I'm sorry to have confused things here. I wanted to make a brief and very general point about the role of the combat photographer, and James Nachtwey is currently one of the best war photographers around. He did not cover Vietnam, but has covered a number of more recent wars, mostly for Time Magazine. I believe the girl burning photograph was made by Nick Ut. (I could be wrong here, and I know less about that photograph and photographer, so I won't add anything there!)
I think Eddie Adams was unfairly criticized when people suggested he should have stopped the execution. He clearly knew what happened, and did his job. I'm not sure he could have predicted at that moment the impact of his photo. And, Alex, you are right about the single moment. Still photographs from wars leave a lasting impression, and an important historical record. Video can't touch it!
Of course, I wonder with digital photography, if we will have a lasting and permanent archive of Iraq, since most of the photographs have been digital. Will the files be accessible in, say, forty years?