After 1945, and especially after 1989, the United States wielded overwhelming power on a previously unimaginable global dimension. The scale and reach of America’s unprecedented power transcends the normal confines of the nation-state. US officials, often in conjunction with private corporations and non-governmental organizations, manage a vast international network of political alliances, legal obligations, diplomatic treaties, economic relationships, and military commitments, all for the purpose of maintaining a world system established by presidents Franklin D. Roosevelt and Harry S. Truman during the 1940s. It is this dominant position that has led observers to describe the United States in imperial terms. This view of the modern United States as an imperialist power is based on the theory that empire does not have to be based on the control of territory. In this sense, if the twentieth-century United States was an imperial power, it was an extra-territorial one. However, this theory of empire is not universally shared, and so this chapter also assesses the competing historiographical and theoretical claims as to whether the modern United States has been an empire; and, if it is, what kind. The most common type of imperialism associated with the modern United States is liberal empire.