1. Introduction

Since the end of the Second World War and the brief unipolar moment that followed, the U.S. has assumed (and claimed) leadership over the world—at least over the “free world.” Sometimes analyzed as hegemony or as empire, this work offers a definition of leadership that is related to, but distinct from these concepts. It relies on the full range of American power in foreign affairs, including attractive “soft” power and traditional military and economic might. The ongoing discussion of leadership by American policymakers confronts a world that is ever-changing and in which this leadership is increasingly challenged. That U.S. leadership is weaker today in many ways than after the Cold War or after the Second World War is only further reason to explore the complexities of American foreign policy that in many ways is facing limits, new and old, but continues to retain special advantages. In an uncertain and crisis-filled world, assessing U.S. leadership is critical for understanding American policy, and global international relations.