Who Controls Diego Garcia?

Qs & As

‘Based on the 2025 agreement, Mauritius could potentially be held legally responsible or face international pressure for US strikes launched from Diego Garcia’

Sovereignty in a Time of War

By Lex

As tensions escalate in the Middle East and global rivalries sharpen, the strategic military base on Diego Garcia has once again moved to the centre of international attention. Reports that UK Prime Minister Keir Starmer initially refused to allow the United States to launch offensive strikes on Iran from the base raise complex legal and geopolitical questions. With sovereignty over the Chagos Archipelago now tied to Mauritius through the 2025 agreement — though not yet fully ratified — the episode highlights unresolved issues about control, legal responsibility, and the future of the Diego Garcia lease. Lex explores the legal, diplomatic, and strategic implications for the UK, the US, and Mauritius.

“Based on the 2025 agreement, Mauritius could potentially be held legally responsible or face international pressure for US strikes launched from Diego Garcia, since sovereignty over the Chagos Archipelago — including the island of Diego Garcia — has transferred to Mauritius. While the UK retains a 99-year lease to operate the base for the US and the UK, Mauritius is the recognized sovereign owner, which alters the legal framework of responsibility under international law…”

* Reports indicate UK Prime Minister Starmer has denied the US permission to use Diego Garcia for the current strikes against Iran. Does the 1966 Exchange of Notes (or the 2025 Treaty) provide the UK with a definitive legal “veto” over US missions, or is this a political stance that Washington could legally challenge?

Reports indicate that in early 2026, UK Prime Minister Keir Starmer initially refused to grant the United States permission to use the Diego Garcia airbase for offensive strikes against Iran, citing international law, and subsequently authorized its use only for defensive purposes. This action rests on the legal framework of the 1966 Exchange of Notes (and the associated, though not yet fully ratified, 2025 treaty), which governs the operation of the joint base.

Under international law, the 1966 Exchange of Notes provides the UK with a definitive contractual right — in effect, a veto — over US operational use of the base.

* If the UK permits the use of the base for strikes — later deemed a “crime of aggression” under international law –, does the UK share legal liability? Does Keir Starmer’s current refusal effectively shield the UK from international prosecution?

The UN Charter (1945) establishes the foundational legal framework governing the use of force in international relations. Its central tenet, Article 2(4), prohibits the threat or use of force against the territorial integrity or political independence of any state. Only two exceptions exist: self-defence against an armed attack (Article 51) and actions authorized by the UN Security Council.

It would therefore appear that if the UK were to allow the US to use the Diego Garcia base indiscriminately to attack another state, the UK could be answerable under international law and before the United Nations.

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* Could Mauritius also be held legally responsible for future US strikes launched from Diego Garcia? Since Mauritius is now the sovereign owner of the islands, does it share the blame for any ‘crimes of aggression’ committed from its territory, even if the UK and US have operational control?

Based on the 2025 agreement, Mauritius could potentially be held legally responsible or face international pressure for US strikes launched from Diego Garcia, since sovereignty over the Chagos Archipelago — including the island of Diego Garcia — has transferred to Mauritius.

While the UK retains a 99-year lease to operate the base for the US and the UK, Mauritius is the recognized sovereign owner, which alters the legal framework of responsibility under international law.

* How does the current conflict redefine the “defense purposes” clause of the Chagos agreements? If the base is used for a war the UK does not officially support, does the lease agreement become functionally void?

It all depends on the clauses of the lease between the UK and the US. The 2025 agreement to transfer sovereignty over the Chagos Archipelago to Mauritius, while leasing Diego Garcia back to the UK/US for 99 years, redefines “defence purposes” so as to accommodate both national sovereignty and the continuation of strategic operations. If the US were to breach this agreement, the original lease could be rendered void. But would the UK have the political will and courage to stand up to the US?

* With the Chagos Bill currently “paused” in the House of Lords, what is the current legal status of the May 2025 treaty? If it remains unratified indefinitely, does Mauritius have a grounds to sue for “bad faith” or “breach of faith” regarding the years of negotiation?

As of March 2026, the May 22, 2025, treaty between the United Kingdom and Mauritius remains unratified and has not entered into force. The parliamentary process for the Diego Garcia Military Base and British Indian Ocean Territory Bill was effectively stalled in late February 2026 after Foreign Office Minister Hamish Falconer confirmed a “pause” to address the Trump administration’s vocal opposition.

President Trump’s recent characterization of the deal as an act of “total weakness” — combined with his public rift with Prime Minister Keir Starmer over the use of the base for strikes against Iran — has left the legislation in a state of suspended animation.

If this impasse continues indefinitely, Mauritius has a compelling case that the UK has violated the international legal principle of “good faith” in negotiations, specifically by failing to implement a settlement designed to resolve the unlawful detachment of the archipelago as identified by the 2019 ICJ and 2021 ITLOS rulings.

* In your view, is there a risk that Starmer’s “principled stand” on Iran will provoke the US administration to withdraw its support for the Mauritius sovereignty deal entirely?

Based on the fallout from ‘Operation Epic Fury’, there is a significant and active risk that Prime Minister Keir Starmer’s recent friction with the Trump administration — specifically his initial refusal to authorize US strikes on Iran from British bases — could derail the Mauritius sovereignty deal.

President Trump has publicly expressed being “very disappointed” in Starmer, mockingly remarking that the UK leader is “not Winston Churchill” after he hesitated to permit the use of Diego Garcia for offensive operations.

