His opposition to the Mexican War including funding it once it started. Sort of an 'oppose the war, support our troops' sort of thing. On the other paw, even though he was extremely racist most of what he said and wrote showed he knew that winning the civil war would ultimately end slavery. (That was the point of a nation not being able survive long half slave and half free. If the Union included free states and not too many special rights for the slave states, slavery could not survive. The attempt at succession was because the South agreed with Lincoln. Slavery could not survive in a nation the slave owners did not dominate.) I think Marx had a thing or two to say on this subject.