underlying cause of the war..And it was a very glaring difference between the two sections of the nation, although there were others as well. And as we have begun to identify these other causes. But the secession of the south was an immediate cause of the war, and one Lincoln had to address. He could either let the south go and there would have been no immediate war (although one might have started later on). Or he could bring them back in to the Union by force. No one on either side anticipated the war would go on for so long, or be so costly in terms of human lives, but war is like that. Once unleashed, it has a momentum of its own.