Why was the period between 1870 and 1914 considered a period of Imperialism for the US?

The period between 1870 and 1914 was marked by the United States' expansionist policies, driven by economic interests, nationalistic fervor, and a belief in Manifest Destiny. The U.S. sought to acquire territories beyond its continental borders, culminating in significant events such as the acquisition of Alaska, the annexation of Hawaii, and involvement in the Spanish-American War, which resulted in control over Puerto Rico, Guam, and the Philippines. This era signified a shift from continental expansion to global Imperialism, as the U.S. aimed to establish itself as a major world power.