The US did not lead the work in ending slavery. Britain did. In fact the US threatened war if Britain interfered with American shipping, which meant that the yankee financed transatlantic slave trade went on 50 years longer than it otherwise have.
The south quit the union mostly because of the prohibition of extending slavery into the free terretories. I suppose you would let that stand.
The south quit the union over the tariff.
Slavery did not end in the north because of happy thoughts. It ended because of economics. The same economic forces were working in the south and slavery would have inevitably ended there as well.
The northern states constructed manumission laws that encourages slave owners to sell their slaves down south rather that free them. They did this to reduce the black population and it largely worked. The southerners wanted to do something along those lines with the western territories.
Everybody knew slavery was going to come to an end. The real question was what to do with all of the blacks. South Carolina for example had a population that was about two thirds black. The whites weren't going to let them vote and take over the state. Lincoln several time proposed mass deportation but could never get congress to put up the money. What Lincoln and the Republicans refused to accept was blacks being resettled in the territories. They wanted to keep the blacks bottled up in the south until they could be gotten rid of somehow. Note that after the war the republican party gave away vast quantities of land in the west to corporations and settlers and even to foreigners immigrants, but nothing for the freed slaves.
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