Some good answers. Its important to remember that at the time of the creation of the nation all the states had legalized slavery and indebted servitude. The fact that it disappeared in the north and stayed on in the south had more to do with climate conditions, crops grown, northern industrial growth, then it had to do with moral idealism.
And there were decades of causes, other then slavery, that had as much or more impact on an eventual conflict. Slavery tended to be the lightening rod that kept opinion red hot, most of all when new territories/states were annexed into the Union and the question of whether to allow slavery in them was raised.
At the same time I'd bet if you asked Union soldiers why they fought 99% would either answer "to preserve the Union", or, "because we were forced to". A southerner would answer the same question, "to preserve our rights". And anyone who thinks millions of white men in the 1860s, on either side, would fight a terrible 4 year war over slavery has a far to high opinion on human beings. The fact is most people on either side couldn't really have cared less about slavery. Those in the north, most of them, didn't think it a good enough reason to fight. And most in the south didn't own any.
The thing is the question of where Federal powers end, and the powers of the States begin, has been a contentious issue from day 1 to today. In 1861 War broke out over it, even today there is constant litigation and political maneuvering over who has power and when. The Federal Govt. or the individual States.
It was the position of The Confederacy that America was a Union ruled by consent and that the Southern states had a Constitutional right to secede from it. The north believed The Constitution formed a Union that was perpetual couldn't be divided and that talk of secession was rebellious.
And that! was the cause of The Civil War.
