If I remember correctly, Marx and Engels were Union enthusiasts:
Karl Marx and Frederick Engels
Writings on the U.S. Civil War
http://www.marxists.org/archive/marx/works/1861/us-civil-war/index.htm
(And if they admitted that tariffs were an issue, perhaps they were just a bit more honest than some of the Union enthusiasts who post here... ;>)
You must remember that Marx and Engels wrote that early in the conflict - October 1861. Before Davis' contempt for his Constitution had become clear, before Davis nationalized industries, trampled civil rights, and seized private property. Had they waited a few more years then perhaps the two could have overcome their dislike of slavery and embraced Jefferson Davis as a kindred spirit.
And if they admitted that tariffs were an issue, perhaps they were just a bit more honest than some of the Union enthusiasts who post here...
But later the two are far more specific on the cause of the conflict - a great deal more honest than the Southern enthusiasts who post here are:
"The question of the principle of the American Civil War is answered by the battle slogan with which the South broke the peace. Stephens, the Vice-President of the Southern Confederacy, declared in the Secession Congress that what essentially distinguished the Constitution newly hatched at Montgomery from the Constitution of Washington and Jefferson was that now for the first time slavery was recognized as an institution good in itself, and as the foundation of the whole state edifice, whereas the revolutionary fathers, men steeped in the prejudices of the eighteenth century, had treated slavery as an evil imported from England and to be eliminated in the course of time. Another matador of the South, Mr. Spratt, cried out: "For us it is a question of founding a great slave republic." If, therefore, it was indeed only in defense of the Union that the North drew the sword, had not the South already declared that the continuance of slavery was no longer compatible with the continuance of the Union?"