Marx and Engels closely followed the American Civil War, writing columns for a London paper which Greeley may have picked up. Or they could have been original pieces for Greeley, I’m just not familiar with the Greeley pieces.
Their columns were collected in the volume ‘The Civil War in the United States’ that I found in a used book store. It all makes fascinating reading. You will find condemnations of America originating with Marx and Engels that have become all too common among our current Politically Correct crowd in both parties, the Obama Left and the neocon/GOPe Right.
Advocating the extermination of southern whites like a couple of freepers are doing is a special touch, I’m not sure that even the two proto-Communists did that although they may well have. But as Lenin is reputed to have said, you have to break a few eggs to make an omelette, and I’m sure that these freepers think that genocide is a small price to pay to further their goals.
Marx wrote 350(!) articles while on the payroll of Greeley’s New York Tribune, which was the leading newspaper of its time (like the NYT in recent decades). He only started writing for a Vienna newspaper about a year before the NY Tribune stopped publishing him.