U.S. Secretary of Defense Ash Carter (L-R) talks with U.S. Army General John Campbell, Commander of U.S. Forces in Afghanistan, as they walk across the tarmac to board their flight to Kandahar, from Hamid Karzai International Airport in Kabul February 22, 2015. The United States is considering slowing a planned withdrawal of U.S. troops from Afghanistan to ensure that "progress sticks" after more than a decade of war, Carter said during an unannounced visit to Kabul on Saturday. REUTERS/Jonathan Ernst (AFGHANISTAN - Tags: POLITICS MILITARY)