4 of 10: U.S. Secretary of Defense Robert Gates is greeted upon his arrival in Kabul by U.S. General David Petraeus, right, Commander of International Forces in Afghanistan, Saturday June 4, 2011. Making his farewell trip to Afghanistan as U.S. defense secretary, Robert Gates said Saturday that success of the U.S. mission here is paramount and that the "most costly thing of all would be to fail." Gates, who is retiring at the end of the month, flew to Afghanistan in an unannounced visit to say farewell to U.S. troops and Afghan leaders. (AP Photo//Jason Reed, Pool) (AP Photo)

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