The Constitution says nothing about secession, but does say something about rebellion. While it does not specifically say all the things that may be done about that rebellion, it implies that something may be done.
Obviously you've given some time to studying the subject, so I don't need to tell you that it is somewhat complicated, since both sides contradicted themselves from time to time, probably on the basis of changing circumstance and quality of the arguments being presented. For instance, why would the states have to vie for readmission if they had no authority to separate in the first place? Why would the south claim it isn't about slavery, yet in many declarations of independence cite slavery as a central issue? Why would the south deny any claim to perpetuality of the union yet explicitly assume it in their Articles of Confederation?
Likewise, it's too simplistic to claim stats as sovereign. Of course they were subject to the laws of the federal/national government. The Constitution allowed for a sort of mixed sovereignty. Part rested with the federal government, part with the states, and part with the people. That in no way implies complete sovereignty or complete independence of the states.
Buchanan didn't send reinforcements, and neither did Lincoln, but he refused to acknowledge any right of the states to secede. His main motivation was to NOT be the president under whom the union dissolved. That makes him weak in character, not a wise sage showing noble restraint.
As to the so-called "Northern Aggression," keep in mind South Carolina fired first. They attacked federal property, attained under valid and mutually respected laws. The fact South Carolina claimed to have validly left (just as they fallaciously claimed the ability to nullify federal laws) the union, does not change the fact that it was federal property on which they fired. Congress has the power, vested by the Constitution, "To exercise exclusive Legislation in all Cases whatsoever, over such District (not exceeding ten Miles square) as may, by Cession of particular States, and the Acceptance of Congress, become the Seat of the Government of the United States, and to exercise like Authority over all Places purchased by the Consent of the Legislature of the State in which the Same shall be, for the Erection of Forts, Magazines, Arsenals, dock-Yards, and other needful Buildings;"
Needless to say they were empowered to protect federal property, as well as quell rebellions, and protect each state against domestic violence. These were powers vested in the federal government, and it likewise had the authority to make laws necessary for carrying out the powers granted in the Constitution, which included the above. To claim it was a war of "Northern Aggression," then, is specious when we consider that South Carolina fired first on federal property. Part of the union or not, the property was properly attained, and the federal government's claim to it was not denied to it in open court. Therefore the attempt to sieze it by force was either an act of rebellion or an act of aggression.
Actually Lincoln did order and dispatch releif missions to Fort Sumter and Fort Pickens in Pensacola.
From http://www.tulane.edu/~latner/AndTheWarCame/AApr12.3.html
Friday April 12, 1861
Fox Expedition Finally Arrives
At 3 a.m., after three days of stormy weather, Fox and the Baltic arrived at a rendezvous point, about ten miles off Charleston Harbor. The weather remained bad, and the seas were still heavy. The Harriet Lane had arrived a few hours earlier, and at 6 a.m. the Pawnee appeared. There was no sign of the three tugboats, the Pocahontas or the Powhatan.
Although without the tugboats or the Powhatan, Fox attempted to make his way into Charleston Harbor with provisions. As he did so, he saw that the fort was already under attack. Turning back, he consulted with the other commanders on the scene. They agreed to wait for the Powhatan until the following morning, and then to attempt another landing, whether it appeared or not.
The Tulane site has an excellent timeline of the Crisis from the election to the onset of the war at:
http://www.tulane.edu/~latner/CrisisMain.html
It is a day by day timeline of the important events and actions of the principals. According to them Lincoln went against the advise of the Military and his own Cabinet, who all advocated abandoning the two forts to keep from precipitating hostilities with the CSA. Lincoln thught this would make the Republican look weak.
Thank you for putting up with my ranting and engaging in a intelligent, thought provoking discussion of the issue. It can be real refreshing.
This thread is a duplicate of http://www.freerepublic.com/focus/news/724692/posts. That thread is now up to 190 posts and can be rather, should I say, heated? Pehaps we should carry on there.