Diego Garcia is one of those places that matters because it is boring infrastructure. It is a runway, a port, logistics, and communications in the right geography. That is why it sits at the heart of the Chagos sovereignty dispute, and why the treaty is designed to separate “sovereignty” from “operational control.”
The UK Parliament briefing describes the base as enabled by a 1966 UK-US agreement following the creation of the British Indian Ocean Territory in 1965. It also lists what the base hosts today: a deep-water port, an airfield, a logistics hub, and communications/surveillance capabilities. The same briefing says the UK government has described Diego Garcia as central to UK emergency planning and notes its use in operations against Islamic State/Daesh and historically Iraq and Afghanistan.
- This is a “sovereignty vs access” treaty: Mauritius gets sovereignty, but the UK keeps deep operational control over the base and its security perimeter.
- Diego Garcia’s value is practical: port, airfield, logistics hub and surveillance roles that support real operations.
- The treaty encodes security review controls over nearby construction, reducing the risk of hostile proximity around a sensitive base.
- Origin: UK created BIOT in 1965; a UK-US agreement in 1966 enabled the base (per Parliament briefing).
- Treaty duration: 99 years from entry into force; extendable 40 years by mutual consent.
- Security rings: 12 nautical miles (about 22km) defence-management zone around Diego Garcia; 24 nautical miles (about 44km) zone with tighter control concepts around maritime structures.
- Base features (per Parliament briefing): deep-water port (incl nuclear-powered submarines), airfield, logistics hub, communications and surveillance; specialised roles include CTBT seismic monitoring and GPS support.
- Resettlement: Mauritius can arrange resettlement on all islands except Diego Garcia.
- Whether any amendments weaken or strengthen the security-review rules around development near the base.
- Whether resettlement and Chagossian-rights debates become the main political risk to implementation.
- Whether the UK publishes more operational detail on how the joint commission and security reviews work in practice.
The treaty in one sentence
Mauritius is sovereign over the Chagos Archipelago, but authorises UK use and UK defence control of Diego Garcia under a long-duration agreement, with specified rights and reserved areas.
| Area | UK position (as summarised in Parliament briefing) | Why it matters |
|---|---|---|
| Diego Garcia base | UK manages and conducts base operations | Continuity for UK-US missions |
| Up to 12nm around the island | UK manages for defence purposes | Limits close-in interference |
| Beyond Diego Garcia (archipelago context) | Rights include navigation/overflight concepts and security review mechanisms | Reduces risk of unwanted development |
| Reserved to Mauritius | Fisheries, environment, natural resources outside base scope | Sovereignty is real even if access is preserved |
Scale math: what the security rings mean in kilometres
Historical context (compressed)
The treaty preamble, as summarised in the Parliament briefing, references decisions of international courts and tribunals including the International Court of Justice (2019) and frames the agreement as completing Mauritius’s decolonisation process. That legal pressure is the background driver. The base’s strategic utility is the operational driver.
| Year | Milestone | Why it matters |
|---|---|---|
| 1965 | Creation of BIOT | Sets the modern sovereignty dispute frame |
| 1966 | UK-US agreement enabling the base | Basing becomes structural |
| 2019 | ICJ advisory opinion referenced in treaty framing | International legal pressure intensifies |
| 2022 | UK-Mauritius negotiations start | Formal pathway to settlement |
| 2025 | Treaty signed and implementing bill introduced | Domestic politics becomes the gate |
| 2026 | Political backlash headlines return | Alliance and cost optics hit the surface |
Bottom line
If you strip the rhetoric away, the treaty is a long-run attempt to swap legal fragility for operational certainty. That is why the details matter: control zones, security reviews, and governance mechanisms decide whether the agreement dampens headline volatility or manufactures it.
Sources (primary)
UK Parliament research briefing (base description, treaty structure, control zones, cost framing): https://researchbriefings.files.parliament.uk/documents/CBP-10273/CBP-10273.pdf
UK/Mauritius joint statement (deal framing and term structure): https://pmo.govmu.org/Documents/Documents/UK%20Mauritius%20joint%20statement.pdf
UK govt written statement (implementation/updates): https://questions-statements.parliament.uk/written-statements/detail/2025-12-15/hcws412