Michael is right the song is influenced from the movie exactly as he describes in the fiirst post here.
Also JF wrote the song when he heard that Richard Nixon was about to run for the presidency so you might combine the film with that.

No JF never went to Vietnamn. Actually the song Proud Mary is about when he got his release paper from the army. He felt so happy that he just did some Cartweeling in his back yard. He had this intro in his head that became Prod Mary. Excellent song by the way

Kaj from Sweden