On December 29, 1890, the U.S. 7th Cavalry, the same division defeated at the Battle of the Little Big Horn by Sioux warriors years before, decided it was time to "take this Indian nation." With the murder two weeks earlier of Chief Sitting Bull, the Sioux Nation's inspirational, moral, and political leader, the U.S. Army felt confident of accomplishing its goal.
The Army was comprised of veteran soldiers who had roamed the Western Plains, many since the Civil War, looking for a new war to fight. In the Civil War, they had been asked to fight against friends and family, almost always other people of European descent But these late-century soldiers rode the frontier because they wanted "villains" whose skin was of a different hue.
Actually the Army had ended what it called the Indian Wars -- a peculiar name for wars that in each case were initiated by the U.S. government -- in 1868, with the signing of a general peace treaty. But then gold was discovered in the Black Hills of South Dakota, land controlled by treaty and considered sacred by the Sioux.
When the Sioux rejected offers by the federal government to sell the land, war again became government policy, in order to ensure access to the resources. For years the only interaction between the United States and the Indian Nations was for concession of land and resources, and skirmishes often resulted from their seizure.