Chagos Archipelago Dispute: ICJ Rulings, Diego Garcia & Strategic Impact

Overview

The legal and political resolution of the Chagos Archipelago dispute marks a turning point in international law. Multilateral judicial rulings successfully dismantled colonial-era partitions while preserving regional security arrangements. Landmark rulings by the International Court of Justice and the International Tribunal for the Law of the Sea confirmed that the decolonisation of Mauritius was legally incomplete after the United Kingdom detached the islands in 1965.

A bilateral agreement in October 2024 officially restored Mauritian sovereignty over the archipelago. At the same time, the accord leased the strategic Diego Garcia military facility back to the United Kingdom for 99 years. For India, this outcome vindicates a long-standing anti-colonial diplomatic stance, deepens its partnership with Mauritius, and secures stability across the Western Indian Ocean.

As of October 2024, the United Kingdom and Mauritius concluded a landmark political agreement recognising Mauritian sovereignty over the Chagos Archipelago. The accord ended nearly six decades of disputed colonial administration.

This breakthrough followed sustained international legal pressure through the United Nations and international courts. Comprising over 58 coral islands in the central Indian Ocean, the territory had remained under British control as the British Indian Ocean Territory (BIOT) since 1965.

The negotiated settlement established a dual-track framework:

• Sovereignty Transfer: Britain recognised Mauritius as the sole lawful sovereign over the entire archipelago.
• Strategic Continuity: The UK secured permission to exercise sovereign rights over Diego Garcia for an initial 99-year lease period.
• Base Operations: The arrangement guarantees uninterrupted operation of the joint UK-US military base while restoring de jure territorial integrity.

The United Kingdom detached the Chagos Archipelago from the Crown Colony of Mauritius in 1965. This took place three years prior to granting Mauritius full independence on March 12, 1968.

British authorities formalised this pre-independence excision through the Lancaster House Agreement 1965, creating the BIOT to retain strategic control over Indian Ocean sea routes.

Between 1968 and 1973, British administrators carried out the forced displacement of Chagossians. Authorities evicted between 1,400 and 2,000 indigenous residents from their homeland to Mauritius and Seychelles. Officials cleared the islands without local consent to construct a joint military facility on Diego Garcia.

For decades, the displaced Chagossian population—often called the Ilois—faced severe economic hardship in exile. Mauritius maintained that the excision violated international law, asserting that consent given under colonial rule was legally invalid.

The International Court of Justice delivered an Advisory Opinion on February 25, 2019. By a decisive 13 to 1 vote, the court determined that the decolonisation of Mauritius was not lawfully completed in 1968.

The world court anchored its ruling in customary international law, establishing key legal findings:

• Peremptory Norm: The right to self-determination had crystallised into customary international law before 1965.
• Territorial Integrity: Under UN General Assembly Resolution 1514 (XV), colonial powers cannot unlawfully divide non-self-governing territories prior to independence.
• Duty to Withdraw: The ICJ concluded that the United Kingdom was under an obligation to end its administration of the archipelago as rapidly as possible.

Following the ruling, the United Nations General Assembly adopted UNGA Resolution 73/295 on May 22, 2019, with 116 member states voting in favour. The resolution set a six-month withdrawal deadline for British authorities and directed international bodies to recognise Mauritian sovereignty.

In compliance, the UN Geospatial Information Section updated its official world map on February 20, 2020, formally depicting the Chagos Archipelago as Mauritian territory.

The International Tribunal for the Law of the Sea rejected British sovereignty over the surrounding waters on January 28, 2021. Its Special Chamber dismissed preliminary objections in Case No. 28.

The dispute arose when Mauritius sought a formal maritime delimitation with the Maldives under the United Nations Convention on the Law of the Sea (UNCLOS). The Maldives challenged jurisdiction, claiming an unresolved sovereignty dispute with the United Kingdom barred ITLOS from proceeding.

The Special Chamber resolved the objection through several core determinations:

• Judicial Weight: Rulings in an ICJ advisory opinion carry authoritative judicial weight when settling preliminary legal questions.
• Absence of British Title: Relying on ICJ decolonisation findings, the tribunal confirmed that Britain possessed no lawful sovereignty over Chagos.
• Binding Precedent: The ruling established that an advisory opinion can definitively settle sovereign status in subsequent binding maritime litigation.

The Special Chamber of ITLOS delivered its final merits judgment on April 28, 2023. The chamber unanimously delimited a single maritime boundary between Mauritius and the Maldives across overlapping Exclusive Economic Zones (EEZ) within 200 nautical miles.

The tribunal applied the standard three-stage maritime delimitation methodology across a disputed zone of 92,563 square kilometres:

• Awarded 47,232 square kilometres to the Maldives.
• Awarded 45,331 square kilometres to Mauritius.

This division gave both island nations legal certainty over their fisheries, seabed resources, and continental shelf boundaries.

