I’ve spent most of this month working on a long article about using data to understand the transatlantic slave trade. It’s on the cover of the Sunday Review this week, if you want to check it out. One of the parts we had to shrink to save space was a paragraph on internal trade in the United States. An important part of the slave system, the internal trade relied on the sale of “surplus” slaves from the Upper Southern states to New Orleans and beyond. Here is what I wrote:

Slave owners in Upper Southern states like Virginia and Maryland produced a “surplus” of enslaved labor, which they sold to traders. These traders brought their captives to the markets of Charleston, Savannah, and New Orleans, where they would be sold to the highest bidder to work and produce a profit. Enslaved women also had to produce new capital, in the form of children, for their slaveholders.

In addition to this point, it is also worth saying why there was such a “surplus” to begin with.

The short answer is: tobacco.

Tobacco, for most of the colonial period, was the agricultural backbone of chattel slavery in the South. Virginia and Maryland were, for all intents and purposes, tobacco colonies, where planters used large numbers of slaves to produce large quantities of tobacco for European consumption. This extractive agriculture—virtually unavoidable to the commodity-driven slave culture—depletes the soil in a relatively short time. This is why slave owners were one of the driving forces of American expansion in the early decades of the republic. They needed more and more land to grow tobacco, cotton, and other commercial staples.

One consequence of this, for the oldest slave economies in North America, was that there were more slaves than were needed to farm the land. Here is none other than Karl Marx giving a succinct description of dynamics in an 1861 essay on “The North American Civil War”:

The cultivation of Southern export articles, cotton, tobacco, sugar, etc., practiced by slaves, is remunerative only so long as it is carried on with large bands of slaves, on a large scale and over vast areas. stretches of naturally fertile soil. , which requires only simple work. Intensive cultivation, which depends less on the fertility of the soil than on the investment of capital, intelligence and the energy of labor, is contrary to the nature of slavery. Hence the rapid transformation of states like Maryland and Virginia, which once employed slaves for the production of export items, into states which raised slaves in order to export those slaves to the Deep South.

Knowing all of this, you can very easily see how the slave system developed in the decades before the Civil War. The advent of the cotton gin made it possible to grow huge amounts of cotton in a variety of different environments. As the demand for cotton grew following the Industrial Revolution in Britain, so did the demand for bonded labor in the South and, in turn, the demand for new land to cultivate.

The demand for labor spurred the slave-trading economy of the Upper South, and the demand for land led to the dispossession of the natives. As historian Joshua D. Rothman notes in “The Ledger and the Chain: How Domestic Slave Traders Shaped America“, “The extension of slavery, seen across the Atlantic world in the 19th century, was both fostered and nurtured by the technological, economic, political and ideological changes that ushered in the modern era.”