"Secretary of Defense Jim Mattis, left, poses for a handshake at Offutt Air Force Base with Gen. John E. Hyten, the head of Strategic Command, in Bellevue, Neb., Thursday, Sept. 14, 2017. Mattis will be receiving classified briefings at Strategic Command, which will help him in his ""nuclear posture review,"" a top-to-bottom reassessment of U.S. nuclear weapons policy. (AP Photo/Nati Harnik)"