British Indian Ocean Territory Military Forces 🇮🇴

Strategic Position

The British Indian Ocean Territory (BIOT) is an archipelago in the central Indian Ocean, centered on the Chagos Islands. Its defense posture is defined by the 2025 UK-Mauritius Treaty, which recognizes Mauritian sovereignty over the archipelago while granting the United Kingdom administrative rights over the island of Diego Garcia for an initial period of 99 years. This treaty, reaching final legislative implementation in 2026, formalizes the long-term presence of the joint UK-US military facility on the island.

The territory’s primary security function is to provide a logistical and operational platform for the monitoring of Indian Ocean sea lanes and the support of military operations across the Middle East, East Africa, and the Indo-Pacific. The strategic framework is governed by the 1966 Exchange of Notes between the United Kingdom and the United States, which has been extended to accommodate current and future operational requirements. Regional security relationships are increasingly coordinated with India and other Five Eyes partners, reflecting a shared priority to maintain maritime stability and counter hostile state activity in the region.

Military Forces

The military structure in the territory is divided between British Forces British Indian Ocean Territory (BFBIOT) and a larger United States presence. The British detachment consists of approximately 40 to 50 personnel. This force is led by a Royal Navy Commander who serves as the Commissioner’s Representative, supported by a detachment of Royal Marines and civilian contractors. The British force is responsible for administrative oversight, customs, and the policing of the territory’s waters.

The United States operates the Naval Support Facility (NSF) Diego Garcia, which hosts a population of approximately 2,000 to 3,000 personnel, including active-duty military, reserve forces, and contractors. The facility functions as a major logistics hub and prepositioning site. Infrastructure includes a 3,600-meter runway capable of supporting heavy transport and strategic bombers, and a deep-water lagoon with berthing facilities for carrier strike groups and nuclear-powered submarines.

Aerial assets on the island are typically rotational rather than permanently assigned. The base frequently hosts B-52H Stratofortress, B-1B Lancer, and B-2 Spirit stealth bombers. Naval assets include the Military Sealift Command’s Maritime Prepositioning Ships Squadron Two (MPSRON 2), which maintains a fleet of vessels loaded with equipment and supplies to support a Marine Corps Expeditionary Brigade. Specialized capabilities hosted on Diego Garcia include a Global Positioning System (GPS) monitoring station, advanced satellite communication arrays, and space surveillance sensors.

Strategic Trends

The primary strategic development in 2025 and 2026 is the legal and administrative transition following the treaty with Mauritius. The agreement includes a 99-year lease back to the United Kingdom, ensuring that the base remains under British administrative control despite the shift in sovereignty. Under the terms established in 2025, the UK provides annual payments to Mauritius to secure these rights and prevent third-party military access to the outer islands of the Chagos Archipelago.

Modernization programs for 2026 prioritize the hardening of communications infrastructure and the expansion of the island's capacity to manage the electromagnetic spectrum. These upgrades are designed to mitigate risks from hostile interference and support the integration of advanced intelligence and surveillance platforms. There is a trend toward increasing the frequency of rotational deployments for carrier strike groups and long-range strike wings, reflecting the base’s role as one of the few permanent points of presence for Western forces in the central Indian Ocean. Challenges remain regarding the long-term environmental sustainability of the atoll and the ongoing domestic legislative processes in both the UK and Mauritius to fully ratify all treaty protocols.