Many countries established a memorial to their war dead after World War I. In the United States, the first burial at the Tomb of the Unknowns took place at Arlington National Cemetery on November 11, 1921, when the unknown soldier from the "Great War" was interred under the large white-marble sarcophagus. Unknown servicemen killed in World War II and Korea were buried in 1958. The unknown serviceman killed in Vietnam was laid to rest on the plaza on Memorial Day 1984, but was disinterred and identified in 1998. Officials then decided to leave the Vietnam War unknown crypt vacant. Soldiers from the Army's 3rd Infantry ("The Old Guard") keep watch over the tomb 24 hours a day, regardless of weather conditions. Each sentinel marches exactly 21 steps, then faces the tomb for 21 seconds, symbolizing the 21-gun salute, America's highest military honor. The guard is changed with a precise ceremony during the day—every half hour from April through September and every hour the rest of the year.
At night the guard is changed every hour.
The Memorial Amphitheater west of the tomb is the scene of special ceremonies on Veterans Day, Memorial Day, and Easter. Mementos from foreign governments are displayed in an indoor trophy room. Across from the amphitheater are memorials to the astronauts killed in the 1986 Challenger space shuttle explosion and to the servicemen killed in 1980 trying to rescue American hostages in Iran. Rising beyond that is the main mast of the USS Maine, the American ship sunk in Havana Harbor in 1898, killing 299 men and sparking the Spanish-American War.