This is one of my long term favorite movies, maybe not because of, but in spite of the fact I was drafted [as an Australian] in 1965 to be part of this American Apocalypse/Genocide.
When the movie was released in 1979 I had shed most of the Rambo style HORROR [of J Doe spitting on us and calling us "baby killers", despite the fact we were simply Universal Soldiers as per Buffy Sainte-Marie] and had become "human" again, having forgiven J Doe for being such an asshole, and no longer wanting to empty a can of his Agent Orange over his sanctimonious head. So I figured another crappy John Wayne style Yippee about American Heroes, and skipped the film.
It was not till 1994, after my divorce [which I guess like for Willard here was par for the course for any of us Vietnam Victims/Veterans] that I watched it on TV and it "put the hooks in me" from that first view, ie it was NOT a "War Yippee" like Full Metal Underpants or Saving Ryan's Privates etc etc, ie the very type of obnoxious "war movies" satirized by John Lennon's movie How I Won the War, which I saw in 1967, just prior to the Blue Bus for Vietnam.
I have watched it since then maybe 50 times for the original I recorded from TV in 1994 and 10 times since the "Detox" version [aka FF Coppola's Superannuation Edition - but he deserved it as seen from his wife's doco Hearts of Darkness].