Written by Defence Research Fellow, Samuel Bashfield
After almost 59 years of controversial existence, the last vestige of the British Empire in the Indian Ocean, the British Indian Ocean Territory, is set to be wound up. On Oct. 3, Britain and Mauritius released a joint statement detailing a historic agreement via which sovereignty over the Chagos Archipelago, including Diego Garcia, will be transferred to Mauritius. The sun is setting on the British Indian Ocean Territory and borders in the Indian Ocean are set to shift.
This will be a significant agreement which will legitimize American use of its critically important military facility on Diego Garcia. This decades-long sovereignty dispute has blighted the U.S. Indian Ocean military presence and hampered its relations with many island and littoral nations as well as multilateral institutions. By running the Diego Garcia base with Mauritius’ consent, the United States will underscore its commitment to international law and the rules-based order.
While the official joint statement was short on details, treaty negotiations will commence post haste. Among the issues that will be crucial to monitor are timelines, financial terms, Britain’s ongoing sovereign rights, base consultation arrangements, and Diego Garcia’s status vis-a-vis the African Nuclear-Weapons-Free Zone Treaty.
Background
The British Indian Ocean Territory, established in November 1965, was created to facilitate a joint U.K.-U.S. Indian Ocean military facility on the largest island, Diego Garcia. While originally a communications station, the facility was expanded throughout the 1970s and early 1980s to create a potent military base. While nominally “joint,” the United States is the primary user and benefactor.
The British Indian Ocean Territory was also intended as part of a broader transfer of power from Britain to the United States. As Britain withdrew “East of Suez,” it was hoped that providing Indian Ocean islands to the United States would cement an American presence in this region and ensure a continued favorable balance of Western power.
Diego Garcia quickly became a lynchpin for U.S. power projection, not just in the Indian Ocean but also East Africa, the Middle East, and South Asia. The islands’ lagoon houses one of America’s pre-positioned military equipment stockpiles, the runway is capable of hosting U.S. heavy bombers, the port supports visiting American and allied ships and submarines, and the islands’ various pieces of intelligence infrastructure provides unparalleled coverage in this vast region.
The British Indian Ocean Territory was created by separating parts of Mauritius and the Seychelles, prior to their ascendency to independence from British rule in 1968 and 1976 respectively. Britain agreed at the time to return the islands when no longer required for defense purposes. The Seychelles’ islands were returned to that country in 1976, as these were never militarized. But Mauritius had agitated for the return of their islands, the Chagos Archipelago, since the 1980s, to no avail.
Mauritius’ efforts culminated in a landmark 2019 International Court of Justice opinion which found that Britain’s retention of the Chagos Archipelago was illegal under international law. While Britain’s Conservative government initially ignored the ruling, the opposition Labour party, then under Jeremy Corbyn’s leadership, stated in 2019 that “the right of return to those islands is absolutely important as a symbol of the way in which we wish to behave in international law.”
A major pivot, however, occurred during Liz Truss’ short-lived prime ministership in late-2022 as then-Foreign Secretary James Cleverly announced that Britain and Mauritius would negotiate the sovereignty of the Chagos Archipelago. According to Cleverly, “it is our intention to secure an agreement on the basis of international law to resolve all outstanding issues.”
Then shadow Foreign Secretary David Lammy responded by tweeting that “it’s welcome the government is finally taking steps to put right a historic wrong on the Chagos Islands. The government must now negotiate a resolution that allows former Chagos inhabitants to return home, while maintaining key commitments on security.” However, despite numerous rounds of negotiations between 2022 and 2024, the successive Conservative governments of Truss and Rishi Sunak were unable to strike a deal with Mauritius, leaving responsibility for a resolution to Keir Starmer’s new Labour government.
At last, on Oct. 3, 2024, news broke of a deal. Described by Britain and Mauritius in a joint statement as “seminal,” the two nations announced “an historic political agreement on the exercise of sovereignty over the Chagos Archipelago.” Under the agreement, “the United Kingdom will agree that Mauritius is sovereign over the Chagos Archipelago, including Diego Garcia.” Both nations will commence treaty negotiations, which will ensure the military base on Diego Garcia will continue uninterrupted for a 99-year period.
Crucially, the joint statement notes that Britain will have a continued stake in Diego Garcia: “For an initial period of 99 years, the United Kingdom will be authorised to exercise with respect to Diego Garcia the sovereign rights and authorities of Mauritius required to ensure the continued operation of the base well into the next century [emphasis added].” Thus, some sort of shared constitutional status for Diego Garcia is to be negotiated.
