This article examines the implications for the U.S. military base in Diego Garcia, located in the Chagos Archipelago, in the event that sovereignty of the atoll is ceded from the U.K. to Mauritius. Diego Garcia has become a critically important military base, facilitating U.S. power projection into the Indian Ocean, Middle East and Africa. The legality and future of British sovereignty over the atoll was challenged in 2019 in both the International Court of Justice and United Nations General Assembly. This article finds that Mauritian sovereignty of Diego Garcia would have considerable adverse consequences for U.S. military activity, potentially prohibiting many of the core functions the facility currently performs, as under U.K. sovereignty. Further, this article examines the strategic utility in Diego Garcia for the U.S. and U.K. governments, which demonstrates why these two nations are willing to endure considerable reputational costs and international backlash for retention of the atoll.