The post-Cold War order established by the United States is at a crossroads: no longer is the liberal order and U.S. hegemonic power a given. The Challenge to NATO is a concise review of the North Atlantic Treaty Organization (NATO), its relationship with the United States, and its implications for global security. Despite seeing its seventieth anniversary in 2019, NATO faces both external and internal threats to its continued survival. This volume examines the organization's past, its current regional operations, and future threats facing the Atlantic Alliance with contributions by well-known academics, former central figures within NATO, and diplomats directly involved in NATO operations. In this volume, Michael O. Slobodchikoff, G. Doug Davis, and Brandon Stewart bring together differing perspectives and orientations to provide a complete understanding of the future of the Atlantic Alliance.