Diego Garcia is the largest island in the Chagos Archipelago, an atoll located just south of the equator in the central Indian Ocean. It is currently administered by the United Kingdom as part of the British Indian Ocean Territory (BIOT). The island's significance is as a major joint UK–U.S. military base, which has been in continuous operation since the 1970s.
The island was historically a dependency of the British colony of Mauritius until 1965, when the UK detached the Chagos Archipelago to create the BIOT, a move Mauritius claims was an illegal dismemberment of its territory before independence. The problem this solved for the UK and US was securing a strategic military foothold in the Indian Ocean. Between 1968 and 1973, the original inhabitants, known as the Chagossians, were forcibly removed to make way for the military base, with many deported to Mauritius and the Seychelles. The base, established by a 1972 agreement between the U.S. and British governments, provides logistics, surveillance, and intelligence support, and has been used for operations in the Gulf War, Iraq, and Afghanistan.
The concept connects to a long-running sovereignty dispute between the UK and Mauritius. In February 2019, the International Court of Justice (ICJ) issued an advisory opinion stating that the UK was obligated to end its administration of the Chagos Archipelago. A recent change occurred in October 2024, when the UK agreed to hand over the Chagos Islands to Mauritius. This was formalized in a treaty signed in May 2025, which granted Mauritius full sovereignty but included a provision for the UK to continue exercising authority over the Diego Garcia military base for an initial 99-year period. However, the UK has since put the bill to ratify this agreement on hold, citing a lack of support from the United States, whose consent is a political and legal requirement for the joint military facility. The special status of the militarized Diego Garcia remains unchanged despite the sovereignty agreement.