What are some economic arguments for expansion?

There are economic reasons that support the concept of expansion. When the United States was expanding westward, first to the Mississippi River and then to the Pacific Ocean, our economy benefited from this expansion. As people moved westward, they needed various products in order to live in these areas. This allowed businesses to expand to provide these products. As businesses expanded, more jobs were created. With more people working, the demand for products also increased. This helped the economy grow. This growth was sustained as more and more people continued to move westward.
Eventually, the United States wanted to get overseas colonies. This also helped the economy grow. As the United States gained control of places such as Guam and Puerto Rico, these colonies provided American businesses with new markets where they could sell their products. These colonies also served as a base where trading ships could stop in order to refuel and bring back materials to the United States. As the United States gained new markets where goods could be sold, the American economy grew even more.
There are various economic reasons to support expansion.
http://americanexpansionism.blogspot.com/2009/11/reasons-for-expansionism.html

http://www.theusaonline.com/history/expansion.htm