Answer: The United States became an imperialist nation at the end of the 19th century because Americans wanted to expand overseas with their belief in manifest destiny. The three factors that started American imperialism were political and military competition including the creation of a strong naval force, economic competition among industrial nations and a belief in the racial and cultural superiority of people of Anglo-Saxon decent. The United States got involved in imperialism because it sought to spread Western values and ideologies, acquire raw materials and new markets in colonies, and establish naval bases outside its borders to become a world power. One major reason that the United States became an imperial power at this time is due to economic prosperity. The country had quickly and successfully been developing its industrial sector throughout the 19th Century. This afforded the country the resources and capital necessary to extend its power overseas.