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.