The boundary dispute also highlighted shifting diplomacy by the Maldives. As recorded by the Observer Research Foundation: The Maldives' Chagos Claim: Implications for India, Male voted against UNGA Resolution 73/295 in 2019 to defend its southern baseline before engaging in ITLOS proceedings. The final judicial outcome fixed maritime boundaries without disrupting wider regional diplomacy.

The political agreement announced on October 3, 2024, reconciles Mauritian territorial sovereignty with the operational continuity of the Diego Garcia military facility.

Under the accord, Britain agreed to transfer de jure sovereignty over the entire Chagos Archipelago to Mauritius. In return, Mauritius authorised the United Kingdom to exercise sovereign functions over Diego Garcia for 99 years, guaranteeing operational access for British and American naval and air units.

The negotiated compromise established specific financial and governance arrangements:

• An indexed annual financial package paid by the British government to Mauritius across the operational lease period.
• A dedicated Chagossian resettlement trust fund administered to support community welfare and infrastructure.
• Strategic environmental cooperation to protect the marine biodiversity and coral atolls across the archipelago.

This functional compromise decoupled basing rights from underlying sovereign title, satisfying international law while maintaining strategic maritime posture.

India's Ministry of External Affairs welcomed the October 3, 2024 agreement, stating that it completed the decolonisation of Mauritius under international law.

The resolution advances India's multi-layered maritime priorities across three distinct fronts:

1. Anti-Colonial Principles: India consistently backed Mauritian sovereignty during written and oral proceedings at the ICJ in September 2018.
2. Regional Security Architecture: The pact aligns with India's SAGAR doctrine, unveiled in Mauritius in March 2015.
3. Strategic Coexistence: A stable Anglo-American presence on Diego Garcia operates in harmony with India's regional surveillance capabilities.

New Delhi's legal backing reinforced its solidarity with the Global South while strengthening its Comprehensive Strategic Partnership with Mauritius.

Operationally, India expanded its role as a regional Net Security Provider on February 29, 2024. Prime Minister Narendra Modi and Prime Minister Pravind Jugnauth inaugurated a 3,000-metre airstrip and the St. James Jetty on North Agalega Island. Maintaining a Western security presence on Diego Garcia provides regional balance while keeping India's primary sea lines of communication secure.

Displaced Chagossian communities have raised serious human rights objections regarding their exclusion from bilateral treaty negotiations between London and Port Louis.

While the October 2024 agreement established a resettlement trust fund and permitted Mauritian-led programmes on outer atolls like Peros Banhos and Salomon, it explicitly barred civilian settlement on Diego Garcia. Chagossian advocacy groups argue that restricting access to their ancestral islands denies their basic right of return.

Beyond humanitarian concerns, the Chagos jurisprudence created major international legal precedents:

1. The right to self-determination achieved the status of customary international law before 1965, invalidating colonial-era land partitions.
2. Regional maritime courts like ITLOS can treat authoritative ICJ advisory opinions as binding guidance on sovereignty questions.
3. Historical territorial disputes can be resolved through pragmatic long-term leases, establishing a template for other contested maritime enclaves.

The Chagos dispute demonstrates the strength of a rules-based maritime architecture grounded in UNCLOS and multilateral diplomacy.

Sustaining regional stability in the Western Indian Ocean requires three practical priorities:

• Transparent Resettlement Execution: Mauritius must run the resettlement trust fund with direct Chagossian participation to guarantee transparent rehabilitation and sustainable livelihoods.
• Enhanced Maritime Domain Awareness: Coastal states—including India, Mauritius, Seychelles, and the Maldives—should expand joint hydrographic surveys and shared maritime radar tracking to counter illegal fishing and piracy.
• Trilateral Security Coordination: Strategic dialogue between India, the United States, and Mauritius should ensure operations on Diego Garcia and Agalega Island complement collective maritime safety.

By aligning judicial determinations with maritime security needs, the Chagos settlement proves that persistent colonial disputes can be peacefully resolved through international law.

• The United Kingdom detached the Chagos Archipelago from Mauritius in 1965 to form the British Indian Ocean Territory, unlawfully evicting up to 2,000 indigenous Chagossians between 1968 and 1973.
• In February 2019, the International Court of Justice determined by 13:1 that Britain's decolonisation of Mauritius was unlawful under customary international law and demanded a rapid end to British administration.
• The ITLOS Special Chamber in 2021 and 2023 rejected British territorial claims, accepted ICJ authority on sovereignty, and delimited a single maritime boundary between Mauritius and the Maldives.
• The October 3, 2024 bilateral agreement acknowledged full Mauritian sovereignty while leasing the joint UK-US military base on Diego Garcia to the United Kingdom for 99 years.
• India consistently backed Mauritius at the ICJ in 2018 and welcomed the 2024 accord, balancing anti-colonial diplomacy with its SAGAR doctrine and Quad-aligned Indian Ocean maritime security objectives.