Beginning in the late 1800s, America began to consider expanding its influence through imperialism. What were four reasons for this change?

In the late 1800s the United States began the process of overseas imperialism. One of the major reasons the United States began to imperialize overseas was the fact that they had already conquered all of the available territory across North America. Having expanded the country from the East Coast to the West Coast, the only way to continue expanding and achieving national goals would be to turn to territories overseas. Now that we know the U.S. had no choice but to look overseas if they wanted to continue expanding, we will look at the factors that motivated the United States to continue expanding.
The first factor we will look at regarding American overseas imperialism deals with military needs and their relation to geography. As the United States grew larger and more powerful, it became more important to protect the United States and its overseas trade interests from attack. In order to do this, the United States began to develop a strong navy. With the development of a strong navy it became necessary to have an infrastructure in place to support the navy. This would involve overseas bases where navy ships could refuel, resupply, and be repaired without having to return all the way to the United States. By colonizing foreign territories, particularly in the Pacific (Hawaii, Guam, etc..) the United States would have these necessary bases in strategic locations.
The second factor we will look at is economic. The United States, much like the European colonizers of the 16th, 17th, and 18th centuries, engaged in imperialism for economic benefits. By colonizing a territory, the United State would be able to extract valuable raw materials that could be used in American manufacturing in the United States. The manufactured goods could then also be sold by American companies to new markets in American colonies. This situation means cheap sources of raw materials and a larger base to sell manufactured goods to. When these two factors are combined, it means greater profit. The ultimate result is a tremendous economic benefit for the United States (or any country that engaged in colonization).
The third factor we will examine is Social Darwinism. Social Darwinism was a popular socio-political theory of the late 1800s. The belief held that just as only strong animals and species survive in the wild due to the characteristics they have developed through evolution, nations and societies would also survive or die out due to their national development. To put it simply, strong nations will thrive and dominate weak nations that will cease to exist. This belief was popular in industrialized and imperialistic nations such as the United States at the end of the 1800s.
The fourth factor we will examine deals with racial and cultural superiority. In the second half of the 1800s, it was a popularly held belief in the United States and England that Anglo-Saxon culture was superior to other cultures around the world. It was believed that English-speaking nations had superior thoughts, views on government, religious practices, and ideas. These ideas and beliefs were supported by writings on the topic by individuals like Josiah Strong. In 1885, Strong published a book titled Our Country. In his publication, Strong argued for Anglo-Saxon supremacy and that it was God's will that Anglo-Saxons would dominate the world. This belief can also be closely linked to the idea of Manifest Destiny that was so central to Westward Expansion across the United States. This belief also stressed the importance of spreading Christianity around the globe. Many in favor of imperialism saw non-Christians as savages in need of saving, which could be done through colonization and conversion to Christianity.
It is my belief that the four factors I have listed played the largest roles in encouraging American Imperialism and justifying the actions of imperialists. When combining these factors with the lack of available land on the North American continent, it is clear to see how this led to a shift in American Imperialism to overseas targets.
http://www-personal.umd.umich.edu/~ppennock/doc-JStrong.htm