Pentagon Arrival Defense Secretary Jim Mattis exchanges greetings with Marine Corps Gen. Joe Dunford, chairman of the Joint Chiefs of Staff, upon arriving at the Pentagon for his first full day as secretary, Jan. 21, 2017. DoD photo by Air Force Tech. Sgt. Brigitte N. Brantley SHARE: Download: Full Size (0.59 MB) Credit: Brigitte N. Brantley VIRIN: 170121-D-GO396-0027A.JPG Photo Gallery