"That being said, after the Republicans won the 1860 election and would soon be able to put in abolitionist policies, should they have let the Confederate Dims break away from the Union and start their own country?First of all, the only abolitionist policies Republicans advocated in 1860 involved US western territories, not Southern states.
I guess that's debatable."
Second, Republicans had nothing whatever to do with letting Southern Democrats break away from the Union and start their own country -- that was 100% the Doughfaced Democrat administration of Pres. James Buchanan.
What Republicans in Congress might have done in December 1860 is given in to Southern demands for "compromises" to persuade some Deep South states not to declare secession, especially Mississippi.
But this would have negated the whole Republican raison d'être and provided no guarantees against future Democrat threats of secession.
Regardless, by the time of Lincoln's inauguration, March 4, 1861, secession and Confederacy were already a fait accompli, and the only issue was whether there would be peace or war.
In his First Inaugural, Lincoln offered peace, but many Southerners called it a "Declaration of War" on the South, and increased their demands, threats & seizures by force of Federal properties.
One key property was Union Fort Sumter in Charleston Harbor.
"And the war came".
So, many have argued that Republican Lincoln should have just submitted and given-in to whatever Southern Democrats demanded.
And actually, Lincoln himself was willing to cut a deal -- to give away Fort Sumter -- if that could guarantee no secession by Virginia.
But the deal could not be struck, and so Lincoln held his ground -- one of the few Republicans ever willing to stand up to radical Democrat demands.
Today we have an opportunity to elect another such Republican, and, hopefully, the results will not be quite as catastrophic as 1860's election.
But I doubt if Pres. Trump's success in 2024 will be 100% free of dire consequences from radical Democrats.
Political violence is always just under the surface with extremist Democrats.
By "after the Republicans won the 1860 election and would soon be able to put in abolitionist policies", what I should have stated better was that the Republicans were set to get the federal government out of the way and let the States decide on abolition/slavery (while still implementing abolition in territories).
And yeah, I agree with your post that in many ways the Civil War started as Democrats vs Democrat (Dim Confederate South while Dim Buchannan in the WH did things like tell U.S. soldiers not to abandon forts within now Confederate areas). And the siege on Fort Sumpter was going on before Lincoln was inaugurated. But there's part of me that thinks that once Lincoln got in office, the Republicans could have done more to just let them go. I don't remember reading any message from Lincoln or the Republicans saying, "Hey, Dims. Now that us Republicans are in charge there no longer needs to be a war. We'll withdraw our troops from old forts that are now in Dim lands. You go your way, we can go ours."
Even with that, I don't know if it would be possible to avoid war. To be fair to Lincoln, it'd be hard to think that war was avoidable with DC surrounded by Maryland and Virginia. And it was earlier in so-called "neutral" Maryland that multiple attempts were made on then President-elect Lincoln's life while he toured there saying that he didn't want a war. And, of course, Virginia had already turned Confederate. So, as you rightly pointed out, the Dims started the war and there may have been nothing the Republicans could have done to prevent it.