Then I assume that any individual or group of individuals can, at will, declare themselves to have disposed of the government and to now be a free and independent nation unto themselves, right?
Regarding fort Sumter.....Yes Lincoln had made promises. For one, he promised the State of Virginia that Fort Sumter would be evacuated if Virginia did not secede.
Lincoln's offer was that he would evacuate Sumter if Virginia adjourned their secession convention. The convention sent a delegation to Washington, which arrived on April 12, just in time to hear the news that Sumter had been attacked, knocking the deal off the table.
The South also sent delegations to Washington, D.C., and offered to pay for the Federal properties and enter into a peace treaty with the United States.
Actually their only assignment was to discuss "matters of mutual interest." There was nothing in their charge that spoke of peace treaties or payment for stolen property. And entering into a discussion with them would have been de facto recognition, something Lincoln was unwilling to give them, along with every other nation in the world.
Bubba Ho-Tep: "Then I assume that any individual or group of individuals can, at will, declare themselves to have disposed of the government and to now be a free and independent nation unto themselves, right?"
The opinion of Northern "Dough-faced" Democrats like outgoing President Buchanan was that secessionists had no legitimate or lawful reason to declare secession, but that the Federal Government had no power to stop them.
That was also Lincoln's opinion, so long as peace was maintained and Federal laws enforced -- i.e., tariffs.
So, had Jefferson Davis been determined to maintain peace, there was good chance of the Confederacy's success.
What changed those opinions were the constant secessionists' provocations of war -- seizures of dozens of Federal properties, forts, ships, arsenals, mints -- and then starting war at Fort Sumter.
But to seal the deal, the Confederacy also formally declared war on the United States (May 6, 1861) and sent military support to Confederates fighting in Union states.
All this happened before a single Confederate soldier was killed in battle with any Union force, and before a any Union army invaded a single Confederate state.
So the Union choice then was whether to conduct a McClellan-like "Dough-faced" losing war, or whether to fight a "total war" for unconditional surrender and destruction of the Slave Power, which had become an abomination to civilized humans.
Lincoln chose total war, unconditional surrender and utter destruction of slavery.