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. 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. In the Indian Appropriations Act of 1871, Congress prohibited any future treaties. This move was steadfastly opposed by Native Americans