I can tell you in one word. "Money."
An independent South represented a horrible financial threat to the wealthy interests of the North East. (Same people controlling the nation today.)
The South produced 73% of all export trade to Europe, but the laws had been jiggered to funnel all that money back through New York, where Washington DC collected it's cut.
With the South leaving, that 230 million dollars per year in trade would move out of New York, and land in Southern port cities, thereby seriously damaging New York shipping, banking, insurance, warehousing, and so forth.
Not only that, but with the South going independent, European goods would be flooding the Southern port cities, and would be distributed along all the transportation networks then in existence, like the Mississippi, where it would then be shipped to all the midwestern states, seriously hurting the manufacturing industry in the North Eastern part of the country.
The Wealthy Liberal money powers of the North East would have been badly hurt if the South became independent, so they exerted their influence over Lincoln to get a war they very badly needed to stop the south from ruining their businesses.
It was about money the North would lose as a result of the South cutting them out of the European trade, and destroying their protectionist laws.
Took me some years to piece all of this together, but that is exactly what the economic evidence shows. The existing money powers in the North would have been destroyed had the South kept it's independence.
That’s a great post, your #37!
I don’t think the Mississippi route could compete with the Erie Canal, but have no other complaint.
Though, of course the NW of the country had shown slavery to be a very inferior economic system.