A Bright Shining Lie (1998) 
HBO made the Pulitzer Prize winning book into a film, starring Bill Paxton as the crazy (and amazing) John Paul Vann.
My dad served two tours in Vietnam, and recommended this book to me as a realistic account of the complicated mess that was the Vietnam War. Dad died two years before this film adaptation came out. That’s a shame, I think he would have enjoyed it.
Paxton is very good in this film. Like he did three years later in the 2001 film Frailty, he really seemed to enjoy sinking his teeth into a megalomaniac leading role.