The idea of territorial expansion was born when America was born. The charters of most British colonies in America granted them dominion as far as the Pacific Ocean. The Articles of Confederation explicitly reserved a place in the new nation for Canada. In 1801, Jefferson looked “forward to distant times, when our rapid multiplication will … cover the whole northern if not the southern continent, with people speaking the same language, governed by similar forms, and by similar laws.”

When Jefferson wrote, the United States possessed 838,000 square miles, an area already about eight times as large as the kingdom from which it had separated. The purchases of Louisiana and Florida more than doubled the national domain, but the grandest acquisitions, geographically at least, took place between 1845 and 1848. Annexation of Texas, settlement of the Oregon boundary, and the conquests of the Mexican War, all accomplished in the administration of James K. Polk, raised the land area of the United States to three million square miles. Later, in 1867, the Alaska Purchase brought holdings on the North American continent to their present extent of three and a half million square miles. Brazil, Canada, and China are about the same size, but only Russia, twice America’s size even after breakup of the Soviet Union, possesses a significantly larger domain.