If you read the actual secession documents passed by the Southern States attempting to leave the Union, it's very clear that the was was about slavery. South Carolina, Mississippi, etc - they knew what the war was about.To expand briefly on this, I would also refer readers to the Cornerstone Speech of Confederate Vice-President Alexander Stephens. "States rights," meanwhile, was the 19th century's version of "activist judges," in that the slave states were more than happy to use the Federal government for their own ends (Fugitive Slave Act, Dred Scott decision).
Was the North fighting to free the slaves? Nope. They were fighting to defend the Union (short for "the United States of America"). But the reason the Union was threatened? The Southern states seceded to defend the right to own slaves. It wasn't even States' Rights, because among the things that they objected to was the actions of non-slave states to impede slavery (slavery transit, blocking the recovery of runaway slaves, etc).
So, to recap (and in so doing I do not ascribe any particular view to anyone else):
1) The cause of the Civil War was slavery.
2) The right side won.
I'm glad we had this little talk. And Mr. P., I will see you tomorrow!