The Civil War created the beginnings of a new world for United States foreign policy, but it was another generation before that future could be realized. Out of the deaths of 600,000 Americans emerged, slowly but with certainty, a different nation, which replaced Jacksonian decentralization with centralization, the presidencies of James Buchanan and Rutherford B. Hayes with those of William McKinley and Theodore Roosevelt, the Jeffersonian agrarian-ideal commercial farmer with the Andrew Carnegie–J. P. Morgan ideal of the billion-dollar U.S. Steel Corporation, and the 1840s laissez-faire capitalism of James K. Polk’s Democrats with the late 1890s corporate capitalism of Senator Mark Hanna’s Republicans. Of special importance, the nation built on these four domestic transformations to construct a foreign policy that replaced the Monroe Doctrine of 1823 with the Open Door policy of 1899–1900; that is, Americans were finished with land expansion from sea to sea. They were confident now in their supremacy over much of the Western Hemisphere and embarked on an imperialist course in parts of Asia and Africa.

These historic changes, of course, did not start cleanly in the 1860s. Jefferson and Polk, for example, had demonstrated the incredible potential for presidential power long before Theodore Roosevelt’s birth. The faith that supplying China’s market could put depression-ridden Americans to work dated back to the mid-1780s, not the mid-1890s. Even the once firmly held belief that the Civil War gave birth to the industrialized United States has been disproved. The annual growth rate of U.S. manufactures was 7.8 percent between 1840 and 1860, but 6 percent between 1870 and 1900. Between 1860 and 1870, the value added by manufacturing increased by only 2.3 percent annually, the lowest rate of increase in the nineteenth century.