Robert Gates and his career in Washington, D.C.

Robert Gates testifies before the Senate Armed Services Committee during his confirmation hearing to become Secretary of Defense on Dec. 5, 2006. The committee unanimously voted to send his nomination to the full Senate.   (Stefan Zaklin / EPA)

President George W. Bush, right, meets with the Joint Chiefs, combatant commanders and the press in the Cabinet Room of the White House in January 2007. Defense Secretary Robert Gates, third from left, is seated opposite the president. In his memoir “Duty,” he writes that he admires Bush, “a man of character, a man of convictions, and a man of action.”  (Charles Dharpak / Associated Press)

Secretary of State Condoleezza Rice and Defense Secretary Robert Gates, leaving the West Wing of the White House.   (Ron Edmonds / Associated Press)

Defense Secretary Robert Gates attends the opening ceremony of the International Institute for Strategic Studies conference in Manama, Bahrain, in 2007. Gates spoke at the conference on “The U.S. and the Regional Balance of Power.”   (Hasan Jamali / Associated Press)

Russian President Vladimir Putin, right, welcomes U.S. Defense Secretary Robert Gates during a meeting in the Kremlin in Moscow in 2007.   (Mikhail Metzel / AFP/Getty Images)

President Obama with Secretary of Defense Robert Gates, right, and the service chiefs during a 2009 visit to the Pentagon in Washington.   (Manuel Balce Ceneta / Associated Press)

Secretary of State Hillary Rodham Clinton and Defense Secretary Robert Gates confer at the conclusion of their testimony before the Senate Foreign Relations Committee hearing on Afghanistan in December 2009. In “Duty,” Gates writes that Senate hearings were a “kangaroo-court environment.”   (Manuel Balce Ceneta / Associated Press)

Defense Secretary Robert Gates stands by an unmanned aerial vehicle -- a drone -- at Kandahar Air Field in Afghanistan.   (Scott Olson / AP)

Secretary of Defense William Gates, left, tours Arlington National Cemetery with Afghan President Hamid Karzai in 2010.  (Cliff Owen / Associated Press)

Former U.S. Defense Secretary Robert Gates’ book, “Duty, Memoirs of a Secretary at War,” went on sale Jan. 14.  (Paul J. Richards / AFP/Getty Images)