Among my eclectic interests is history. Thus, recent discussions over at Balkanization that have a historical bent is of some interest. They also touch my "uh ..." buttons, even though if they are written by people above my credential grade. This not haven stopped me before (and not quite as arrogant as this person), I offer some thoughts. First, Sandy Levinson:
I think main justification for Lincoln's decision to join in going to war--it takes two to tango, after all--is along "humanitarian intervention" grounds emphasizing the necessity of eliminating chattel slavery, but this justification, as I've also argued earlier, calls into question the weakness of the ensuing "reconstruction" that utterly failed to achieve genuine "regime change" and permanently harmed American politics by giving the South even more political power in the long run than it had had prior to the War.
The justification originally given was union. This was a sound reason to go to war. Our country was established, "a more perfect union," for various purposes. See, the Preamble of the Constitution. And, it was that -- a union -- not a band of states that could easily break apart, form illegal confederations (Art. I, sec. 10), and declare themselves independent when a national election doesn't go their way. This would put the union at grave risk, make it weak and open to fracture and invasion. Furthermore, the country was set up as a republican government.
Even slavery was factored in here, as shown by various provisions that dealt with the issue (in a way that allowed the institution to die out), imperfectly or not. And, slavery was to be dealt with that way as well. Did not slavery die out in Britain, a free nation? So here. The territorial issue was for years dealt by legislative action. Ditto fugitive slaves. Yes, by the 1850s, the nation as a whole was starting to firmly go against the institution. As Don Fehrenbacher's The Slaveholding Republic noted national policy was (by choice, not constitutional demand) pro-slavery up to that point, but could switch if Republicans gain political control.
But, even here, not only did the South have to accept all that republican government already gave them in the antebellum years, but the alternative was quite dubious. The Constitution in various ways still "served their interests," in the words of this somewhat convoluted post. I do think they took a "bet" of sorts in 1787 to follow the rules. But, yes, the Declaration of Independence voiced a revolutionary theme that justified rebellion if the rules were carried out in such a way that their liberty was severely threatened. That is, if "tyranny" was in place, according to the spirit of the land. A stream of abuses and so forth would underline the point. The Revolutionary War, in this view, was no "preventive" war.
[Update: The referenced post is part of a debate, the reply here. Brad's first post on the matter is particularly on the money. MG provides a thought experiment that doesn't work.]
The South had reason to believe that the balance of the government was going against them. More specifically, some forces in the South. After all, and again the confused post doesn't really underline the point, many Southern Founders realized (or even hoped) that slavery might eventually die out. They wanted it done the right way though, hopefully peacefully, and without a threat to basic republican governmental principles. So, yes, the war was connected to slavery. The South wanted to "solve" the problem outside of the rules. And, without much to stand on -- was slavery directly threatened where it existed? Just how dubious was the idea that Congress could regulate slavery in the territories (and would slavery really thrive there anyway)?
Finally, what was the likelihood if rebellion took place? War was quite possible. And, the net result quite likely -- see the French Revolution / Haiti -- would be a direct threat to slavery itself. Is this a reason to threaten "a more perfect union," reject a fair election, and so forth? After all, even after the Civil War, the South retained much control over their "domestic relations," plus the balance of national/state power -- even when the Fourteenth Amendment seemed to suggest otherwise -- remained much the same in many ways into the 20th Century. As the DOI notes, independence is a last resort. Equity did not seem to warrant it in 1861. Many in the South even agreed, though a majority most likely did support secession. [The final four only after the battle of Fort Sumter.]
Was the North right to go to war to fight the rebellion? I think it useful to point out that many did not think it would go on for so long. Likewise, many -- including top generals like McClellan -- supported limited war aims. Fighting armies, and not to the death, not civilians. This helped the Confederacy given that it made the great imbalance of resources and arms less dangerous for the rebels. The alternative was shown with the "total war" sentiments of Sherman and Grant. Also, it bears noting that the hard fighting often came from the rebels. For instance, after Grant and company fought to regain key western forts (a "limited war" end), the rebels tried to gain the advantage at Shiloh. Lee was the one who invaded Northern territory twice, taking advantage the first time around especially from M's timidity and limited war aims.
As I noted earlier, the Copperheads had something of a point. The carnage and so forth of the war became rather hard to take. But, it had a "snowball" effect to it, really -- sending troops to try to regain forts or battle volunteer soldiers on the other side in the early battles had widespread support for a reason. So, I don't think "chattel slavery" per se was needed as a war aim to make things just, surely not at first. And, even later, it was clearly intimately connected to a broader theme.
As to the first post, this reply hit the spot for me for a somewhat different reason. See here as well.