Similar to so many tribes whose lives were disrupted by settler incorporation of Western Lands, the Nez Perce tribe and the Sioux were greatly impacted by Westward Expansion. The Nez Perce social orders are interwoven with connection to the land. Natural geography found its way into cultural belief systems and practices, such as development of rites of passage and even the development of calendars. The Sioux practice of roaming freely on the land from place to place is also a very strong cultural practice within their social order. Both realities are strongly disrupted with Western Expansion towards the end of the 19th Century and start of the 20th. The natural encroachment was a given, in terms of finding their own tribes being relegated to specific locations. At the same time, the process of the removal of voice and denial of cultural traditions greatly impacted both the Sioux and the Nez Perce. Finally, I would submit that the Massacre at Wounded Knee, where the presence of the United States Military was needed to physically escort Native Americans off of the land upon which they lived for generations, was caused in large part due to the incorporation of Western territories into the column of White Settlers and away from Native Americans.
The formal addition of Indian lands into territories of the United States served as a population magnet for land hungry immigrants, prospectors, and railroad barons. The frontier always represented opportunity for America, and especially in the Gilded Age of laissez faire capitalism. This meant that population pressure on Indian tribes would inevitably grow, resulting in conflict, the spread of disease, and the decimation and conquest of the native peoples, including the Sioux and the Nez Perce.
The Sioux were particularly vulnerable to such expansion, as they were a nomadic buffalo culture, roaming the Great Plains, and living on prime land for white settlement, farming and the intercontinental railroad. In short, the addition of the western territories to the US was the beginning of the end for native tribes and their independence in America, as well as their cultural integrity.