Mexican-American War: A conflict between Mexico and the United States from 1846 to 1848 that resulted in significant territorial gains for the U.S., including California, Nevada, Utah, Arizona, New Mexico, Texas, Colorado, Wyoming, Kansas, and Oklahoma.
Gadsden Purchase: An additional piece of land purchased from Mexico by the U.S. government in 1853 that completed present-day Arizona and New Mexico.
Alaska Purchase: The acquisition of Alaska from Russia by Secretary of State William Seward in 1867.