In 1965, the United Kingdom split the Chagos Archipelago from Mauritius and the islands of Aldabra, Farquhar and Desroches from the Seychelles to form the British Indian Ocean Territory. The islands were formally established as an overseas territory of the United Kingdom on 8 November 1965. On 23 June 1976, Aldabra, Farquhar and Desroches were returned to Seychelles as a result of its attaining independence.
The Chagos has been part of Mauritius since the 18th century when the French first settled the islands. All of the islands forming part of the French colonial territory of Isle de France (as Mauritius was then known) were ceded to the British in 1810 under the Act of Capitulation signed between the two countries. Chagos always was part of Mauritius, and was generally recognised as being so, until the UK purported to split it off in 1965, prior to Mauritian independence in 1968 and leased Diego Garcia, the main island of the archipelago, to the United States under a 50-year lease (which expires in 2016 and comes up for renewal in 2014).
After initially denying that the islands were inhabited, British officials forcibly expelled approximately 2,000 Chagossians who had lived on those islands a century to mainland Mauritius to allow the United States to establish a military base on Diego Garcia. Since 1971, only the atoll of Diego Garcia is inhabited only by some 3,000 UK and US military and civilian contracted personnel.
Mauritius has repeatedly asserted that the British claim that the Chagos Archipelago is one of its territories is a violation of United Nations' resolutions banning the dismemberment of colonial territories before independence. The UK has stated that it has no doubt about its sovereignty over the Chagos but has also said that the Chagos will be returned to Mauritius once the islands are no longer required for defence purposes. Given the absence of any progress with the UK, Mauritius has decided to "internationalise" the dispute and take up the matter at all appropriate legal and political forums. The African Union and the Non-Aligned Movement have expressed unanimous support for Mauritius on the Chagos issue.
The British Government established a marine protected area (MPA) around the Chagos Islands known as the Chagos Marine Protected Area was created on the 1st April 2010 and enforced on the 1st November 2010. It is the world's largest fully protected reserve, twice the size of Great Britain. The designation proved controversial as the decision was announced during a period when the UK Parliament was in recess.
On December 1, 2010, WikiLeaks release a leaked US Embassy London diplomatic cable dating back to 2009  exposed British and US calculations in creating the marine nature reserve. The cable relays exchanges between US Political Counselor Richard Mills and British Director of the Foreign and Commonwealth OfficeColin Roberts, in which Roberts "asserted that establishing a marine park would, in effect, put paid to resettlement claims of the archipelago’s former residents." Richard Mills concludes:
Establishing a marine reserve might, indeed, as the FCO's Roberts stated, be the most effective long-term way to prevent any of the Chagos Islands' former inhabitants or their descendants from resettling in the [British Indian Ocean Territory].
The cable (reference ID "09LONDON1156" ) was classified as confidential and "no foreigners", and leaked as part of the Cablegate cache.
The Government of Mauritius initiated proceedings on 20 December 2010 against the UK Government under the United Nations Convention on the Law of the Sea (UNCLOS) to challenge the legality of the ‘marine protected area’. Mauritius argues that Britain breached a UN resolution when it separated Chagos from the rest of the colony of Mauritius in the 1960s, before the country became independent, and that Britain therefore doesn’t have the right to declare the area a marine reserve and that the MPA was not compatible with the rights of the Chagossians.