I'm not sure what the context of this argument is. But I do know that Marx viewed the American Civil War as "the one great historical event" of his lifetime, and had nothing but contempt for those who believed, like Wojtek (I believe?) here, that it was an imperial war fought by the North over the tariff or for other narrow commercial reasons.
In the well-known quote from Capital Vol. 1, "In the United States of North America, every independent movement of the workers was paralysed so long as slavery disfigured a part of the Republic. Labour cannot emancipate itself in the white skin where in the black it is branded But out of the death of slavery a new life at once arose. The first fruit of the Civil War was the eight hours' agitation..." Etc.
I'm sure Michael Perelman among others can supply plenty more on the subject.
Josh