The Civil War wasn't about slavery, for either side, and all the educated leaders on all sides of the parties involved knew that the Cotton Kingdom had already reached its geographical limits by 1845-50, and wasn't going to expand. Daniel Webster knew this when the battle of the Wilmot Proviso was being argued; it didn't matter whether or not New Mexico or any other western state was a 'slave' or 'free' state, since the issue was moot; slavery wasn't economically feasible outside the cotton industry.
It wasn't feasible in the industrial North because it was cheaper to just let your workers starve or die from disease during economic slow downs than keep feeding and sheltering them; they could easily be replaced by immigrants at any time. In fact, white labor was valued less than slaves in the South as well; Irish and German labor was used in clearing the swamps and building the levees along the Mississippi, and the most dangerous jobs on the docks; slaves were too valuable, while the Irish and German laborers could simply be left where they fell and died, and covered over as building and filling progressed; nobody would miss them. The levees have tens of thousands of skeletons in them of these laborers.
The Civil War was about money, changing the tax structure and funding of the Federal government, tariffs, and railroad routes, along with massive corporate welfare programs for northern manufacturers and huge land giveaways to railroads, the Homestead Act, Morrill Tariff Acts, and where the continental railroad and its branches would run to and from. This would have put a far larger tax burden on the South than the North, with no offsetting benefits whatsoever. There were slave states in the Union as well, and Northern armies took strong steps to keep 'free' blacks from fleeing North as the Union armies moved south, so I don't know where the Yankees get off claiming to be 'Great Liberators'; they weren't even close to that.