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President Barack Obama walks down the stairs of Air Force One upon his arrival to Andrews Air Force Base on March 29, 2010 near Camp Springs, Maryland. Obama was returning from a surprise visit to U.S. military stationed in Afghanistan

President Barack Obama walks down the stairs of Air Force One to a waiting U.S. Secret Service vehicle, Monday, March 29, 2010, at Andrews Air Force Base in Maryland, after making a surprise visit to Afghanistan.

Troops strike a pose as President Barack Obama, seen background, greets military personnel in Dragon dining facility at Bagram Air Base in Afghanistan, Monday, March 29, 2010

President Barack Obama greets military personnel in Dragon dining facility at Bagram Air Base in Afghanistan, Monday, March 29, 2010

US President Barack Obama (R) visits with troops at Bagram Air Base on March 28, 2010. Obama thanked the Afghan people and US troops for their sacrifices in the war in Afghanistan, and vowed to reverse the Taliban's momentum

US President Barack Obama (R) visits with troops at Bagram Air Base on March 28, 2010. Obama thanked the Afghan people and US troops for their sacrifices in the war in Afghanistan, and vowed to reverse the Taliban's momentum

U.S. President Barack Obama meets with troops at Bagram Air Base in Kabul, March 28, 2010. Obama arrived unannounced in Afghanistan on Sunday, his first visit to the war zone that could define his presidency since his election as U.S. commander-in-chief. Air Force One landed in darkness at Bagram airfield north of the Afghan capital, and Obama was whisked by helicopter to Hamid Karzai's palace in Kabul, where he was greeted by the Afghan president and a band playing the U.S. national anthem. Reuters