Historian Frederick Jackson Turner presented his “frontier thesis” in an address in Chicago, the site of the 1893 World’s Columbian Exposition. Turner pointed to expansion as the most important factor in American history. He claimed that “the existence of an area of free land, its continuous recession, and the advance of American settlement westward explain American development.” In 1890, however, the Census Bureau stated that all the land within the United States was claimed, and there was no longer a frontier. “Now, four centuries from...