The forces pushing American expansion overseas included the idea that America had already exhausted its frontier but needed the frontier to allow people in the democracy to continually reinvent themselves. This idea, referred to as the "Turner Thesis," came from Frederick Jackson Turner, a historian who presented the idea in the 1890s that the American frontier had closed but that the frontier had made the country distinctly American. As a result, people felt that they needed to expand overseas in search of a new frontier.
Another force compelling American imperialism was the ideas of Alfred Thayer Mahan, a naval historian who wrote The Influence of Sea Power Upon History in 1890. His thesis was that great empires possess sea power, and this idea also compelled the American government to establish naval bases abroad.
In addition, the economic depression that started in 1893 convinced experts such as Mahan that the U.S. needed markets overseas to absorb the excess domestic goods that were being produced. These impulses, along with the desire for the raw materials that overseas empires could produce, propelled American imperialism.
The U.S. began expanding oversees for many different reasons. One of the earliest factors was lucrative overseas trade opportunities. By the mid 1800’s, American merchants carried on a profitable trade with Asia and hoped to expand the market into other areas. Japan was opened to American trade in 1854, and the new trade opportunities got the U.S. more interested in further expansion, which led to greater U.S. involvement in Asia.
The search for markets and materials drove the U.S. to more expansion. Some politicians, like Secretary of State Seward, dreamed of an American empire similar to those in Europe. This would allow the U.s. to take what they saw as their rightful place at the center of power. Seward purchased Alaska in 1867, which soon revealed a fortune in gold and natural resources.
Some Americans believed that a good reason for expansion into other countries was to “lift up” those races that were “uncivilized” by sharing Christianity and western civilization. This also promoted movement into Africa, Asia and the Caribbean.
Also, naval power was essential to a nation projecting its power overseas. The U.S. needed overseas colonies where ships could dock to maintain this navy, which eventually allowed the U.S. to become more involved in foreign affairs.