Marx on Slavery in his 1846 Letter to Annenkov

Marx had a curious analysis of slavery in a letter of December 28, 1846 to the Russian scholar P. V. Annenkov:

“Freedom and slavery constitute an antagonism. There is no need for me to speak either of the good or of the bad aspects of freedom. As for slavery, there is no need for me to speak of its bad aspects. The only thing requiring explanation is the good side of slavery. I do not mean indirect slavery, the slavery of proletariat; I mean direct slavery, the slavery of the Blacks in Surinam, in Brazil, in the southern regions of North America.

Direct slavery is as much the pivot upon which our present-day industrialism turns as are machinery, credit, etc. Without slavery there would be no cotton, without cotton there would be no modern industry. It is slavery which has given value to the colonies, it is the colonies which have created world trade, and world trade is the necessary condition for large-scale machine industry. Consequently, prior to the slave trade, the colonies sent very few products to the Old World, and did not noticeably change the face of the world. Slavery is therefore an economic category of paramount importance. Without slavery, North America, the most progressive nation, would he transformed into a patriarchal country. Only wipe North America off the map and you will get anarchy, the complete decay of trade and modern civilisation. But to do away with slavery would be to wipe America off the map. Being an economic category, slavery has existed in all nations since the beginning of the world. All that modern nations have achieved is to disguise slavery at home and import it openly into the New World. After these reflections on slavery, what will the good Mr Proudhon do? He will seek the synthesis of liberty and slavery, the true golden mean, in other words the balance between slavery and liberty.”
Letter from Marx to Pavel Vasilyevich Annenkov, December 28, 1846.
https://marxists.anu.edu.au/archive/marx/works/1846/letters/46_12_28.htm

It is rather odd indeed to see Marx declare slavery as “the pivot upon which our present-day industrialism turns,” and furthermore regard this as part of the “good side of slavery.” Britain, the greatest capitalist economy in the 1840s, had, as everybody knows, moved to abolish slavery in virtually all parts of its empire by the time of the Slavery Abolition Act of 1833, and the Slave Trade Act of 1807 had abolished the slave trade in the British Empire.

Peculiar too is the view that if slavery were abolished in America, then this would “wipe America off the map.” When the US abolished slavery in the Civil war, this did not result in the collapse of the US economy which was in fact quickly industrialising and on the path to becoming the largest economy in the world.