On 30 December 1966, the UK Government entered into an agreement with the US Government to make the British Indian Ocean Territory available for the defence purposes of both Governments "as they may arise". 1966 Exchange of Notes US-UK.pdf.
Under the agreement, the Territory remained under United Kingdom sovereignty, but was to be available to the US for an "indefinitely long period". The initial period was to be 50 years (until 2016), but to continue in force automatically for a further 20 years (2036) unless terminated by either Government under notice given between 2014 and 2016.
The Exchange of Notes referred to "administrative measures that may be necessary to enable any such defense requirement to be met", a reference to the Secret talks that had been taking place about the need to clear the Territory of its indigenous Chagossian population.
Due to the unavailability of funds from the US Congress, no defence facilities were constructed until 1971. A further agreement 1972 Exchange of Notes US-UK.pdf laid down the basis for a "limited Naval Communications Facility" on the island of Diego Garcia.
Under the agreement, the US were 'given' the western arm and southern end of the atoll, shown in black on the map, for construction purposes.
The agreement somewhat unrealistically required that "As far as possible the activities of the facility and its personnel shall not interfere with the flora and fauna of Diego Garcia." It did not require the land or lagoon area to be re-instated if the US ceased to use the island.
1976 Exchange of Notes US-UK Naval Support Facility DG.pdf, expanded the limited Naval Communications Facility into a US Navy Support Facility. Otherwise, the land area allocated and other provisions remained largely the same.
In 1976, following the independence of the former British Colony of the Seychelles, the islands of Aldabra, Farquhar and Desroches, which had formed part of the BIOT, were returned to the Seychelles and this was reflected in the 1976 Exchange of Notes US-UK.pdf.
In 1987, the 1987 Exchange of Notes US-UK.pdf, laid down the basis for construction contracts on Diego Garcia to be awarded to joint ventures between US and UK firms.