I've often speculated that the first few minutes of the film--up to and including the "Saigon...
." sequence--is actually set
after the main events of the film. I thought this after noticing the stone head in the sequence of images set to Jim Morrison. It's the same head we see in the very last scene...as Willard and Lance sail away.
Anyway--in the opening sequence, Willard is completely drained, disenchanted, and
very cynical. The events in the movie show him groing increasingly...drained, disenchanted, and cynical.
And if you listen carefully to Willard's narration--during the opening sequence, it's in the present tense. For the rest of the film, it's in
past tense.
The "thinking back" begins, in my interperetation, with Willard doing the
mok'bara (as it were) in his room (culminating in his injuring his hand--which the two soldiers take note of when they go to him, to take him to his briefing). Everything before that is set after the film's storyline.
Just my $0.02.
