Beginning in the 1840s, the nation’s swelling population, booming economy, and boundless confidence propelled a new era of rapid westward migration. Until then, the overwhelming majority of Americans lived east of the Mississippi River. Under the banner of manifest destiny, Americans encountered Native Americans, who inhabited the plains, deserts, and rugged coasts of the west; the British, who claimed the Oregon Country; and the Mexicans, whose flag flew over the vast expanse of the Southwest. Nevertheless, by 1850 the United States stretched to the Pacific and included the Utah Territory with its Mormon settlement.

Frontier settlers took the land and then, with the exception of the Mormons, lobbied their government to acquire the territory they had settled. The human cost of aggressive expansionism was high. The young Mexican nation lost a war and half of its territory. Two centuries of Indian wars, which ended east of the Mississippi during the 1830s, continued for another half century in the West.