Answer: The Age of Imperialism began around 1870, when modern, relatively developed nations took over less developed areas, colonized them, or influenced them in order to expand their own power.
The United States abandoned its century-long commitment to isolationism in the late nineteenth century and became an imperial power. Following the Spanish-American War, the United States gained significant control of Cuba, annexed Hawaii, and claimed territories in Guam, Puerto Rico, and the Philippines.