One of most well-known historical theories about America is the “frontier thesis,” advanced by Frederick Jackson Turner in 1893. It states that the long existence of a “frontier,” a zone between the settled and unsettled regions of the United States, is responsible for distinctive aspects of the American character: independence, self-reliance, egalitarianism, a certain disdain for high culture and learning, etc. The United States was, at independence, a fairly narrow strip of land east of the Mississippi, but the frontier was pushed continually west until eventually it reached the Pacific and vanished altogether.
You can see this movement on a sequence of population density maps, based on decennial census results, published in the 1898 Statistical atlas of the United States. Here’s the 1790 to 1860, when they didn’t even both showing the west: