No offense meant but I just had to chuckle at that response. If you look at the newspaper articles, especially from Greeley's paper, of the period that's pretty much the same position they were taking in 1861 before the Southern states started the war. Everything old is new again.
My point is, maybe the north should have taken that position. Just let the south secede and gave them Sumpter. If they could run things ok and become a good neighbor, fine. Slavery would have been abolished anyway. It was just to big a travesty to continue. The goodness in people would have eventually elected politicians who wound have abolished slavery. Actually, the vast majority of people in the south did not own slaves and were against it.