Defense Secretary Robert Gates and President Obama salute during a farewell ceremony for Gates on June 30, 2011. Evan Vucci/AP hide caption

itoggle caption Evan Vucci/AP

U.S. soldiers carry a comrade injured by an improvised explosive device, or IED, in Logar province, south of Kabul, on Oct. 13. Roadside bombs are one of the biggest threats facing U.S. and Afghan troops, and insurgents keeping finding inventive ways to disguise them. Munir Uz Zaman/AFP/Getty Images hide caption

itoggle caption Munir Uz Zaman/AFP/Getty Images