World War I.
Korea
In most countries, the tomb of the unknown soldier is exactly that. The remains of an unknown and unidentified are contained in it.
The first unknown soldier was said to be buried in "The Tomb of The Unknowns"(there were more unknown soldiers that died and were buried in the same tomb, I won't say what it was called before that because some school teachers request this for homework) :P
1921.
no
No one knows they are unknown.
Tomb of the "Unknown Soldier"
Tomb of the Unknown Soldier
The Tomb of the Unknown Soldier contains the remains of one unidentified soldier. It was deliberately ensured that a chosen dead soldier should remain unknown, as to represent all those forces who have no known grave,
...No one knows who he was. That is the reason for the name it is the tomb of the unknown soldier. It is where unidentified remains could be remembered, if they could identify who the people were they wouldn't be in the tomb of the unknown soldier instead they would be buried with a headstone that bore their name.
The unknown soldier buried under the actual Tomb is from WWI. There are crypts in front of the Tomb for soldiers from WWII, Korea, and Vietnam. The unknown soldier from Vietnam was removed and identified as Michael J Blassie. He is now buried in Jefferson Barracks National Cemetery in St Louis, MO.
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.
The Tomb of the Unknown Soldier honours all those who died but it particularly honours the dead who could not be identified and buried with a name or returned to their families