The first half of the 19th century is often seen as dominated by Removal. While it is obviously true that Removal impacted many Native nations in this era, it should not be forgotten that it is also during this time that the United States expands. With the Louisiana Purchase, the government looks westward, and simultaneously to Removal, it starts to get involved in the affairs of Native nations in the upper midwest and on the plains.
The ideology of Manifest Destiny, derived from early Puritan notions of being the Chosen People, is given shape during these years, and the war with Mexico, ended in 1848, further expands the United States' claims over territory and over indigenous peoples.
The relations with American Indian nations are defined, both in hindsight and for the future, in the decisions of the Marshall Trilogy. Native nations are sovereign, as implied in the constitution, but they are also defined as wards of the government. Thus, they remain sovereign in theory, but their sovereignty is unilaterally limited by an imposition of guardianship. Johnson v McIntosh also contains a legitimization, the legal justification for American title on the continent, based on the supposed absence of agriculture in Native societies (which is why it is so important to keep up the stereotype that Native peoples were hunters and gatherers).
In the 1850s, the federal government increases its treaty-making with the Chippewa and Sioux as well as with nations in Oregon Territory. The ground is thus being prepared for the expansion of federal control over the future 48 states after the Civil War.