The following is a guest post by Clare Feikert-Ahalt, a senior foreign law specialist at the Law Library of Congress covering the United Kingdom and several other jurisdictions. Clare has written numerous posts for In Custodia Legis, including Revealing the Presence of Ghosts; Weird Laws, or Urban Legends?; FALQs: Brexit Referendum; 100 Years of “Poppy Day” in the United Kingdom; and most recently Mr. Bates vs. The Post Office Spurs Possible Law Change.
A small, but important, island known as Diego Garcia has given rise to a number of legal challenges and international agreements that date back to Britain’s colonial era. The challenges surround whether the detachment from Mauritius, and subsequent colonization of the Chagos Archipelago, which consists of several islands and atolls remotely located in the center of the Indian Ocean, including the island of Diego Garcia, was lawful, and whether the removal and prohibition on the return of its inhabitants occurred within the bounds of the law. A recent agreement between the United Kingdom (UK) and Mauritius settles the disputes, by returning Chagos Archipelago to Mauritus and providing the UK with continued use of a military base, which I will describe in a post tomorrow. Today I will look at the history that preceded the agreement.
UK Colonization of Chagos Archipelago
One of the driving forces for the UK colonization of Chagos Archipelago was the establishment of a defense facility, to be operated jointly with the United States (US). Almost immediately upon detaching the Chagos Archipelago from Mauritius and establishing the colony of the British Indian Ocean Territory (BIOT) the UK, after undertaking a survey to determine the most appropriate location for a defense facility, entered into an agreement with the US to allow Diego Garcia to be used for defense purposes. The US subsequently constructed, and jointly operated with the UK, a defense facility that according to the UK government provides “crucial strategic capabilities, which have played a key role in missions to disrupt high-value terrorists, including Islamic State threats to the UK.”
History of the Chagos Archipelago and Diego Garcia
The BIOT, which includes Diego Garcia, was the last colony established by the British as its colonial era entered into its waning days and Mauritius was on the verge of obtaining independence. In 1965, the government of the UK and a representative of Mauritius signed an agreement detaching the Chagos Archipelago from the territory of Mauritius.
The agreement between the UK and Mauritius provided the legal foundation for the UK to establish the BIOT as new colony in the Chagos Archipelago, which initially included three other islands detached from Seychelles that were later ceded back to the Seychelles upon their independence in 1976. In return for the detachment of the Chagos Archipelago, the UK government provided Mauritius with a grant of £3 million (approximately US$4 million), along with a commitment to return the islands to Mauritius at a later date when it no longer needed the territory for defense purposes. Once under UK control, in 1966, the UK signed an agreement with the US to establish a military base on the largest island, Diego Garcia.
Independence of Mauritius Leads to Legal Dispute over Territorial Definition
Mauritius was granted independence from the UK in 1968, but the definition of Mauritius, contained in the Mauritius Independence Act 1968, which became its constitution and was promulgated by the government of the UK prior to Mauritius’ independence, does not include the Chagos Archipelago. Instead “Mauritius” is defined in section 5 of the 1968 Act as “the territories which immediately before the appointed day constitute the Colony of Mauritius.” The Mauritian government later claimed that its independence was made conditional upon the detachment of the Chagos Archipelago from its territory and disputed the sovereignty of the UK over the Chagos Archipelago.
This bilateral dispute progressed through numerous meetings, international exchanges, courts and tribunals for a period of 60 years until the UK and Mauritius signed the recent agreement providing sovereignty over the Chagos Archipelago to Mauritius..
United Nations Resolution of 1966
In 1966, the General Assembly of the United Nations (UN) adopted a resolution condemning the British for exercising sovereignty over the Chagos Archipelago and calling for it to be returned to Mauritius. In the same year, the UK and US reached an agreement providing for the use of an island in the Chagos Archipelago for defense purposes. The agreement provided that the UK government would take any administrative measures necessary to ensure the defense needs were met, which included the resettlement of the inhabitants of the islands.
Challenges Regarding Status Continue
The challenges faced by the Chagossians, along with their efforts to reclaim Diego Garcia are well detailed and documented in the decisions of the courts in which they lodged their claims.
The UK entered into an agreement with Mauritius in 1972 whereby it agreed to pay Mauritius £650,000 (approximately US$875,000) for the cost of resettlement of people displaced from the Chagos Archipelago. The UK reached an additional agreement with Mauritius in 1982, under which it paid a further £4 million (approximately US$5.4 million) to be placed into a trust fund for the Chagossians removed from the islands as a final settlement of all claims, without admitting liability.
Despite these agreements and settlement, Mauritius continued to challenge the legitimacy of British sovereignty over the Chagos Archipelago and the Chagossians challenged the legality of their resettlement and exile from Diego Garcia. During these challenges, and in response to a judgment from England’s High Court, the UK government conducted a feasibility study in 2002 into the return of the Chagossians to Diego Garcia. The study concluded that if the Chagossians were permitted to return to live on Diego Garcia, the costs of long-term inhabitation would be prohibitive and that natural events, such as flooding and seismic activity “would make life difficult for a resettled population.”
Advisory Opinion from the International Court of Justice (ICJ)
In 2019, the ICJ issued an advisory opinion that the decolonization of Mauritius was not completed lawfully and that an international agreement was not possible when one territory was under the authority of the other. The ICJ stated that the UK “has an obligation to bring to an end its administration of the Chagos Archipelago as rapidly as possible.” The UK government acknowledged the opinion, but noted it was not legally binding. It stated that it did “not share the court’s approach” and asserted that it has exercised sovereignty over the Chagos Archipelago since 1814. The UK affirmed that it stood by its commitment “to cede sovereignty of the territory to Mauritius when it is no longer required for defence purposes.”
While advisory opinions from the ICJ are not binding, the UK government in 2025 acknowledged that they do “carr[y] significant weight; in particular it is likely to be highly influential on any subsequent court/tribunal”. This advisory opinion had a “meaningful real-world impact on the sustainability of UK sovereignty and the operation of the Base.” In particular, the UK government determined that if Mauritius made another legal challenge, its “… longstanding legal view is that [the UK] would not have a realistic prospect of success.”
The advisory opinion was followed in 2021, by a case heard by the Special Chamber of the International Tribunal for the Law of the Sea relating to the delimitation of the boundary between Mauritius and the Maldives and the court ruled that the sovereignty of Mauritius over the Chagos Archipelago could be inferred from the advisory opinion made by the International Court of Justice.
The Congress of the Universal Postal Union also recognized Mauritius as responsible for making decisions regarding international postal services in the Chagos Archipelago. The UK government determined these decisions “confirmed the risk that a future (binding) case could be brought successfully against the UK” and that this “would create serious real-world operational impacts for the Base.”
Between the years 2021-2022, the UK used diplomacy and bilateral initiatives to attempt to steer Mauritius away from commencing further legal challenges, but these were unsuccessful and “… it became clear by mid-2022 that the only viable means to halt the process was to enter negotiations” and the start of these were announced in November 2022. They resulted in the May 2025 agreement, which I will describe in tomorrow’s post. Stay tuned!
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