There were several different factors that caused the US to become a part of the imperialist movement, including:
1) Industrialization- The invention of technologies that allowed for the mass production of goods allowed businesses to make more of their products than ever before. Due to their increased ability to make large sums of goods, these individuals wanted the US to expand their global influence so that they may have new consumers to sell their products too.
2) Social Darwinism- This idea revolves around the concept of the "survival of the fittest." Many Americans felt that the US was a superior society that should spread its ideas and influence in order to help less civilized countries/territories.
In terms of studying that attitude and mentality, it would be helpful to study the journals of someone moving Westward during that time period (mid to late 19th century).
