Two main factors that led to American Imperialism were the creation of the United States Navy and the expansion of America's economic interests overseas. American Imperialism began in the 1890s, increased into the early 1900s, and began to slow down after that. It refers to the influence of the United States in other countries, specifically in cultural, economic and military spheres.
According to the Merriam-Webster dictionary, imperialism is defined as "the policy or practice by which a country increases its power by gaining control over other areas of the world." One of the first concrete examples of American Imperialism is when the United States gained control of Hawaii in 1898. From there, the imperialistic ideas snowballed, and continued until just after the end of World War II.
As American industrial production increased in the mid-1800s, the general population was unable to keep up with it. Americans simply could not purchase everything that was produced. This was the catalyst to expand the country's overseas trade, and seek out foreign markets.
Since the country was expanding trade and gaining control over other areas throughout the world, the United States Navy was created to protect America's interests overseas. In addition to taking control of Hawaii, the United States also annexed the Philippines in the late 1800s. There was a need for a strong naval presence in those areas.