Although it's not at the top of the piece, he blows to smithereens the idea that the Civil War was about states' rights. Sadly (but by no means unexpectedly), I have family members who still believe this.
I've referenced the CSA constitution, and Confederate state articles of secession, before. He goes beyond that to quote CSA Vice President Alexander Stephens' in his Cornerstone speech, between Lincoln's inauguration and Fort Sumter:
The evidence is overwhelming that Southern states seceded and fought to maintain slavery. Don’t believe me; believe the words of secessionists and Confederate leaders. Among the most often cited is Confederate vice-president Alexander Stephens who in 1861 declared the Founders “fundamentally wrong” in judging all humans equal. “Our new government is founded upon exactly the opposite idea; its cornerstone rests upon the great truth, that the negro is not equal to the white man; that slavery—the subordination to the superior race—is his natural and normal condition.”
I want to go a bit further than Horwitz’s quote, per Wiki’s further quote of the speech:
Many governments have been founded upon the principle of the subordination and serfdom of certain classes of the same race; such were and are in violation of the laws of nature. Our system commits no such violation of nature's laws.
I’m not very optimistic that the debate over South Carolina’s flag will bring a deeper reckoning. Furling the statehouse flag may bring temporary relief to South Carolinians, but what we truly need to bury is the gauzy fiction that the antebellum South was in any way benign, or that slavery and white supremacy weren’t the cornerstone of the Confederacy. Only then, perhaps, will we be able to say that the murdered in Charleston didn’t die in vain, and that the Lost Cause, at last, is well and truly lost.
Most flag defenders, however, are sincere when they say they cherish the banner as a symbol of their ancestors’ valor. About 20 percent of white Southern males of military age died in the Civil War. In South Carolina the toll was even higher, and thousands more were left maimed, their farms and homes in ruins. For many descendants of Southern soldiers, the rebel flag recalls that sacrifice, and taking it down dishonors those who fought under the banner. No one wants to be asked to spit on their ancestors’ graves.