Specifically conquering the Native Americans, the Mexican-American War, and the Spanish-American War.
American Imperialism: Lumen Learning. Read the first section on American Imperialism.
Introduction to the age of empire: Khan Academy. After more than one hundred years of isolationism, at the end of the nineteenth century the United States became an imperial power. Khan Academy article and video.
American Imperialism The Spanish-American War: Digital Public Library of America.
Age of American Imperialism: Ancestry. A collection of newspaper articles and clippings.
American Imperialism Internet Modern History Sourcebook: Fordham University.