In the middle of the 19th century, America essentially stopped at the Mississippi River, with a wide gap from there to California. But then miners poured in, hoping to uncover gold and other metals. They were followed by corporations who finished what they had started. The mining industry opened different areas of the west and blazed many trails for others to follow.

The Homestead Act of 1862 and other similar legislation encouraged more settlement by giving away free land in the Great Plains to individuals. Like the miners, many of them failed, but this didn't stop the rush of people into new land. Ranchers pushed even farther west than the homesteaders, opening up the Rocky Mountains. By 1890, the gap had closed; the frontier was erased. This prompted historian Frederick Jackson Turner to consider the impact of the American West. Turner's Frontier Thesis argued that its very existence had done more to shape American society and character than any other factor and that its closure would create a need for American expansion in other ways.