Starmer said the deal to secure the U.K.-U.S. military base in Diego Garcia — to be leased for more than $100 million a year — is vital for Britain's defense, intelligence and national security.

Diego Garcia is part of the Chagos Archipelago, located about 4,700 km. southwest of Iran and about 1,800 km. south of India. It is not an independent country but a British territory leased to the ...