The United Kingdom agreed Thursday to return all but one of the Chagos Islands to Mauritius.

Mauritius is an island country in the Indian Ocean off the southeastern coast of East Africa. In 1965, Britain siphoned the Chagos Islands—an archipelago of over 60 islands in the Indian Ocean—away from Mauritius, a former U.K. colony that gained independence three years later.

The British government made the deal to give back all but one of the Chagos Islands to secure the operation of a U.K.-U.S. naval base at Diego Garcia, the largest of the islands off the tip of India.

The U.S. built the base in the 1970s thanks to a lease agreement with Britain. It is now home to roughly 2,500 personnel, mainly Americans, and has been described by the U.S. as “an all but indispensable platform” for security operations in the Middle East, South Asia and East Africa.

According to the British government, if they didn’t give the rest of the Chagos Islands back to Mauritius, the Diego Garcia base would be threatened by contested sovereignty and legal challenges, including through international courts and tribunals.

“It will strengthen our role in safeguarding global security, shut down any possibility of the Indian Ocean being used as a dangerous illegal migration route to the U.K., as well as guaranteeing our long-term relationship with Mauritius, a close Commonwealth partner,” British Foreign Secretary David Lammy said.

The deal still has to be signed off in a treaty and is dependent on legal processes being finalized. Both sides have agreed to complete the process as quickly as possible.

Under the deal, the British government will keep Diego Garcia for an initial 99-year period and will pay Mauritius rent. The cost of the rent has not been disclosed.

Additionally, the deal will create a “resettlement” fund for the natives of the Chagos Islands who were displaced decades ago to help them move back to the islands other than Diego Garcia.

Chagossian Voices, a U.K.-based group representing the Chagossian diaspora around the world, demanded that the roughly 1,500 natives who were displaced in the creation of the Diego Garcia base be included in the deal.

“Chagossians have learned this outcome from the media and remain powerless and voiceless in determining our own future and the future of our homeland,” the group said in a social media post. “The views of Chagossians, the indigenous inhabitants of the islands, have been consistently and deliberately ignored and we demand full inclusion in the drafting of the treaty.”

Meanwhile, President Joe Biden applauded the “historic agreement” in a statement, adding, “The agreement secures the effective operation of the joint facility on Diego Garcia into the next century.”

This article includes reporting from The Associated Press.