The westward migration of people affected the United States in a number of ways. Let us look at a few of them.
First, one historian named Frederick Jackson Turner argued that the availability of land in the west, and the fact that Americans moved onto it, made the United States more democratic. There were no masses of poor people in American cities, Turner argued in his "frontier thesis," because people could always move west onto cheap land. This thesis has been mostly rejected by historians for a number of reasons, but there is no doubt that westward migration created very real economic opportunities for millions of Americans.
Second, westward migration secured for the United States the vast lands and natural resources of the West. American businessmen were quick in exploiting the timber, precious metals, and energy resources of the west, not to mention the vast lands that were put to farming. These lands, of course, were conquered from Mexico and Native Americans.
A third effect of westward expansion was the near-destruction of Native peoples. This process, which began in the colonial period, is one of the great tragedies of American history. From Narragansetts in Massachusetts to Shawnees in the Ohio Valley to Tuscaroras in North Carolina to Apaches in the Southwest, Native peoples were driven from their lands to make way for white expansion.
Finally, westward expansion proved to be very politically divisive in the mid-nineteenth century. This was because westward expansion became intertwined with the toxic political issue of slavery. As the United States took lands from Mexico, the issue of whether slavery would be allowed in these territories quickly emerged, and political compromise proved powerless to resolve it. The issues of slavery and westward expansion would ultimately be a major factor contributing to the outbreak of the Civil War in 1861.