Frederick Jackson Turner was a historian who introduced the highly influential "Frontier Thesis" in an 1893 paper he gave to the American Historical Association. In it, he argued that the American character and culture were shaped by the fact that the United States was located on the border of a frontier that people were pushing into. While the earliest settlers were European in social structure and outlook, as Americans moved west in successive waves—to the Appalachians and onward—the frontier shaped and changed the American mindset to be different from that of the European. The frontier made Americans rugged individualists who always had a new space to conquer outside of the confines of civilization. It made Americans violent, and it made them distrust hierarchy and reject aristocracy in favor of an every-man-for-himself democracy and egalitarianism.

Turner's theory was masculinist, rejecting effete "culture" as not part of the American ethos and thus plugging back into a long—and ironically literary—history going back at least to Washington Irving in defining red-blooded, vigorous American men in contrast to timid European intellectuals (think of Ichabod Crane). Turner's theory, appearing in 1893, also raised alarm bells, because according the US Census, the American frontier closed in 1890. People therefore worried that the end of the frontier would have a negative effect on American energy and individualism and rob people of the outlet of "untamed" land.

Frederick Jackson Turner was and is important in American history because he originated the "frontier thesis" that guided American views of history for a long time.  Turner argued that the presence of the frontier made America what it was.  It allowed Americans to become individualistic, self-reliant, and democratic.  This thesis has been very important in American historical thinking because it has influenced the way that Americans think about themselves and the ways in which they imagine the days of the frontier and of westward expansion.  In these ways, Jackson changed the way that Americans thought of themselves, their character, and where that character came from.