Imperialism is defined as "when a country expands beyond its natural borders either through the acquisition of land and territory or through the influence or control of a foreign government." This period is sometimes known as the "American Empire Period." The period lasted about 50 years but many argue that it continues into the modern day.
By 1890, the United States was the largest producer of goods in the world (today, its China). However, the US military was one of the smallest in the world with only about 30,000 soldiers total. Thus, American Imperialism was motivated by two factors: 1) the US needed new markets to sell their goods and 2) there was a desire to expand military might.
There was also a secondary motivation - racism. Many in Congress believed that most of the world lived in darkness and only Anglo-Saxon Christians could cause those in darkness to live in the light.
These motivations caused the United States to begin the Imperial period in the Kingdom of Hawaii.