City (pop., 2000: 486,699), southeastern Arizona, U.S. It lies along the Santa Cruz River on a hilly plain of the Sonoran Desert rimmed by mountains. In 1700 the Spanish founded several missions nearby, and in 1776 the small walled pueblo of Tucson was made a Spanish presidio (fort). It remained the province's military headquarters under Mexican rule. The U.S. acquired the territory through the Gadsden Purchase (signed in 1853). Tucson was the territorial capital from 1867 to 1877. It grew with the arrival of the railroad in 1880 and the discovery of silver at nearby Tombstone and copper at Bisbee. Its dry, sunny climate and desert locale have made it a popular tourist and health resort and retirement community. It is the seat of the University of Arizona (1885).