From Halopedia, the Halo wiki
Due to the atoll's proximity to India, Hong Kong, Pakistan, and Africa, Britain established a military base on Diego Garcia for both training and refueling purposes. The base has been in use ever since. The base later came under the control of the Unified Earth Government's United Nations Space Command.
Battle of Earth
On October 20, 2552, when the Covenant's Fleet of Sacred Consecration arrived at Earth, the military base on Diego Garcia was immediately alerted to the Covenant's presence. The base launched the first counterattacks against the Covenant when they invaded New Mombasa in Kenya, Africa during the subsequent Battle for Earth.
These attacks proved effective as later in November, a Covenant capital ship was sent to Diego Garcia to destroy the base. While most of the Earth's surface was later brutally bombarded by a reinforced Covenant armada, consisting the remaining Jiralhanae-controlled ships of the High Charity defense fleet, led by the Prophet of Truth's Forerunner Dreadnought on November 17, the base nevertheless survived the Human-Covenant War and is still in use by the UNSC as of 2553.
Diego Garcia was mentioned in the Halo 2 E3 demo as being the base of the "405th", a Marine unit that became A Company in the finished game. Diego Garcia's existence as a military base, with units involved in the fighting at Mombasa, was later made official by the Halo Encyclopedia (2009 edition).
List of appearances