of it and its right-of-way. A Transcontinental Railroad crosses Southern Arizona via Tucson and Phoenix. Also, there is a major railroad south from Northern Arizona, via Phoenix, Casa Grande, and Tucson, to Nogales, where it crosses the border and meets the Mexican railroads. The primary major airport of southern Arizona, Tucson International Airport, is located just south of Tucson. History Southern Arizona has been inhabited by human beings for several millennia. The Hohokam were the principal Pre-Columbian inhabitants of the area. In more recent times, the dominant indigenous group of Native Americans have been the O'odham tribe, which continues to reside in this area in their traditional homeland. Several Spanish missions were established in the early 18th century by Father Eusebio Francisco Kino in what was then known as the Pimería Alta. After the Mexican–American War of 1846–47, much of Southern Arizona was purchased by the United States from Mexico for $15,000,000 in the Gadsden Purchase of 1854. More recently, Southern Arizona played an important role during the Cold War. Davis-Monthan AFB was the home base of an air force wing of 18 heavy Titan II Intercontinental Ballistic Missiles dispersed over a wide area of launch sites – south, southwest, and southeast of Tucson. About 20 years ago, due to their advancing age and the advent of several important Strategic Nuclear Weapons Reduction treaties, all of these missiles were retired from service. All but one of their launch silos and command shelters were demolished with explosives. The remaining site, Titan II ICBM Site 571-7, about 15 miles south of Tucson on Interstate 19, serves as the Titan Missile Museum, a National Historic Landmark. Secession Beginning in 1987, a group of southern Arizonans have considered seceding from the rest of Arizona to form a new U.S. state, potentially named Baja Arizona or Gadsden. A political push to that effect began in February, 2011, led by a group of attorneys largely in opposition to what they perceived as a nativist, conservative dominance in the Phoenix area in contrast to the Democrats of the south. The organizers of this movement had aimed to hold a referendum vote on the matter in 2012 in Pima County (and, possibly, Santa Cruz County and Cochise County). , no ballot initiatives had been generated. References Geography of Arizona