The United States Army.
The Tomb of the Unknown Soldier is guarded by the Third United States Infantry Regiment, the "Old Guard." The regiment has diverse ceremonial as well as real-world missions. Soldiers of the regiment include the U.S. Army Drill Team, the Caisson Platoon, the Continental Color Guard, the Presidential Salute Battery, and, of course, the Tomb of the Unknowns.
Well... a soldier first! If you are looking for a real answer, a soldier at the Tomb of the Unknown Soldier in Arlington Cemetery is a member of the 3rd Infantry Guard command stationed at Ft. Myer which is adjacent to the Cemetery, Guard of Honor, Tomb of the Unknown Soldier. The unit is part of the corps that makes up the Tomb Guards, Color Guard for official funerals in the cemetery (The Caisons), US Army #rd Infantry Drill Team, and the US Army Fife and Drum Corps among some of their official duties. This is a very prestigious duty assignment for any Army infantryman and there is a reather lengthy selection and training process to be part of this high honor. You can gete more details at:
http://www.tombguard.org/
Since ancient times, a warrior's most sacred duty has been to guard the person that a society views as most worthy of honor. Throughout most of history, that has been the nation's monarch. Thus, those warriors specifically assigned the duty of protecting that person were ALWAYS considered to be the best of the best of a society's military. Thus the term, "honor guard."
In the last 150 years or so, nations have begun to view members of the military, that have died in combat, to be equally worthy of honor. As the Tomb of the Unknown Soldier is, for the U.S., symbolic of ALL such American military personnel, the tradition of an honor guard for a nation's leaders has now been transferred to an honor guard for those fallen military.
Note in the video below that, at 2:26, the U.S. President bows to Unknown Soldier -- symbolically representing that (at least in honor) these fallen military people out-rank the President! As such, they recieve a constant honor guard.
On a more practical level, a guard also ensures that visitors maintain the respect and decorum that such a places deserves.
The Sentinels at the Tomb of the Unknowns are part of a platoon in the 3rd U.S. Infantry Regiment in the U.S. Army. These men, and some women, are put through rigorous training and are very carefully chosen for this position. The Tomb of the Unknowns is guarded 24 hours a day, seven days a week, 365 days a year, in all weather.
The US Army's 3rd IR (Infantry Regiment) called "The Old Guard" pulls that duty.
The soldier at Arlington National Cemetery is guarding the tomb of the unknown soldier.
The Old Guard of the Third Infantry Regiment, US Army.
It is a platoon from th Old Guard of the 3rd Infantry Regiment, US Army.
There is a special unit put together from all the services. So, each service sends men to guard the tomb. These men are hand picked and considered the top people in their units.
Barracks under the tomb
The Tomb of The Unknown Soldier was named The Tomb of The Unknown Soldier because the soldiers are unknown and they are honored in a tomb.
Tomb of the Unknown Soldier - Moscow - was created in 1967.
Tomb of the Unknown Soldier in Bucharest was created in 1923.
Tomb of the Unknown Soldier - Damascus - was created in 1994.
The US government paid for the tomb of the unknown soldier.
In most countries, the tomb of the unknown soldier is exactly that. The remains of an unknown and unidentified are contained in it.
The Tomb of the Unknown Soldier is guarded by Soldiers, not Marines.
The Tomb of the Unknown Soldier is located in Arlington, Virginia. It is also sometimes referred to as the Tomb of the Unknowns.
Tomb of the Unknown Revolutionary War Soldier was created in 1954.
The Tomb of the Unknown Australian Soldier is located at the Australian War Memorial in Canberra. The tomb is located in the Hall of Memory.
They would shoot them, yes. The guns are unloaded but ready to be used at all times, with a bayonet attached.