He makes the argument that the Old South, its finest intellectual hour seen in Thornwell, was much superior intellectually than the North. By the way, this is published by the scholarly Mercer University Press, so it ain't no hick from the sticks writing.
Thornwell also warned of the coming religious crisis, that which we call the Civil War. While criticizing unbiblical aspects of Southern slavery, he primarily warned against the rising humanistic and unitarian tide from the North which would overwhelm the South, not only militarily, but also--and more deadly--spiritually.
If I can restate the thesis in different terms: with the fall of the Confederacy, we saw the last bastion of a thoroughly Christian civilization destroyed. America would move from a decentralized republic to a consolidated Empire (which subsequent decades proved chillingly).
Wavering between two and three stars, but tried to err on the side of being nice. More comments later.
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Ah, never mind the niceness. After reading Clyde Wilson's excellent analysis of another great Carolinian, one realizes what this book could have been but was not.