US President Barack Obama and outgoing US Defense Secretary Robert Gates salute troops during the Armed Forces Farewell Tribute to honor Gates June 30, 2011 at the Pentagon in Washington, DC. Obama hailed retiring Defense Secretary Robert Gates Thursday as an honest, humble man who loved his troops and had one of the best tenures of any civilian Pentagon chief. AFP PHOTO/Mandel NGAN (Photo credit should read MANDEL NGAN/AFP/Getty Images)