This original and well-written work takes up an intriguing question: Why did the United States not expand for over three decades after the Civil War, when its power was such as to present few external obstacles? Expansionist projects proved abortive, Zakaria argues, because of the institutional weakness of the American state from 1865 to 1889. Growth of national and presidential power, in turn, accounts for expansion in the 1890s. Zakaria skillfully develops that explanation for the postbellum era, but his framework does not fit the expansionist 1840s, when the American state was much weaker than at any time after the Civil War (and when, as David Potter once observed, the nationalists were not expansionists and the expansionists were not nationalists). The author challenges the claims of "defensive realists," who see expansion as a response to external threats, but his harsh judgment of these theorists obscures the belief he shares with them -- that the task of theory is to offer explanations on the model of the physical sciences that are abstract and deterministic. Raymond Aron demonstrated a generation ago why such a conception of theory is impoverished. 'Tis a pity that political scientists seem intent on pursuing it.