The Army. It is the 3rd U.S. Infantry Regiment, also known as “The Old Guard”.
United States Army, 3d Inf. Regt.
There's a great James Can movie called Gardens of Stone that gives a good representation of the rigorous training and discipline that unit goes through
U.S. Army Old Guard
Army
3rd infantry Marines
United States Army
There are also sentinels that walk back and forth
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