It doesn't portray the South as inhuman barbarians say the way an Allied propaganda film from the 1940s might portray the Germans or the Japanese with no subtlety and zero humanity.
Rhett tells the secessionists very early in the film they're going to get their teeth kicked in by the Yankees and... they're going to deserve it. They go on and on about states' rights and wanting to keep their slaves and Rhett lets them know in no uncertain terms where their political arrogance and the racial caste system that underlies their backwards economy and society is going to lead. It's going to be a disaster.
Heck, it even acknowledges the South was the one to escalate a political crisis into a civil war.
The film told 'em. They didn't listen. The film (rightfully) trashes the hubris of the planter class. Ashley tries to be diplomatic about it, but Rhett goes in hard and doesn't care how much the truth offends.
This scene is probably my favorite one in the film. Clark Gable kills it. Rhett's look of horrified disbelief in the idiotic display in front of him before sighing into resignation at 0:33 is how one acts.
It's still a problematic film, but it doesn't let the South off the hook.