Which branch of the US military guard the 'Tomb of the Unknown Soldier' in Arlington, Virginia?
The tomb guards are soldiers of the United States Army. The first military guards were troopers from the 3rd Calvary, "Brave Rifles", who were posted nearby on Fort Myer. Since April 6, 1948, (known as "Army Day"), when the regiment was reactivated, it has been guarded by soldiers from the 3rd Infantry Regiment, "The Old Guard".
It is considered one of the highest honors to serve as a Sentinel at the Tomb of the Unknowns. Fewer than 20% of all volunteers are accepted for training and of those only a fraction pass training to become full-fledged Tomb Guards.
The solider "walking the mat" does not wear rank insignia, so as not to outrank the Unknowns, whatever their ranks may have been. Non-commissioned officers, usually the Relief Commander and Assistant Relief Commanders, do wear insignia of their rank but only when changing the guard. They have a separate uniform (without rank) that is worn when they actually guard the Unknowns or are "posted.
There is a meticulous routine that the guard follows when watching over the graves. When marching, exactly 21 steps complete the routine. When completed, the soldier waits 21 seconds before starting the next step in the routine. In all of the individual routines, there are that same number of 21 steps and the same wait time of 21 seconds. Twenty-one was chosen because it symbolizes the highest military honor that can be bestowed- the 21-gun salute.