The Sentinels at the Tomb of the Unknown Soldier stand guard 24 hours a day year-round to protect those that gave everything in service of their nation. What does it take to become a Sentinel? In our new series "Behind the Old Guard," we take an in-depth look at some of the specialty units in the 3d U.S. Infantry Regiment.(Video by Staff Sergeant Chris Ballentine)(Historic and aerial photos courtesy of U.S. Library of Congress http://www.loc.gov/)