President James Monroe’s 1823 message to Congress declaring opposition to European colonization in the Western Hemisphere became the cornerstone of nineteenth-century American statecraft. The Monroe Doctrine proclaimed anticolonial principles, yet it rapidly became the myth and means for subsequent generations of politicians to pursue expansionist foreign policies. The crucial episodes in nineteenth and early-twentieth-century foreign relations—westward expansion in the 1840s, Civil War diplomacy, the imperialism of 1898, entrance into World War I, and the establishment of the League of Nations—were framed by the Doctrine as its meaning evolved to suit the dreams and fears of an American empire. In Jay Sexton’s adroit telling, the Monroe Doctrine provides a new lens through which to view a great paradox at the center of American history: the nation’s conflicting traditions of anticolonialism and imperialism.