The government broke treaties pure and simple. They allowed settlers to
cross the Black Hills and seek gold. The Sioux considered the Black
Hills scared and the plains tribes were often attacked by troops. Read
Black Elk Speaks about an attack he experienced. In 1861 the Dakota
Sioux were rounded up by the military after an attack on a settlers
family. They took over 2,000 men, women, and children to Ft. Laramie.
The government was going to hang 300 Dakota Sioux men, but Lincoln
stopped it. Instead they hung 10 and this is the largest mass hanging in
United States history, but no history book has it in it. Over and over
treaties were broken, lands taken, and the Native Americans moved off
ancestral lands.
I hope this helps!!!