A U.S. Air Force KC-135 Stratotanker assigned to the 100th Air Refueling Wing prepares to refuel a U.S. Air Force E-4B transporting the U.S. Secretary of Defense Feb. 20, 2017. In case of national emergency or destruction of ground command and control centers, the E-4B provides a highly survivable command, control and communications center to direct U.S. forces, execute emergency war orders and coordinate actions by civil authorities. (U.S. Air Force photo by Staff Sgt. Micaiah Anthony)