As I recall, Stewart was a bonafide war hero, and even attained very high rank (General?).
He also had a son who served, and was killed, in Vietnam. The only angry or negative thing I ever heard him say publicly was in an interview where he essentially said of the Vietnam war protesters "Damn them all to hell."
IMHO, most of the flip-flop to the current belief system occurred in the 1960's. By demonizing heroes, and painting them as fools, baby-killers, and ticking time bombs, those who cowardly refused to serve could seem, to themselves at least, to be "better" than those they attacked and ridiculed. It was an absolutely sick and evil way to justify and validate an inexcusable, self-centered cowardice. Unfortunately, it played directly into the hands of those whose motives were the destruction of America and those positive values for which she stands.
This forced "paradigm shift" ran its course by 1980 or so in every segment of society except those with the most invested in its perpetration: the entertainment industry and academia. Sadly, these were the very segments with the ability to keep that Leftist concoction of discredited beliefs on life-support. Those holdover '60s liberals will be around, still pushing their self-justification for craven cowardice, for at least another 20-30 years. They have infected one, maybe two generations with their selfish poison. The best we can do is to reward those, like Gibson and Milius, who do not slur their own country to salve their vestigial consciences. Besides, their movies are more fun.