Once South Carolina declared its independence, the Federal government had no reason to maintain an armed force at Fort Sumter.
A small compromise to prevent a war. Relinquish it.
See #18.
Methinks peaceful compromise comes better from “you made X, it’s yours, keep it” than “you made X, now give it to me & GTFO.”
Anyone in any position of influence & control, whether it politics or business, whether an American or European, understood at least by the 1850s, if not earlier, the promise a unified US held in terms of world power. Any nominal projection of immigration and birth rates could easily predict the sum total population and economic power the US would exert in a relatively short amount of time. (Which was proven 50 years later in 1917.)
Banking, finance and commercial interests all had a stake in this outcome. It was inevitable - as long as the Union held. So, what occurred in 1861 is essentially the same thing that happened in Catalonia last weekend. There is no way on earth that a separation was going to be allowed. One side can advocate abolishing slavery, the other self-determination or whatever cause du jour floats their boat.
It was always about preserving the Union, and not exactly the noble reasons espoused by Lincoln, et al. Rather, it was simply a raw exercise in power, no different than GBs attempt at defeating the Revolution. If you think you can pull it off, go ahead and try, but that doesn't mean the controlling power is going to let you escape without a fight.