This book investigates the major influences on U.S. decision-making regarding the enlargement of the North Atlantic Treaty Organization (NATO) following the end of the Cold War. After the collapse of the Soviet Union, many questioned the need for the Alliance's continued existence. It was not obvious that NATO would survive, and indeed thrive in the twenty-first century. The United States has been the driving force behind NATO's surprising endurance and growth. This book identifies key factors that have motivated American decision-makers to support the expansion of the Alliance's membership since the end of the Cold War in 1989-1991. Time and again, evolving threats to transatlantic security have revealed the need to sustain the Alliance. Cold War fears of communist aggression were replaced by the dangers of instability created by ethnic and religious conflicts, as demonstrated in the Balkans. These dangers in turn gave way to menacing transnational terrorist organizations, including al Qaeda. As the threats changed, the importance of close political association at times trumped that of enhanced military capability. Cultivating the international community of free democracies by expanding NATO membership provided a framework to counter the emerging threats.