In 1787, the Northwest Ordinance made the Ohio River the dividing line between slavery and freedom in the West, yet in 1861, when the Civil War tore the nation apart, the region failed to split at this seam. In Slavery's Borderland, historian Matthew Salafia shows how the river was both a physical boundary and a unifying economic and cultural force that muddied the distinction between southern and northern forms of labor and politics.

Countering the tendency to emphasize differences between slave and free states, Salafia argues that these systems of labor were not so much...