I've never been able to figure out how Hollywood could make such a 180 in such a short time.
I've got my own theory...after the 1950's, the Leftists in H'Wood (supported of course by the Communist Party) became dominant; they were the younger, more vigourous crowd. Add to that that the "baby boom" actors and producers were just becoming rich and famous while living the life of drugs, booze, and sex prevalent in the '60s. The "John Wayne" image of an American Hero, with its implied courage, sense of duty, and honor was a direct slap at them, so their only option to avoid being seen as what they were was to slander that image as false; and those who truly lived it (the American military) as fools, dopers, and corrupt baby-killers. Often back then, they were heard to claim that those who refused to join up were the REAL patriots (a sentiment echoed today...by some of the same people) and those who did their duty were nothing but suckers at best and murderers at worst.
In short, it was a total perversion. But it really happened, and it happened deliberately.