The South seceded when during the election of 1860, not a single southern state gave their electoral votes to Abraham Lincoln, but still had his Presidency assumed upon them.

The federal government began the fracture that culminated in a clean break with the Tarriff of Abominations in the 1830's. This placed a tariff on imported manufactured goods. What foreign nations countered with was a tariff on one of the few things the USA exported---agricultural products. This caused the divide to begin. It was about economics primarily.

Lincoln also had this to say:

I have no purpose to introduce political and social equality between the white and the black races. There is a physical difference between the two, which, in my judgment, will probably forever forbid their living together upon the footing of perfect equality, and inasmuch as it becomes a necessity that there must be a difference, I, as well as Judge Douglas, am in favor of the race to which I belong having the superior position. - Abraham Lincoln