What motivations and justifications explain U. S. expansion between the Civil War and World War I?

After the declaration of independence, the United States did not stop growing. Immigration was incessant at the same time that the discovery of gold, the territorial annexation and the relatively easy access to the land made possible an advance towards the west. The population expanded throughout the territory: the waterways also contributed to this.

At the beginning of the 19th century, what in the history of the United States is known as the "march towards the west, the colonization of the agricultural Midwest by the so-called farmers who were family production units but who were absolutely involved in a capitalist economy, that is, that they were specialized producers that produced for the market.