If you're looking for an entire movie about how things were in Vietnam for the infantry, I recommend Oliver Stone's Platoon, based on his own experiences. If you're looking for a film illustrating how an individual soldier and the nation were scarred and forever changed by the war, look no further than Stone's Born on the Fourth of July (1989).
Based on his autobiography, the movie follows Ron Kovic (Tom Cruise), a young, idealist whose heart always sings a patriotic hymn. As a child, he became inspired John F. Kennedy's inauguration, and when the fighting breaks out in Vietnam, he feels it's his duty to do join. He enlists in the Marines, but overseas, he finds the war confusing and disheartening. Soon, he's injured in a firefight and paralyzed from the chest down. Back in the States, he tries to recover, and we see his progression from bitter, angry reclusive to a leading anti-war protester.
Casting Cruise was a wise choice. Early on, he has those pretty-boy, youthful Hollywood looks and a fresh, open, naive face, and as the film progresses, we see him harden and transform, for the better and worse. The story is essentially how a generation of veterans felt betrayed by the country they defended: lied to about why they went, what they were doing, and how they'd be treated and hurt by the reception they got back home from families and friends who didn't understand their pain and the protesters who called them "baby killers." To take one of Hollywood's most recognizable stars and reduce him to such a level captures some of that.
The film around Cruise convincingly changes as well. One thing I've always admired about Stone is his sense of history. The set designs of his films always feel like they're straight from the time periods they're set in. This one goes from the 1950s nostalgic suburbia to the sweltering beaches of Vietnam to the chaotic college campuses of the sixties.
While the combat is not featured as much as it was in Platoon, aspects of the film are just as disturbing and shocking. When he's brought back to the States after his injury, Ron stays for months in a dank, rundown veteran's hospital in Brooklyn that is underfunded, scummy, filthy, and run by people who don't care. It's enough to make your stomach twist. The film is also painful when Ron lashes out at those around him or confronts his actions in the war.
Stone has always been a bold, visionary director. He does not make quiet, small motion pictures, and he's not afraid to take chances. Sure, he strikes a out a few times, but he always swings for the fences, and when he's on, he's makes stellar films. Born on the Fourth of July is one of his classics.