## Facts

- The United Nations General Assembly requested an advisory opinion on the legal consequences of the 1965 separation of the Chagos Archipelago from Mauritius and the United Kingdom’s continued administration of the territory.
- The Chagos Archipelago had been administered by the United Kingdom as a dependency of the non-self-governing territory of Mauritius.
- In 1965, during negotiations leading to Mauritian independence, the United Kingdom detached the Chagos Archipelago and created the British Indian Ocean Territory (BIOT).
- The United Kingdom later arranged for BIOT to be made available for defense purposes, and the Chagossian population was removed from the islands.
- Mauritius became independent in 1968 and consistently maintained that the 1965 detachment was unlawful and that decolonization was therefore incomplete.
- Numerous states and the African Union participated through written statements and oral observations.
- The Court unanimously found it had jurisdiction; by 12–2 it decided to answer; and by 13–1 it adopted its principal conclusions on the merits.

## Issues

1. Whether Mauritius’s decolonization was lawfully completed when it became independent in 1968, given the prior separation of the Chagos Archipelago and applicable international law, including relevant General Assembly resolutions.
2. What legal consequences under international law arise from the United Kingdom’s continued administration of the Chagos Archipelago, including effects on Mauritius’s ability to implement a resettlement program for Mauritian nationals of Chagossian origin.
3. Whether the Court should decline to answer the request in the exercise of its discretion.

## Decision

- The Court held it had advisory jurisdiction because the request came from a competent UN organ and posed legal questions.
- The Court declined to refuse the request, finding no compelling reasons to withhold an opinion despite political aspects and despite the matter’s connection to a bilateral dispute.
- The Court concluded that, by 1965, the right of peoples to self-determination had crystallized in customary international law and required respect for the territorial integrity of non-self-governing territories.
- The Court found the Chagos Archipelago was an integral part of Mauritius at the time of detachment and that the separation was not based on the free and genuine expression of the will of the people concerned.
- The Court advised that Mauritius’s decolonization was not lawfully completed in 1968.
- The Court advised that the United Kingdom’s continued administration of the Chagos Archipelago is a continuing internationally wrongful act engaging its international responsibility.
- The Court advised that the United Kingdom must end its administration as rapidly as possible, and that all UN member states must cooperate with the United Nations to complete Mauritius’s decolonization.

## Legal Principles

- An ICJ advisory opinion may be given on legal questions requested by an authorized UN organ; alleged political implications do not remove a question’s legal character.
- The Court may decline an advisory request only for compelling reasons; a request is not improper solely because it relates to a dispute between states.
- The right to self-determination, as customary international law by the mid-1960s, includes protection of the territorial integrity of a non-self-governing territory unless alteration reflects the free and genuine will of the people concerned.
- A colonial detachment not grounded in a free and genuine expression of the people’s will breaches self-determination and prevents lawful completion of decolonization.
- Continued administration maintaining an unlawful territorial situation constitutes a wrongful act of a continuing character, requiring cessation.
- Because self-determination is owed to the international community, other states have obligations to cooperate through the United Nations to bring the unlawful situation to an end and not to hinder completion of decolonization.

## Conclusion

The Court advised that separating the Chagos Archipelago from Mauritius in 1965 violated the customary international law right of self-determination and territorial integrity, leaving Mauritius’s decolonization incomplete; the United Kingdom must end its administration promptly, and all UN member states must cooperate with the United Nations to complete decolonization.


Legal Consequences of the Separation of the Chagos Archipelago from Mauritius in 1965, Advisory Opinion, 2019 I.C.J. 95 (Feb. 25)