Complex sentence: The Sioux and other Plains tribes accepted the reservation system, but encroachments by whites in search of gold in the Black Hills led to renewed war by the end of Grant's second term, ending the understanding that had developed between Grant and Sioux Chief Red Cloud.
Split into simpler sentences: The Sioux and other Plains tribes accepted the reservation system, but encroachments by whites in search of gold in the Black Hills led to renewed war by the end of Grant's second term. The war ended the growing understanding that had developed between Grant and Sioux Chief Red Cloud.