This book was written during a time of unprecedented U.S. power, measured not only in terms of the material resources the United States can marshal to project influence abroad but also, and perhaps more important, by the absence of any significant external constraints on how it might use that power. Under these conditions, it is fair to argue, constraints on how the United States uses its power in the international system will depend on limits operating within its own domestic political system. The nature of those internal constraints and the actual effects they might have on U.S. behavior should be of great interest to any contemporary observer of American foreign policy and international relations.