Why did the U.S. decide to engage in expansionism?
The former Colonies were able to expand into North America simply because they had the military backing and a desire for additional resources and land. As colonial immigration increased, it simply became advantageous for the numbers of people, who, landing on the East Coast, kept pushing westward.
After the formation of the United States, the displacement of Native Americans occurred wholesale, as they no longer had any European power, British or French, who could support them. Ultimately they could not resist the onslaught of immigration. As more territories became states, boundaries kept pushing across the continent.
By 1890, the Frontier was gone; the expansionist tendency then left the continental United States and began to push into the Caribbean and Pacific. After World War II, the creation of new states and territories was no longer required, but only the incorporation of those areas into the American Sphere of Influence.