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In his book, “The Promise,” about President Obama’s first year in office, Jonathan Alter describes a brief conversation between the president and Vice President Joe Biden that took place last November at the end of Mr. Obama’s long deliberation about what to do in Afghanistan. Mr. Biden asked whether the new policy of beginning a significant withdrawal of U.S. troops from Afghanistan in 2011 was a direct presidential order that could not be countermanded by the military. The president said yes.