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.  
<p>(Video by Staff Sergeant Chris Ballentine)
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(Historic and aerial photos courtesy of U.S. Library of Congress <a rel="nofollow" class="external free" href="https://www.loc.gov/">https://www.loc.gov/</a>)
<a rel="nofollow" class="external text" href="https://www.youtube.com/user/OldGuardVideo">3d U.S. Infantry Regiment (The Old Guard)</a>
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