Please identify this Vietnam war film? I think it was from the 1980s

I think it's the Vietnam war, and it opens with a scene where an American soldier has (presumably) been captured. He then (seemingly very casually) has a knife drawn across his throat - there doesn't look to be too much damage. But then from a different angle we see his head just "hinge" back off his neck, revealing that the cut was obviously very deep!

I thinks it's the opening scene from The Exterminator.

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