What Baptist has done is simple, if laborious and painful: he has studied many, many slave narratives and post-slavery interviews with the institution's survivors and discerned patterns of horror which render a story quite different from what we may have learned in school. Then he has woven this material into a powerful narrative of the pre-Civil War United States.
His central assertion, which I would say is unlikely to be refuted, is that southern planters in the then-southwest (Alabama, Mississippi, Texas and so on) invented a new system for exploiting bound human labor that yielded previously unimagined productivity. Enslaved African Americans literally created the wealth of the emerging United States, under the lash. Here's a longish, but I hope understandable, set of quotations that summarize Baptist's case:
Southern planters could sell any amount of cotton to English mills; their riches rapidly increased. Northern financiers and budding industrialists got in on the bounty by buying up securitized shares of slave "hands" who worked on the plantations, financing yet further geographical spread of the system. This financial tie-in, in turn, meant that in the Panic of 1837, Northern bankers ended up owning the debt of some of the the biggest planters. Now it was their system.
From the perspective of the Southern planter class, the means by which their slave-enabled boom could be revived and made to last forever was through expansion to new lands. They sought (and obtained) Texas and yearned to seize the entire U.S. Southwest, Mexico, Cuba, and Central America for the slave production system.
So why did resistance emerge in the North to extension of slavery? Baptist makes a solid case for the North's complicity in the profits of slave production, but is less clear on how countervailing forces emerged. Certainly most Northerners were not abolitionists until perhaps the middle of the Civil War and even then their abolitionism did not extend to sympathy with enslaved African Americans.
My understanding is that a critical fraction of Northerners came to resent being impeded by the "Slave Power" from developing the country in a different direction. The South's insistence on its "institution" stood in the way of "free labor," of "internal improvements" like railroads aided by the federal government, of a Homestead Act offering public land to settlers, and of the launching of land grant colleges. This vision had significant popular force behind it; Congress enacted much of it within a year of Confederate secession. Meanwhile, the South saw Lincoln's election by northerners with these ideas as a sign that they and their system had lost. Hence the wild throw of the dice that was the Confederacy and the Great Rebellion.
Baptist insists that we misunderstand how defining slavery was to the mid-nineteenth century United States.
I think Baptist would question, as we all must, whether the rebels of 1860 have even yet definitively lost their war to retain driven, powerless labor. This is what Harold Meyerson questions this month in How the American South Drives the Low-Wage Economy. The country is still fighting it out; at least some of the time, the contemporary "we" is a broader, more inclusive one than in 1860.