* Critics argue that “returning” Chagos makes the base vulnerable to Chinese influence. How can Mauritius legally or diplomatically “de-risk” this perception to satisfy the strategic anxieties of the US?

To “de-risk” its sovereignty claim, Mauritius can implement a multi-layered security framework that codifies a “no-go” policy for foreign militaries on the outer islands and enforces a 24-nautical-mile “security shield” around Diego Garcia, where all administrative control remains with the base operators.
By establishing a permanent joint security council for regional monitoring and providing a 99-year “guaranteed use” clause immune to domestic political shifts, Port Louis can transform a potential flashpoint into a collaborative partnership that offers the US greater strategic certainty than the previous British administration.

* President Trump has called the deal a “big mistake.” Is there a viable legal pathway for Mauritius to negotiate a bilateral deal directly with the US, effectively cutting the UK out of the middle?

Based on recent developments, there is a viable, but politically complex, legal pathway for Mauritius to deal directly with the United States regarding the Diego Garcia military base. Although the 2025 Treaty formally installs the UK as the lessee for 99 years, the US-Mauritius Bilateral Security Discussions held in Port Louis in February 2026 signal a pivot toward a more direct strategic partnership.

* Since the 2019 ICJ Advisory Opinion was non-binding, what mechanisms are available to Mauritius to “harden” that ruling into a mandatory one if the UK unilaterally walks away from the treaty?

While the 2019 International Court of Justice (ICJ) advisory opinion on the Chagos Archipelago is formally non-binding, Mauritius has several mechanisms to “harden” its conclusions into mandatory obligations — particularly if the UK walks away from the 2024/2025 treaty framework. These mechanisms rely on transforming the advisory opinion into binding international law through General Assembly resolutions, arbitration, and institutional pressure.

* Can Mauritius leverage international bodies (like ICAO or IMO) to “de-register” Diego Garcia or disrupt BIOT’s telecommunications and mail to create a functional administrative blockade?

Based on the 2024-2025 UK-Mauritius agreement, Mauritius is legally barred from creating an artificial blockade of the Chagos Archipelago, particularly Diego Garcia. While the treaty returns sovereignty to Mauritius, it simultaneously grants the UK a 99-year lease with total administrative authority over Diego Garcia.Crucially, the terms require Mauritius to guarantee unrestricted access for UK and US military vessels and aircraft.

Under this framework, the UK retains “unrestricted ability” to control the flow of individuals, goods, and communications to the base.

*If the deal collapses, should Mauritius push for a new, more aggressive UNGA resolution calling for international sanctions or “non-recognition” of the BIOT administration by all UN member states?

If the Mauritius-UK deal regarding the Chagos Archipelago collapses, pursuing a UN General Assembly (UNGA) resolution for the formal non-recognition of the British Indian Ocean Territory (BIOT) would be a potent, aggressive legal strategy to enforce sovereignty. Such a move aligns with established international norms designed to prevent the legitimisation of unlawfully contested borders.

By securing a collective “duty of non-recognition” from the international community, Mauritius would effectively transform the BIOT into a diplomatic pariah, making it increasingly difficult for the UK to maintain its administration under the weight of global censure.

* Following Mauritius’s suspension of ties with the Maldives yesterday (Feb 28), does this regional tension affect the legal weight of Mauritius’s maritime boundary claims, or does it signal a necessary shift to aggressive diplomacy?

The suspension of diplomatic ties between Mauritius and the Maldives on February 27–28, 2026, over the Chagos Archipelago dispute, constitutes a major geopolitical escalation. This move shifts the conflict from the realm of legal arbitration into a phase of high-stakes, confrontational bilateral diplomacy.

While this rupture does not diminish the legal weight of Mauritius’s existing international victories — such as the ICJ Advisory Opinion and ITLOS rulings — it directly challenges the practical implementation of those mandates. Ultimately, it signals Mauritius’s pivot toward a more aggressive foreign policy aimed at forcing recognition of its sovereign rights.

* If the treaty is abandoned, should the Mauritian government officially support and protect Chagossian “civilian return” missions to the islands as a way of asserting de facto sovereignty?

The question of whether the Mauritian government should support and protect Chagossian civilian return missions to the Chagos Archipelago — particularly if the May 2025 treaty were to be abandoned — remains a central issue in the ongoing dispute over the islands.

Proponents argue that physically inhabiting the islands (specifically the outer islands of Peros Banhos and the Salomon Atolls) is the most effective way to establish de facto sovereignty, effectively transforming de jure recognition from the ICJ and UN into concrete control.

Following the 2019 International Court of Justice (ICJ) advisory opinion — which declared the separation of the Chagos Archipelago unlawful and mandated that the UK end its administration — supporting a return could be viewed as a necessary step in upholding international law and finally completing the decolonisation of Mauritius.

* At the end of the day, a fundamental question remains: given the current geopolitical volatility, do you think 99 years is a sustainable timeframe for a lease if the “landlord” (Mauritius) has no say in — or even prior knowledge of — the wars launched from its soil?

The 99-year lease agreement signed in May 2025 between the United Kingdom and Mauritius — which secures the US-UK military base on Diego Garcia while returning sovereignty of the Chagos Archipelago to Mauritius — remains a highly contested, long-term geopolitical arrangement. Whether such an expansive timeframe is truly “sustainable” amidst today’s extreme geopolitical volatility is a subject of intense and ongoing debate.

Mauritius Times ePaper Friday 6 March 2026