President Donald Trump has come out against British Prime Minister Keir Starmer’s plan to cede Diego Garcia, a small island in the Indian Ocean that is also a largely important U.S. military base. Trump called the proposal a “big mistake.” He’s right. Diego Garcia may be tiny, but its strategic importance looms large in a world where the United States and its allies face growing threats.

Diego Garcia is the largest of the Chagos Islands, and the U.S. shares the military base there with Britain. Under the terms of the deal now in dispute, the United Kingdom intends to grant sovereignty of the Chagos archipelago to Mauritius while maintaining access to the base with a 99-year lease. In theory, this might sound fine. But in practice, it is wholly unacceptable.