In what was the frontier it the US pushed west of the Mississippi River?

The frontier in the United States west of the Mississippi River was characterized by the expansion of settlers, farmers, and traders into new territories during the 19th century. This movement was fueled by the belief in Manifest Destiny, the idea that Americans were destined to expand across the continent. Key events, such as the Louisiana Purchase in 1803 and the subsequent westward migrations, facilitated the establishment of new settlements and the development of infrastructure. This period also saw significant conflicts with Native American tribes and changes in the landscape and demographic makeup of the region.