Answer:
Tribal sovereignty in the United States is the concept of the inherent authority of indigenous tribes to govern themselves within the borders of the United States. Originally, the U.S. federal government recognized American Indian tribes as independent nations, and came to policy agreements with them via treaties. As the U.S. accelerated its westward expansion, internal political pressure grew for "Indian removal", but the pace of treaty-making grew nevertheless. Then the Civil War forged the U.S. into a more centralized and nationalistic country, fueling a "full bore assault on tribal culture and institutions", and pressure for Native Americans to assimilate.[1] In the Indian Appropriations Act of 1871, without any input from Native Americans, Congress prohibited any future treaties. This was steadfastly opposed by Native Americans.Currently, the U.S. recognizes tribal nations as "domestic dependent nations" and uses its own legal system to define the relationship between the federal, state, and tribal governments.
Map of the contiguous United States with reservation lands excluded as of 2003
Reservation lands in the contiguous United States as of 2019