I personally don't get the impression that slavery was a major motivating factor in the civil war.
Actually, lots of the more radical abolitionists were glad to see the "south" go, because that would mean fewer states in the United States that still had legal slavery, like New York and New Jersey, that would need to be changed.
The Confederates, by being a different country, would not be able to stop it as effectively.
Here's how it went: Lincoln wanted his tariffs paid, to cover his and his friend's internal improvements. Those tariffs made the raw materials exported by the South, and the relatively greater import of manufactured goods, more expensive both ways. The North, since they imported raw materials and exported manufactured goods, were less bothered by the way the tariffs were structured.
The South seceded over the tariff, not slavery. Ft Sumpter was being resupplied by Lincoln, which meant that a FOREIGN GOVERNMENT was obviously going to continue to collect the tax in a Confederate port, that's why it was fired upon.
Since it was obviously a war of aggression by the United States against the Confederate States, and the US was maintaining a blockade against trade with the CS, both England and France were considering coming into the war on the side of the CS.
But then, Lee made a BIG MISTAKE: He attacked into Pennsylvania. That mistake opened a window of opportunity for Lincoln to change the entire tone of his war. The US were no longer the attackers, they now could claim defense.
Jumping on this, Lincoln made one of the best public relations triumphs of all history: The Gettysburg Address and the Emancipation Proclamation. He _made_ he war about slavery, and by doing so undermined public opinion in France and England, whose people were against slavery, thereby preventing anyone from coming in and breaking the blockade.
So much for government highschool history.