The Indian Removal Act, signed on May 28th, 1830, by President Andrew Jackson authorized the negotiation with Native American tribes by the president, with the intent of moving southern tribes to federal territory west of the Mississippi River so the country could occupy their land.
Before the act, there was a lot of effort into encouraging the Indians to assimilate American culture, like converting to Christianity and learning English. There was also a lot of conflicts involving land ownership of land and properties.
The South's population strongly endorsed the act once it would allow for and occupation and the expansion of the settler's economic activities. This act faced resistance from Indian tribes such as the Cherokee, but eventually got removed by the government, in a march known as Trail of Tears.
After the Removal Act, the land became part of the official U.S. territory and got occupied by settlers, promoting the growth and establishment of cities and routes, paving way for the country's expansion, both in land and economically.