According to one non-exhaustive survey (http://www.slipcue.com/music/jazz/warsongs_01.html): "The patriotic war song pretty much died a lonely death during the Vietnam War with Sgt. Barry Sadler's 'Ballad Of The Green Berets' and Tony Orlando & Dawn's 'Tie A Yellow Ribbon' proving the last gasp of this once-great genre." Plenty of WWII songs though.

Also, I wouldn't want to take this class (http://www.arts.adelaide.edu.au/personal/DHart/ResponsesToWar/Lecture/20thCOther/VietnamUSA.html), whose syllabus lists "Born in the USA" under "Partriotic [sic] Rock And Pro-War Country And Western".