Military service in the United States is voluntary, although conscription may occur in wartime through the Selective Service System. From 1940 until 1973, conscription was mandatory even during peacetime. Today, American forces can be rapidly deployed by the Air Force's large fleet of transport aircraft, the Navy's 11 active aircraft carriers, and Marine expeditionary units at sea with the Navy, and Army's XVIII Airborne Corps and 75th Ranger Regiment deployed by Air Force transport aircraft. The Air Force can strike targets across the globe through its fleet of strategic bombers, maintains the air defense across the United States, and provides close air support to Army and Marine Corps ground forces. The Space Force operates the Global Positioning System, operates the Eastern and Western Ranges for all space launches, and operates the United States' Space Surveillance and Missile Warning networks. The military operates about 800 bases and facilities abroad, and maintains deployments greater than 100 active duty personnel in 25 foreign countries.