The scene in which the NVA tunnel is discovered by a soldier accidentally sitting on a nail is based on a true event from the war. During Operation Crimp (8 - 14 Jan '66, a joint operation between the U.S. and Australian troops that was the largest operation of the war at that time) a U.S. serviceman discovered a bunker complex by sitting on a nail protruding from something covering a tunnel entrance.
Opening blurb: THE VIETNAM WAR WAS THE LONGEST WAR IN AMERICAN HISTORY