The Nazis and the secessionists have a lot more in common than the neo-rebs like to admit.
The issue was how well slavery might do past the 1860's. And the record shows that the Nazis used it quite effectively in the 1940's. There's no reason to think it was going away or fading in 1860, and even if mechanization would have reduced the gang labor growing of cotton, it was mechanization that had made it profitable in the first place-- the cotton gin.
Walt