You said it far better than I would've.I would beg to disagree.
Lincoln’s message in the years before his election was anti-slavery, and he was elected on an anti-slavery agenda. Yes, it’s true that in the period immediately after his inauguration he spoke of preserving the Union rather than fighting slavery, but this was in the context of minimising the number of slave states which might secede – in the political realities of the time, if he had led the Union to war in 1861 on the grounds of fighting slavery, the border slave states like Kentucky and Delaware would almost certainly have seceded as well, which would have made the Union’s job much harder.
Lincoln benefited in another way from making preservation of the Union the issue – that way he forced the secessionists to fire the first shot, giving the Union the moral high ground. Had abolition been the issue, he would have been morally obliged to invade the secessionist states to free the slaves, and thus would have been cast as the aggressor.
With respect, I think this is taking a pedantically narrow view of the issues at stake. Yes, Lincoln said, “You have no oath registered in Heaven to destroy the Government, while I have the most solemn one to preserve and defend it.” That makes it sound like he was presenting secession as the casus belli. But this is to ignore the wider context of Lincoln’s views, as expressed in the years prior to his election. It also ignores the reason that many people volunteered for the Union armies. Yes, many enlisted to preserve the Union, or because service in the Army paid and fed well, or because they’d get citizenship at the end. But there were many Abolitionists in the Union Army as well, who went to war to end slavery. It was these attitudes which transformed the war from its narrow, technical basis to its broader goal of abolishing slavery.
I’d suggest that a useful comparison would be World War One. The various nations of Europe technically went to war over Austria-Hungary’s aggression against Serbia. Does that mean that Germany, Turkey and Bulgaria supported the right of A-H to invade Serbia, and that Russia, France, Italy, Britain and the USA fought to oppose that right? Of course not. There were all sorts of other issues at stake, but it was A-H’s threat to invade Serbia which started the whole thing off.
I won’t argue that economics played a part. But I think you’re wrong to ignore the issue of the creation of new states from out of unorganised Federal territory in the west. Which of these states would be free, and which would allow slaves, and how would this decision be made? Lincoln opposed the spreading of slavery anywhere outside where it existed at the time, which I understand the slave states considered intolerable, because of the long term consequences for the continuation of slavery. This isn’t an economic issue, but a combined political/moral one.
To summarise: Lincoln was morally opposed to slavery, and the people who elected him and who opposed him were well aware of it. The fact that he used secession as the issue to justify the start of the war, rather than slavery, was a consequence of his shaky political position at the time. It wasn’t until the strategic situation had improved (thus reducing his requirement to rely on moderate pro-slave support) that he could cast the war in more stark moral terms.
I'm both grateful and somewhat resentful.