How did the incorporation of western territories into the United States affect Indian nations such as the Sioux or the Nez Pierce?
Even before the United States officially acquired Western territories, settlers began to stream into them. One has to remember that land boundaries are often not clearly marked, and some of these settlers may have honestly believed that they were still on United States soil when in reality they were in British, Spanish, French or Russian territory. When the United States acquired the titles to these Western lands, they did not ask the indigenous people there for permission. The settlers streamed into the new land and began to affect the wildlife of the new territory. This especially affected the Sioux, who occupied a large swath of the Great Plains. The Sioux hunted over most of this territory in order to follow the buffalo herds. Settlers who were used to fences and private property saw the buffalo as a nuisance and began to shoot them indiscriminately. This led to conflict with the Sioux. The federal government first sought to create treaties that protected the Western routes to Oregon, but soon railroads overtook the Plains, themselves, thus putting the Sioux onto smaller and smaller reservations. The Sioux resisted through war but were overcome with superior opposing numbers, as well as the systematic destruction of the buffalo. While some Sioux escaped to Canada under the leadership of Sitting Bull, they quickly returned as the Canadian government was no more accommodating than the Americans.
The Nez Perce made similar treaties with the U.S. government, but, when the reservations were underfunded, the Nez Perce sought escape, rather than outright war. Chief Joseph led the Nez Perce in an escape attempt to Canada. Finally cornered, Chief Joseph's small band of Nez Perce surrendered and were forced back onto the reservation.
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.