Remaining Uncertainties
The announcement omits several important issues. For example, no timeframe is given for sovereignty transfer. The statement noted that a treaty will be inked “as quickly as possible,” but did not elaborate further. When sovereignty negotiations commenced in late 2022, both parties expected an outcome in several months, but this decision took around two years. As the deal is subject to the finalization of a treaty, this process could unfortunately become likewise protracted.
Additionally, no detail is given as to the payment Mauritius expects in exchange for renting Diego Garcia to the British (and by extension to the Americans). The joint statement notes that Britain will provide Mauritius a “package of financial support,” which will include an “indexed annual payment” as well as support for Mauritius’ infrastructure. However, the dollar figure, and whether the Americans will contribute, is less clear.
Following this announcement, the United Kingdom and Mauritius will now commence negotiating a treaty. Several key points will have significant implications for the crucial base on Diego Garcia.
First, detailed consultation agreements over military use of the Diego Garcia base will need to be struck between Mauritian, British, and American officials. Britain and the United States have shared a “special relationship” for decades with high levels of mutual trust, which has translated into liberal access arrangements for American forces. Whether Mauritius will offer such permissive rights to the United States is uncertain, especially as it relates to intelligence gathering and combat operations.
Second, Mauritius is a signatory to the African Nuclear-Weapons-Free Zone Treaty (better known as the Pelindaba Treaty) that seeks to prohibit the research, development, manufacture, stockpiling, or possession of nuclear explosive devices. Britain has controversially excised Diego Garcia from the treaty. When Mauritius gains sovereignty, it would need to allow some sort of exemption for the United States to continue its mission of supporting nuclear-capable bombers and nuclear-fueled naval assets. These two issues could be mitigated by Britain’s continued “sovereign rights.”
Third and finally, Mauritius has committed to allow the forcibly displaced former inhabitants of the Chagos Archipelago to return. The return of the Chagossians has historically been opposed by British and American officials for military security reasons. According to the Anglo-Mauritian joint statement: “Mauritius will now be free to implement a programme of resettlement on the islands of the Chagos Archipelago, other than Diego Garcia [emphasis added].” Whether this solution will be agreeable to the Chagossians is unclear. Without access to Diego Garcia and its myriad modern facilities by virtue of the military base, returning to the archipelago’s outer islands, including Peros Banhos and the Salomon Islands, will be made more difficult.
Conclusion
Despite these ongoing issues, the eventual Anglo-Mauritian treaty will simultaneously end a decades-long controversy that has haunted a key element of the Anglo-American military presence in the Indian Ocean and help prove that the West is still committed to the rules-based order and international law. As China continues to expand its presence and influence in the Indian Ocean, the Chagos Archipelago sovereignty dispute has symbolized Western double standards and duplicity to detractors. With this new deal, critics can no longer assert that America’s Indian Ocean presence on Diego Garcia is based on colonialism.
On the face of it, Britain will lose a bargaining chip in its relations with the United States. Since the British Indian Ocean Territory’s creation, Britain has used the islands to maintain close ties with the United States. For instance, U.S. access to Diego Garcia was deployed as a bargaining chip three times during the Cold War to further Britain’s nuclear weapons capabilities. According to former British Prime Minister Boris Johnson, a vocal critical of ceding sovereignty to Mauritius: “The Americans don’t give us crucial nuclear secrets just because they love little old England. They don’t share intelligence because they adore our quaint accents. We have a great and indispensable relationship because we have important things to offer — including Diego Garcia.” However, it seems that under this new treaty, Britain will simultaneously cede the Chagos Archipelago while retaining “sovereign rights and authorities” over Diego Garcia, by authorization of Mauritius. Thus, perhaps London can have its cake and eat it too.
History should harshly judge Britain’s illegal and immoral retention of the Chagos Archipelago. Strategists will debate Britain’s willingness to yield sovereignty over the Chagos, at a time when China’s influence is on the rise. While the strategic rationale for a Western military presence on Diego Garcia was sound, the military ends did not justify the colonial means. All eyes will now turn to treaty negotiations.
As treaty talks commence, various groups are vocalizing opposition to a deal. Some Conservative members of parliament are railing against the announcement and some Chagossian groups are likewise dismayed. These efforts to obstruct a deal should be watched closely as treaty talks commence. Amid this resistance, it’s now up to Mauritius and the United Kingdom to conclude a treaty that simultaneously safeguards the Anglo-American joint military facility on Diego Garcia, finalizes Mauritius’ decolonization process in accordance with international law, and allows the forcibly displaced Chagossians to resettle.
Samuel Bashfield is research fellow with the Australia India Institute’s Defence Program, based at the University of Melbourne and PhD Scholar at the Australian National University’s National Security College. This piece arose from a recent United States Studies Centre workshop on Australian Indian Ocean strategy.