Gadsden Purchase
The United States acquired the southwestern boot heel of the state and southern Arizona below the Gila river in the mostly desert Gadsden Purchase of 1853. This purchase was desired when it was found that a much easier route for a proposed transcontinental railroad was located slightly south of the Gila river. This territory had not been explored or mapped when the Treaty of Guadalupe Hidalgo was negotiated in 1848. The ever-present Santa Anna was in power again in 1853 and needed the money from the Gadsden Purchase to fill his coffers and to pay the Mexican Army for that year. The Southern Pacific built the second transcontinental railroad though this purchased land in 1881.