Three factors that spurred American imperialism were the need to expand even after the frontier had vanished, with industrialization warranting further claim to more resources, the growing importance of foreign trade to the American economy and the desire to compete with European empires for power. Because of this mixture of cultural, economic and political factors, the U.S. was influenced to practice imperialism, particularly during the late 19th and early 20th centuries.
As the frontier was thoroughly explored and colonized by the end of the 19th century, it was still many Americans' belief that the United States needed to continue expanding. This became entirely evident as the U.S. was able to lay claim to a crop of new territories at the end of the Spanish American War, including Hawaii, Guam, Puerto Rico and the Philippines.
American imperialism also stemmed from economic need. Laying claim to new territories meant that the U.S. could continually increase its stock in raw resources, an important commodity during this period in history as the U.S. began to industrialize quickly. Furthermore, stock market crashes made overseas markets more attractive to investors, so territories outside of the states became more entrenched with the American economy and American trade, in particular, came to rely heavily on foreign nations.
The lead up to the First World War was a time of prodigious colonial expansion by various European Empires, especially Germany. The U.S. wanted to compete with the European empires to gain significant footing as a new empire. This prompted numerous people to suggest the U.S. should claim territories in Central America and the Pacific for naval power, trade dominance and control of foreign market interests.