If you truly think the civil war was fought over slavery, you are grossly mistaken.
We detached this subthread from https://news.ycombinator.com/item?id=12463191 and marked it off-topic.
The Confederacy, quite overtly, seceded over and fought the war solely and entirely to protect slavery from what they expected (quite probably correctly) was an inevitable abolition within the United States.
The Union, OTOH, its true wasn't particularly fighting to eliminate slavery (especially not the slave states in the Union), but for the idea that admission to the United States was a one-way door. But, then again, the Union was heavily in favor of abolition, just not enough that it was worth splitting the country over (perhaps largely because many of them, like the Confederacy, could see where the long-term trend was headed.) Once the rebels caused the split anyway, that concern went out the door.
"My paramount object, is to save the Union, and not either destroy or save slavery. If I could save the Union without freeing the slaves, I would do it. If I could save the Union by freeing some and leaving others in slavery, I would do it. If I could save it by freeing all, I would do that. What I do about slavery and the colored race, I do because it helps save the Union." - Abraham Lincoln
The "States' rights" argument is trotted out by the Lost Cause folks (https://en.wikipedia.org/wiki/Lost_Cause_of_the_Confederacy) who want to pretend their affection for the South is somehow noble or genteel.
However, the case in the States was that at the turn of the 19th Century, the States were all pretty much fine with slavery. By the middle of the 19th Century, the North had decided it was not fine with slavery. The South was still fine with slavery. The North decided the South was going to do what the North wanted it to do, whether the South wanted to or not. To do that, it was going to use a strengthened federal government.
How you feel about slavery is beside the point (obviously it’s a barbaric practice). The point is that one group of people had a change of heart, and used force to compel another group of people to kowtow to its whims. And that changed the structure of the country.
No, they weren't. The protections of slavery written into the original constitution (including the fact that slaves, despite not being property rather than represented citizens, were counted at all in apportioning House seats -- and therefore also electoral votes) were written into it specifically because at that time the slave states of the South were already convinced that the North -- and the majority of the represented citizenry of the country -- would be inclined to at least greatly restrict if not abolish it in short order, based on the abolitionist movements that actually existed at that time and had been successful in the North, some even before the Revolution.
The North tolerated slavery in the South as the price of Union, right up until the South decided to sunder the Union over it, but the idea that the whole country was "pretty much fine with slavery" even at the turn of the 19th Century is just plain historically indefensible.
One of the reasons that the South broke away from the union was that in the new states being formed out of unincorporated territory, immigrants from the North were outnumbering those from the South, and subsequently rejecting slavery.
The South discovered that no one outside the South was fine with slavery, and those people were willing to take up arms to keep it contained there.
[1] "The new Constitution has put at rest forever all the agitating questions relating to our peculiar institutions--African slavery as it exists among us--the proper status of the negro in our form of civilization. This was the immediate cause of the late rupture and present revolution [...] The general opinion of the men of that day [Revolutionary Period] was, that, somehow or other, in the order of Providence, the institution [slavery] would be evanescent and pass away [...] Our new Government is founded upon exactly the opposite ideas; its foundations are laid, its cornerstone rests, upon the great truth that the negro is not equal to the white man; that slavery, subordination to the superior race, is his natural and normal condition."
(here's one: http://www.ucs.louisiana.edu/~ras2777/amgov/secession.html)