Thanks for the recc, I've read Marx's letter (on behalf of the English IWMA) to Abraham Lincoln, but hadn't considered there was a collection of writings from that time. I think of the years before 1867 as Marx being like shut up in the library of England so yeah I'm curious about his correspondence and organizing outside of that work.

Dubois mentions some early American Marxists before the civil war but said they didnt really know what to make of the issue of slavery. I'm actually not very aware of the early history of American Marxists, I don't know of any real serious showing of American Marxists until the St. Louis commune of 1877