A B-1 bomber leaves Ellsworth Air Force Base, S.D., May 10, 2012, bound for Green Flag, a training exercise at Nellis Air Force Base, Nev. Green Flag occurs approximately 10 times a year. Since 1975, not only Air Force aircrews, but other military branches and U.S. allies have taken part in the two-week exercises. (U.S. Air Force photo by Airman 1st Class Kate Thornton/Released)