U.S. Army Gen. Martin E. Dempsey, chairman of the Joint Chiefs of Staff, takes a photo with a U.S. sailor while aboard the USS Ross on Naval Station Rota, Spain, Dec. 6, 2014. Dempsey is visiting U.S. service members who are deployed outside the United States during the holidays. DoD photo by D. Myles Cullen

U.S. Defense Secretary Chuck Hagel visits U.S. service members at the International Security Assistance Force dining facility in Kabul, Afghanistan, Dec. 6, 2014. DoD photo by U.S. Air Force Master Sgt. Adrian Cadiz

U.S. Defense Secretary Chuck Hagel, left, and Afghan President Ashraf Ghani conduct a joint press conference at the Presidential Palace in Kabul, Afghanistan, Dec. 6, 2014. DoD photo by U.S. Air Force Master Sgt. Adrian Cadiz

U.S. Defense Secretary Chuck Hagel, center left, meets with Afghan President Ashraf Ghani, center right, to discuss matters of mutual importance at the Presidential Palace in Kabul, Afghanistan, Dec. 6, 2014. DoD photo by U.S. Air Force Master Sgt. Adrian Cadiz

Defense Secretary Chuck Hagel briefs reporters on board an aircraft as they travel to Afghanistan, Dec. 5, 2014. DoD photo by U.S. Air Force Master Sgt. Adrian Cadiz

Defense Secretary Chuck Hagel waves goodbye as he boards an aircraft to depart Joint Base Andrews, Md., Dec. 5, 2014, to travel to Afghanistan. DoD photo by U.S. Air Force Master Sgt. Adrian Cadiz

Deputy Defense Secretary Bob Work, left, stands with Army Gen. Charles H. Jacoby Jr. during a change-of-command ceremony for U.S. Northern Command and North American Aerospace Defense Command on Peterson Air Force Base in Colorado Springs, Colo., Dec. 5, 2014. Jacoby relinquished command to Navy Adm. William E. Gortney. DoD photo by U.S. Navy Petty Officer 2nd Class Sean Hurt

U.S. Defense Secretary Chuck Hagel, left, speaks with U.S. Army Gen. John F. Campbell, commander of the International Security Assistance Force and U.S. Forces-Afghanistan, as he arrives in Kabul, Afghanistan, Dec. 6, 2014. DoD photo by U.S. Air Force Master Sgt. Adrian Cadiz