Permanent Court of Arbitration ruling in the arbitration initiated by the Philippines, the UK’s recognition and acceptance of the ICJ advisory in support of Mauritius — despite its critical security interests in the region and the internal pressures of domestic politics — signals the resilience of the notion of a free and open rules-based order. The role of global institutions remains vital to the evolution of a forward-looking agenda for global and maritime governance. The UK’s commitment to the ICJ advisory will indeed strengthen efforts to establish a rules-based and norm-governed order in the Indian Ocean and the wider Indo-Pacific. The return of sovereignty over the Chagos Islands to Mauritius serves as a critical reminder that, in a purported rules-based order in the maritime domain, global norms are paramount and afford all stakeholders, including small island countries, an opportunity to pursue their strategic and geopolitical objectives, even as they confront counterparts that surpass them in economic and security prowess.
Sayantan Haldar is an Associate Fellow with the Strategic Studies Programme at the Observer Research Foundation.
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Sayantan Haldar is an Associate Fellow with ORF’s Strategic Studies Programme. At ORF, Sayantan’s work is focused on Maritime Studies. He is interested in questions on ...
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