Several years following World War I, Congress approved the burial of an unknown soldier at Arlington Cemetery. The process for selecting the soldier to be honored has remained essentially the same since that time. Several identical caskets containing the remains of an unidentified soldier are prepared, and a highly decorated veteran of that war makes the selection at random. That coffin comes to Arlington; the others are buried with honors at foreign battlefield cemeteries.
The procedure has been followed for the World War II, the Korean War, and Vietnam. Surprisingly, considering the all-consuming nationwide effort that led to victory in the second world war, the unknown soldier for that war and for Korea weren’t interred at Arlington until 1958.
The tomb itself is guarded 24 hours a day, and the guard is changed each half hour in summer, each hour the rest of the year. Those soldiers performing this prestigious guard duty don’t wear rank insignia on their uniforms, so they don’t outrank the Unknowns.
One factor unknown to the world when the monument was revealed in 1921 is DNA. According to the Arlington National Cemetery website:
The remains of the Vietnam Unknown were exhumed May 14, 1998. Based on mitochondrial DNA testing, DoD scientists identified the remains as those of Air Force 1st Lt. Michael Joseph Blassie, who was shot down near An Loc, Vietnam, in 1972. It has been decided that the crypt that contained the remains of the Vietnam Unknown will remain vacant.