Could the U.S. rest easy if its only military base in the Indian Ocean were under the sovereign authority of an African government instead of the U.K.? At center stage is Naval Support Facility Diego Garcia, a critical hub for U.S. forces. The base is the sole reason that the U.K. clings onto the 58 small Indian Ocean isles that make up the Chagos Archipelago, of which Diego Garcia is the largest. For decades, London has made Diego Garcia available to the Pentagon on the most favorable terms imaginable: no rent, few restrictions and little oversight. The island is an...