In Frederick Jackson Turner's famous 1893 paper, "The Significance of the Frontier in American History," he argued that the closing of the American frontier in 1890 was a watershed moment in US history. What had made the US different from European countries, he argued, was the existence of a frontier that provided an outlet for American energies and desires. Turner asserted that the existence of an untamed frontier on the edge of "civilization," needing to be settled (from a white point of view: the Native Americans believed it already was settled) and the abundance of land and resources this frontier symbolized was integral to the American Dream and the formation of the American character.

Americans early on developed such traits as self reliance, independence, resourcefulness, individualism, and hopefulness because there were always successive waves of new frontiers opening up that could test the spirit and make dreams comes true, at least in the American mythology.

With the completion of railway travel and the planting of towns and growing urbanization across the continent, that era of US history had now ended. The United States would have to adapt and find new projects and dreams to absorb the vibrant, restless energies that had once been absorbed by the frontier. The frontier had been a safety valve, a place where people who couldn't find a place to fit in could find a wilderness to conquer.

Turner's thesis was popular in part because it was straightforward and easy to understand. It also asked questions that were particularly relevant to that period of history, a time when a great influx of immigrants were raising concerns about where outlets could be found to absorb a new and growing generation.