Manifest destiny — the belief that the United States had a “God-given” right to aggressively spread the values of white civilization and expand the nation from ocean to ocean — dictated U.S. policy toward Indians and other nations with claims in North America. In the name of manifest destiny, Americans forced the removal of the Five Civilized Tribes of the South (the Cherokee, Chocktaw, Chickasaw, Creek, and Seminole peoples) to Oklahoma in the 1830s; colonized Texas and won its independence from Mexico in 1836; conquered California, Arizona, New Mexico, and parts of Utah and Colorado in the Mexican-American War of 1846–1848; and invaded Oregon in the mid-1840s.

By midcentury, hordes of settlers crossed the Great Plains on their way to the goldfields of California or the rich farmland of Washington and Oregon. In their path stood a solid wall of Indian land, much of it in reservations granted by the U.S. government in its policy of Indian removal. In 1851, some ten thousand Plains Indians came together at Fort Laramie in Wyoming to negotiate a treaty that ceded a wide swath of their land to allow passage of wagon trains headed west. In return, the government promised that the remaining Indian land would remain inviolate.

The Indians who “touched the pen” to the 1851 Treaty of Fort Laramie hoped to preserve their land and culture in the face of the white onslaught. Settlers and miners cut down trees, polluted streams, and killed off the bison. Whites brought alcohol, guns, and something even more deadly — disease. Between 1780 and 1870, the population of the Plains tribes declined by half. “If I could see this thing, if I knew where it came from, I would go there and fight it,” a Cheyenne warrior lamented. Disease also shifted the power from Woodland agrarian tribes to the Lakota (Western) Sioux, who fled the contagion by pursuing an equestrian nomadic existence that displaced weaker tribes in the western plains.

Poverty and starvation stalked the reservations. Confined by armed force, the Indians eked out an existence on stingy government rations. Styled as stepping-stones on the road to “civilization,” Indian reservations closely resembled colonial societies where native populations, ruled by outside bureaucrats, saw their culture assaulted, their religious practices outlawed, their children sent away to school, and their way of life attacked in the name of progress and civilization.

To Americans raised on theories of racial superiority, the Indians constituted, in the words of one Colorado militia major, “an obstacle to civilization … [and] should be exterminated.” This attitude pervaded the military. In November 1864 at the Sand Creek massacre in Colorado Territory, Colonel John M. Chivington and his Colorado militia descended on a village of Cheyenne, mostly women and children. Their leader, Black Kettle, raised a white flag and an American flag to signal surrender, but the charging cavalry ignored his signal and butchered 270 Indians. Chivington watched as his men scalped and mutilated their victims and later justified the killing of Indian children with the terse remark, “Nits make lice.” The city of Denver treated Chivington and his men as heroes, but a congressional inquiry eventually castigated the soldiers for their “fiendish malignity” and condemned the “savage cruelty” of the massacre.