Complex sentence: He spent most of his life on the Great Sioux Reservation, which was established in 1868 in the Fort Laramie Treaty; more specifically he lived in the region, which is the Rosebud Indian Reservation today.
Split into simpler sentences: He spent most of his life on the Great Sioux Reservation, which was established in 1868 by the Fort Laramie Treaty. More specifically, he lived in the region which is the Rosebud Indian Reservation today.