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President Barack Obama steps off Air Force One after arriving at Bagram Air Field for an unannounced visit, on Sunday, May 25, 2014, north of Kabul, Afghanistan. (AP Photo/ Evan Vucci) ( Obama Afghanistan )

President Barack Obama shakes hands during a troop rally at Bagram Air Field during an unannounced visit, on Sunday, May 25, 2014, north of Kabul, Afghanistan. (AP Photo/ Evan Vucci) ( Obama Afghanistan )

U.S. Army soldiers look on as President Barack Obama speaks during a troop rally after arriving at Bagram Air Field for an unannounced visit, on Sunday, May 25, 2014, north of Kabul, Afghanistan. (AP Photo/ Evan Vucci) ( Obama Afghanistan )

President Barack Obama greets troops at Bagram Air Field, north of Kabul, Afghanistan, during an unannounced visit on Sunday, May 25, 2014. (AP Photo/ Evan Vucci) ( Obama Afghanistan )

Country music singer Brad Paisley performs during a troop rally with President Barack Obama during an unannounced visit at Bagram Air Field, on Sunday, May 25, 2014, north of Kabul, Afghanistan. (AP Photo/ Evan Vucci) ( Obama Afghanistan )

President Barack Obama gestures while speaking to troops at Bagram Air Field north of Kabul, Afghanistan, during an unannounced visit, on Sunday, May 25, 2014. (AP Photo/ Evan Vucci) ( Obama Afghanistan )

US President Barack Obama addresses US troops during a surprise visit to Bagram Air Field, north of Kabul, in Afghanistan, May 25, 2014, prior to the Memorial Day holiday. SAUL LOEB/AFP/Getty Images ( 530131129 )

US President Barack Obama attends a military briefing with General Joseph Dunfore, Commander of ISAF and US Forces Afghanistan, at Bagram Air Field, north of Kabul, in Afghanistan, May 25, 2014, during a surprise trip to visit US troops prior to the Memorial Day holiday. SAUL LOEB/AFP/Getty Images ( 530129619 )

President Barack Obama gestures as he speaks to troops at Bagram Air Field north of Kabul, Afghanistan during an unannounced visit, on Sunday, May 25, 2014. (AP Photo/ Evan Vucci) ( Obama Afghanistan )

US singer Brad Paisley greets US troops during a surprise visit by US President Barack Obama to Bagram Air Field, north of Kabul, in Afghanistan, May 25, 2014, prior to the Memorial Day holiday. SAUL LOEB/AFP/Getty Images ( 530131196 )

President Barack Obama shakes hands at a troop rally at Bagram Air Field, north of Kabul, Afghanistan, during an unannounced visit, on Sunday, May 25, 2014. (AP Photo/ Evan Vucci) ( Obama Afghanistan )

US President Barack Obama greets US Ambassador to Afghanistan James Cunningham (C) and General Joseph Dunfore, Commander of ISAF and US Forces Afghanistan, after disembarking from Air Force One upon arrival at Bagram Air Field, north of Kabul, in Afghanistan, May 25, 2014, on a surprise trip to visit US troops prior to the Memorial Day holiday. SAUL LOEB/AFP/Getty Images ( 530127151 )

US President Barack Obama does a Hawaiian "shaka" as he greets US troops during a surprise visit to Bagram Air Field, north of Kabul, in Afghanistan, May 25, 2014, prior to the Memorial Day holiday. SAUL LOEB/AFP/Getty Images ( 530131635 )

US troops cheer President Barack Obama during his surprise visit to Bagram Air Field, north of Kabul, in Afghanistan, May 25, 2014, prior to the Memorial Day holiday. SAUL LOEB/AFP/Getty Images ( 530131158 )

US President Barack Obama greets US troops during a surprise visit to Bagram Air Field, north of Kabul, in Afghanistan, May 25, 2014, prior to the Memorial Day holiday. SAUL LOEB/AFP/Getty Images ( 530131544 )

US President Barack Obama arrives to address troops during a surprise visit with US troops at Bagram Air Field, north of Kabul, in Afghanistan, May 25, 2014, prior to the Memorial Day holiday. SAUL LOEB/AFP/Getty Images ( 530130524 )

US President Barack Obama speaks to US troops during a surprise visit toBagram Air Field, north of Kabul, in Afghanistan, May 25, 2014, prior to the Memorial Day holiday. SAUL LOEB/AFP/Getty Images ( 530131782 )

US President Barack Obama boards Air Force One before departing Bagram Air Field, north of Kabul, in Afghanistan, May 26, 2014, following a surprise trip to visit US troops prior to the Memorial Day holiday. SAUL LOEB/AFP/Getty Images ( 530132258 )