Slavery.
But not why you think.
The north hated blacks, probably more than the south. In the south, people lived next to blacks all the time. It's hard to hate the guy who works for you (and most slaves just worked. They even got wages, albeit lower than freemen. If they worked for someone outside of their owner, they got paid freemen wages, but had to give part of the wages to their owner.)
The northern economy was based on food farming and manufacture. Slaves aren't great for that, and northerners didn't want blacks around anyway, so slavery wasn't a thing there. In the south, it was about cash crops -- primarily cotton -- which sucks balls to pick, so you hand that job to waves of slaves.
The north was mainly working, middle class folks. The south was rich plantation owners, dirt poor sustenance farmers, and slaves. In the senate, the slaves states matched the free states number for number, and in the House, the south got to count the slaves as 3/5 for population, even though none of them got to vote. That meant that the rich plantation owners (the planters) could essentially run Congress the way the Jews run things now. (Also, a healthy percentage of the planters WERE jews, and the jews handled the slave trade to them.)
So, it was about slavery, but not about freeing the slaves. It was about wrecking the political power that the planters had through the easy money of having people you paid practically nothing pick your cotton, get to count them towards your vote, and then run the country that way. Without slaves, the population numbers all change (that's why the SCOTUS decisions saying slaves weren't citizens, they wanted to zero them out in the census), the economy of cotton changes, and the planters lose their power.
It would be like telling Amazon they have to pay all the warehouse workers $40/hour. It's couched as worker's rights, but it's really about breaking Amazon.
Also of note is that the cotton gin had already started seriously winnowing down the need for slaves in the south. It's because of that that the war was ripe, because when you can do the work by machine, the north is ready to take over the work.
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