"The Unknown Soldier" has been perceived as Jim Morrison's reaction to the Vietnam War and the way that conflict was portrayed in American media at the time.

[2] According to author Richie Weidman, Morrison was inspired to write the lyrics after visiting the Tomb of the Unknown Soldier, at the Arlington National Cemetery, on November 25, 1967; the same day when the band performed at the Hilton Hotel, International Ballroom.

[3] When playing the track in live concerts, the Doors usually approached a cinematic performance, with Morrison pretending to be shot by Robby Krieger onstage, illustrating the death of the soldier that is mentioned in the lyrics.

[3] Critic Charles S. Gardner of Bridgeport Telegram, reviewing the Doors concert in JFK Stadium, called "The Unknown Soldier" a "desperately anti-war ballad climaxing with Morrison's being thrown to the floor in a burst of exploding electronic feedback".

"[10] Cash Box said that the "strong beat, instrumental majesty and a midway break unlike any dramatic effect on a single put together a smash outing," and also praised the "sheer passion" of the song.