The Indian Removal Act of 1830 required many American Indian tribes in the eastern United States to move to the Great Plains west of the Missouri River, a region identified as the Permanent Indian Frontier and also called the Great American Desert, where only the military, licensed traders, and missionaries were legally allowed to live if they were not Indians. From the late 1830s to the early 1860s, tens of thousands of freight and emigrant wagons crossed these lands on the way west, and emigrants leaving Missouri often wrote that they were leaving the United States and entering the wilderness, a prospect many viewed with apprehension. By the mid-1850s, Indian nations living in territories west of here felt their lifeways increasingly threatened as western expansion reduced the vast herds of big game, vegetation, and other resources critical to survival and tribal independence. Twenty-four years after the Indian Removal Act, the Kansas-Nebraska Act of 1854 opened the Kansas region to non-Indian settlement, ending the Permanent Indian Frontier.