Expansionism shaped the United States in very important ways. Let us look at two of these ways.
First, expansionism helped to bring about the Civil War. Expansion repeatedly forced the North and South into conflict over the degree to which slavery would spread into the new lands. Second, expansionism helped to create an American spirit, a feeling of superiority and "manifest destiny." As America expanded, an American spirit and attitude developed in which Americans came to feel that they were a very special people who were destined to lead the world. It also led (according to Frederick Jackson Turner) to a situation in which Americans became more democratic and more individualistic.
In these ways, expansionism shaped America both in tangible ways and in terms of their attitudes.