Mobilizing an interdisciplinary group of scholars and practitioners, this book reviews the history and consequences of NATO's post-Cold War enlargement into Central and Eastern Europe. It offers a nuanced discussion of the merits and drawbacks of NATO enlargement across the different actors involved and compares the results of the policy against potential alternatives that were not chosen. Particular attention is given to NATO enlargement's influence on the course of U.S. foreign policy, democracy and security in Central and Eastern Europe, NATO's own development as a political and military institution, and relations with China and Russia (including the 2022 Russia-Ukraine War). Written for an engaged audience, the book is designed to appeal to students, researchers, and policymakers alike while offering both policy insights and avenues for future scholarship.