If you read about the economics of this period you'll find that, as with most of history, the reality isn't this simple. A lot of Southern capital went into financing Northern industry, for example. It's difficult to neatly divide the nation's economy in this way as if they were two separate spheres, as much they as they may have want to think of themselves that way.

Not one southern city industrialized.

Investors tend to invest in things they know about and can keep an eye on, which tends to be local. Investment capital uniformly fleeing the slave states to the free ones suggests that investing in slave states was a bad investment.

In those days, different states were like different countries today.

> Not one southern city industrialized.

There's also the climate of the South to consider — until the invention of air-conditioning, factory work in the South would have been at a significant disadvantage. Malaria, too, was an issue within living memory.

People don't realise nowadays how very inhospitable warm climates can be.

And yet millions of people lived and worked there. Agricultural work is backbreaking.

The South didn't even have a shoe factory.

Really, how productive can a workforce be that can't read, that have to be forced to do anything, that will not contribute any improvements, that have to be guarded at all times, that will sabotage the work if they can get away with it, etc.?

How productive would you be if you were a slave?

Slavery is a terrible economic system when compared with free labor. It isn't remotely a surprise that free economies economically bury slave economies.

Right. Likewise with India and Kenya and a hundred other examples. It is the slavery in the colonies that allows the center to industrialize. The colonies send resources to the center. The center exports its goods to the colonies. The center also exports the unemployment it might otherwise face from rapidly increasing productivity. India was one of the wealthiest civilizations on Earth for thousands of years, but it is still recovering from what happened in the 1700s, when England industrialized and forced products into India at gunpoint.

Britain had no competition at the time. Slavery cannot compete with free labor.

> but it is still recovering from

I.e. slavery did not benefit the Indian economy, nor the Kenyan economy, etc. Neither industrialized.

The US economy did industrialize (except for the slave states), and did not rely on slave labor from other countries, either, and it grew to economic dominance.

I know there's a theory that the northern economy depended on the southern slave economy, but when it was cut off from the southern economy in 1860, it prospered while the slave economy crashed.

> I know there's a theory that the northern economy depended on the southern slave economy, but when it was cut off from the southern economy in 1860, it prospered while the slave economy crashed.

The Confederacy crashed economically because they tried to pull a 19th century equivalent of the Arab Oil Embargo, cutting off their main export crop in the hopes of compelling Europe to intervene. The Confederacy suffered from what we'd call in modern times the resource curse--ironically, one enabled entirely by industrialization.

"slavery did not benefit the Indian economy, nor the Kenyan economy, etc."

Right, obviously. The reason why Britain invaded is because they wanted India and Kenya to work for Britain. Slavery benefited Britain.