U.S. Air Force crew chiefs, with the 177th Fighter Wing of the New Jersey Air National Guard, prepare their unit's F-16 Fighting Falcons for flight during Thracian Star, a bilateral training exercise to enhance interoperability with the Bulgarian air force, at Graf Ignatievo Air Base, Bulgaria, on July 13, 2015. (U.S. Air National Guard photo by Master Sgt. Andrew J. Moseley/Released)