One thing that seems obvious to me, but hasn't been mentioned, is this:
If the States had been allowed to secede peacefully, it would not have necessarily ended the Union forever. Slavery, for instance, was a damned expensive way of raising cotton. Economic forces and technology would have done away with it a few years down the road.
Any threat from abroad would have been met with some sort of Alliance.The Union was formed originally out of necessity... strength in numbers.
Trade between North and South would have continued, but on more even terms.
The War was caused because self-perpetuation is the first rule of government and Lincoln followed it. His successors have doubled down on it.
