On June 23, 2016, a referendum will decide whether Britain will leave the European Union (EU) or remain a member. Britain’s departure from the EU would affect the rest of the world, because it would have implications for a broad spectrum of international concerns–very importantly, international security. For the United States, Britain remains among the most important allies across the security spectrum, but the prospects of a Brexit leave the future of UK-US security cooperation uncertain.

In this piece, Walter B. Slocombe explores how Britain leaving the EU would affect US security interests, both directly and indirectly, from the secondary effects of the departure on other international relationships. Slocombe outlines the probable adverse effects a Brexit would have on Britain’s ability to maintain its international clout; the strength of the European Union’s diplomatic and foreign policy agenda; and Britain’s relationship with NATO.