Complex sentence: Ranchers led by William Henderson repeatedly tried to drive Kamiakin from his ancestral lands, but superintendent of Indian Affairs, Robert Milroy, intervened and vowed (successfully) to allow Kamiakin to live out his days there.
Split into simpler sentences: Ranchers led by William Henderson repeatedly tried to drive Kamiakin from his ancestral lands, but Robert Milroy, superintendent of Indian Affairs, intervened. He followed through on his vow to allow Kamiakin to live out his days there.