The recent comments discussion in Lydia McGrew's post at "Right Reason" is a good example of how Americans are still living with the legacy of the Civil War. People's passions run very high on the subject. I'm not an expert on these matters, but I thought I'd put down my thoughts:
(1) I confess I can understand the position of those who want to deny that the War Between States was over slavery. I'm a Southerner myself; it's hard having one's cultural heritage constantly reduced to one thing, and that among the worst things our civil discourse can name. But slavery clearly was an issue for the South's secession; there's nothing to do but cowboy up and move on from there. The States themselves said it. Mississippi and Texas (shame to say) were most forthright:
Our position is thoroughly identified with the institution of slavery-- the greatest material interest of the world. Its labor supplies the product which constitutes by far the largest and most important portions of commerce of the earth. These products are peculiar to the climate verging on the tropical regions, and by an imperious law of nature, none but the black race can bear exposure to the tropical sun. These products have become necessities of the world, and a blow at slavery is a blow at commerce and civilization. That blow has been long aimed at the institution, and was at the point of reaching its consummation. There was no choice left us but submission to the mandates of abolition, or a dissolution of the Union, whose principles had been subverted to work out our ruin. (Mississippi)
In all the non-slave-holding States, in violation of that good faith and comity which should exist between entirely distinct nations, the people have formed themselves into a great sectional party, now strong enough in numbers to control the affairs of each of those States, based upon an unnatural feeling of hostility to these Southern States and their beneficent and patriarchal system of African slavery, proclaiming the debasing doctrine of equality of all men, irrespective of race or color-- a doctrine at war with nature, in opposition to the experience of mankind, and in violation of the plainest revelations of Divine Law. They demand the abolition of negro slavery throughout the confederacy, the recognition of political equality between the white and negro races, and avow their determination to press on their crusade against us, so long as a negro slave remains in these States. (Texas)
(2) Was secession legal? Like Lincoln, I hold that the Constitution is what it was put forward to be, a revision of, and not a mere replacement for, the Articles of Confederation. And the Articles of Confederation clearly state that "the Union shall be perpetual." Not only is this not eliminated by anything in the Constitution, it seems implicitly supposed in the Preamble (nor will it suffice to say that the Preamble establishes no law; it establishes the purpose of the document, and those who advocated secession also appealed to it in their arguments for secession). On this view there is room to think, if you think the evidence warrants the conclusion, that Lincoln erred in invading so soon after secession, rather than trying negotiation; after all, the Articles clearly state that there shall be a league of friendship among the States. But whether Lincoln was justified in forwarding the War as he did is an entirely different issue from whether the States were justified in what they did.
(3) Was the secession really about states' rights? Of course; but anyone can see, particularly given a look at the various Declarations of the Causes of Secession, that this does not exclude its being about slavery. It's unfortunate that such an important issue as states' rights became so closely connected to such an odious issue as slavery. But the South has itself to blame for that; now everyone has to deal with it.
There is so much misinformation on this issue; people spread inaccurate information (e.g., they are constantly talking as if the Confederate Battle Flag were the Confederate Flag), and people are continually making things up in a 'high priori' way. Regardless of the details, let everyone take the right lesson to heart: that there can be no compromise on the principle that everyone was created equal and endowed with inalienable rights.