The Plains Indians were ultimately defeated by the American government and its representatives through a series of events including numerous interactions of guerilla and organized warfare, systematic killing of the American buffalo, and the cultural assimilation process.
In response to the Homestead Act of 1862 and the invasion of people looking to settle their lands, the Plains Indians first engaged in attacks against white settlers. These killings brought military action by the United States government and full-blown warfare between the two sides. When conflict alone did not see the submission of the Plains Indians, the American government turned to removing the people's main food source, the American buffalo. The final decline of Plains Indian's cultural and lifestyle came as a result of the assimilation programs that completely eliminated traditions and culture, instead replacing them with Anglo customs and education.
The most important factor in the decline could be the loss of the American buffalo, this style of military warfare is barbaric and left the Plains Indians with no way to sustain themselves or their lifestyle.
