History of racial segregation in the United States
In 1887, Congress passed the Dawes Act.  This law stopped giving land to whole tribes, and broke up the land into small pieces for individual families to use for farming.  Indians who took the land, started living alone instead of with their tribes, and started farming were viewed as "civilized," and they were made United States citizens.  Indians who refused to segregate themselves even more on small pieces of land were not allowed to be citizens.  Whatever land was left was sold to white settlers, which made the reservations even smaller.