In a nutshell? Stone was saying Vietnam was a waste of time, lives, money, etc. Basically it was all for nothing and the true cost occurred to the soldiers who survived and are still paying the price to this day, decades after it ended.
It's amazing to me that when I saw the film when it came out, I didn't pick up any anti-US vibe. I was just blown away by the intensity of the film. But when I saw it again decades later, I definitely felt a strong anti-US vibe to it:
Chris/Stone says at the end of the film that instead of fighting the enemy, "we fought each other". As I mentioned in an earlier post, that is a criticism of the US competence, but it doesn't necessarily imply that the motives of the US for being there were immoral.
There's only one scene that really focuses on the Vietnamese, towards the end of the film, you see about 30 of them marching in a slow run, and they are quiet, organized and efficient, like a well-oiled machine. Quiet a contrast from the verdict Stone gives of the US troops.
As you mention in BOT4OJ, Stone is far more explicit there of his criticism of the US role in Vietnam.
If a private venture fails it's closed down. If a government venture fails it's expanded. M Friedman
Taylor's character is based off of Stone's experience. The village massacre is based off of My-Lai. The chaos and fog of war is based on LBJ running the war from the Whitehouse instead of letting the Generals do their job. The infighting is based upon well, what always happens in a Platoon of mostly draftees. The Vietnamese being efficient just tells me we were in their backyard and they were very good at defending it.
It is telling to me that the soldiers won that final battle by a kill factor of more than 10-1 but we still managed to lose in the end.
Oliver Stone wrote and directed an Oscar winning movie based on not only his experiences in Vietnam but also on how the war really played out and how it effected boots on the ground on foreign soil. Really they all were left twisting in the wind.
While the film is as close a representation of Vietnam as any of its genre, it's Stones way of also saying Eff The US. Throughout the film, "Politics Man, Politics"... It's on the surface. It's not like Kubrick or FFC, where the deeper meanings are subdued and hidden. Stone puts it on Blast. I don't disagree with his view on the Vietnam War, I mean, that's pretty much the Common Thought Process nowadays... BUT, Stone has gone way past this. He's become a Self Hating American. It's ridiculous... But he'll go and hang with Dictators, no big deal. They're great guys.