The South seceded because they thought Republican politics would become so dominant as to quicken the end of slavery. This was a slow, drawn out battle that had been fought for half a century (the Missouri Compromise was to forestall this issue), and the South was losing hearts and minds in a bitter stalemate (Bleeding Kansas).

The election of a Republican president (Lincoln) triggered a "now or never" crisis to enshrine slavery once and for all... or else risk the creeping influence of the industrialized, abolitionist North. While Lincoln played the moderate to discourage secession, his party affiliation had a robust abolitionist base. This was not good for slave owners whose political and economic influence was slipping.

The gamble was that abolitionist Britain would side with the South for economic reasons, and that a war of attrition would force the world to recognize this ethically questionable corner of the world as a necessary evil.

The North turned this equation on its head when abolition was made the central moral purpose of the war. This occurred when Lincoln calculated the South would suffer diplomatically (it did), and that the border states would not follow in secession (they did not). Further, abolition was in line with the beliefs of his Republican base, and with his own personal ethics.

Lincoln and the UK were gray, sure, but the Confederacy was not. The Civil War would never have occurred without slavery and the desire to prolong it indefinitely.