Let's just imagine you in south carolina's shoes for a seconde...you just declared your independence as a state with a new government which you elected, yet the army of the old government INSISTS without compromise that it is still entitled to camp out with an arsenal on a piece of your land smack dab in the middle of the entrance to your biggest seaport. Further, when you ask him kindly to leave, he responds by saying "make me" and calls for a fleet of warships to reinforce him. Tell me then, do you feel threatened? Do you want the old army's presence there to continue?
Even with the reinforcements and supplies could Sumter have threatened the survival of the confederacy?
Probably not, but it certainly could have severely impeded commerce into and out of charleston, one of the main ports in the confederacy and the single main port of south carolina. That aside, it all comes down to the fort's proper use, owner, and function. Any way you look at it, it's on south carolina's land. It guard's south carolina's seaport. It is of absolutely no use whatsoever to the northern government for any purpose other than the less than honorable motive of impeding commerce into a port of another.
Simply put, the north had no business being in Sumter unless that business was to cause trouble with the south.
Could the 500 or so men in the fort launched an invasion of South Carolina?
Most likely not, but they could have severely impeded commerce into the state, not to mention the fact that they had no other business or interest in being there other than to be a force that is capable of impeding that commerce.
Would closing the third largest port in the south caused irreparable economic harm?
It certainly wouldn't help.
Had the troops in Sumter issued any threat at all?
Lincoln sure did. He did so in the form of ordering 3 warships to "provision" the fort after repeated and perfectly peaceful requests by the southerners to the fort's occupants that they leave the garrison, and in doing so be given safe passage home without interference. In fact, they were even given safe passage home without interference after the battle!
The answer to all those is no.
Not exactly, and to suggest otherwise is to grossly oversimplify the situation. So why the rush to open fire if not because you wanted a war all along?
Because (1) peaceful requests for the turnover of the fort had been made since the preceding december, (2) occupants of the fort had repeatedly indicated that they would not leave without a fight as they had been ordered to stay put, (3) occupants of the fort had no business being there in the first place other than to retain the capability to cause trouble for southern commerce, and (4) Lincoln had dispatched three warships to reprovision the fort with increase ammunition and supplies, indicating preparation for future and increased military capabilities.
Or, if you wish, we can make an analogy. Suppose you had recently bought yourself a new house, yet when you moved in, the previous occupants were still residing in a van parked in your driveway. Now suppose that the van threatened to completely shut off your driveway from any access by you and its occupants had indicated that they would bash the hood in on any car you tried to park in the vicinity of their van. Further, lets suppose that the van's occupants were completely unwilling to leave despite the fact that you had made several polite requests of them and even invited them in for dinner on occasion. And to make matters worse, what if you knew that they were storing ammunition in the van and, on top of that, a bunch of redneck friends were driving in from out of state with more ammo with the intent of joining them on your driveway. Tell me, if faced with this scenario, would you take action to get the van moved?
If you are sane, probably. You would probably call the police to come tow them. But what if we suppose that this hypothetical situation exists in a state where there are no police, and all such matters are left entirely to you to enforce. See where I'm getting at?
The people in the van have no place being in your driveway in the first place. Similarly, the yankees in fort sumter had no place being in the middle of charleston south carolina's harbor in the first place. Additionally, the presence of both is at least an inconvenience if not a greater threat, and it also happens to be a presence that is unwilling to remove itself and in fact is staked in ready to use force to impede any who try to make it leave. And on top of that, there are reenforcements coming to them soon to help them stake it out and fight, plus no authority other than yourself to make them leave. So what do you do? You make them leave!
That is exactly what South Carolina did, and for that I can in no way fault them.
Because in the end it boils down to which of the two different viewpoints you hold to. You claim that South Carolina had a right to secede and to sieze federal property within it's borders. Lincoln, on the other hand, said that South Carolina's actions were illegal and that federal property was federal property and would remain so. I agree with that position and so did the Supreme Court in 1869.
So what were South Carolina's motives? Surely none of this came as a surprise to them. On his way to Washington for his inaguration Lincoln had, in speeches in Indanapolis and New Jersey and Philadelphia, made it clear that he intended to hold on to federal property in the south and resupply it if necessary. This was no secret. In private correspondence to Seward and Chase and Winfield Scott and Major Anderson he repeated his intention to hold Sumter if at all possible. The Davis administration had been appointed a month before Lincoln was inagurated so they would have known this. They would also have known that the idea of military force to put prevent secession was not popular with the majority of the Northern press and much of the political leadership. So the south really had the opportunity to wait out Lincoln if they really were interested in a peaceful solution to the crisis. But instead they chose not to. Why not? Again I'll point out that the troops in Sumter were in no position to threaten the confederacy or to harm Charleston much. Shutting the port at Charleston wouldn't have harmned the confederate economy. Indeed, had the troops in Sumter shut down the port that would have placed the onus for the first hostilities on the North and would have weakened Lincoln's political support even further. So the south really had nothing to lose and everything to gain by waiting out Lincoln - if they had really been interested in a peaceful soulution. But many of their actions to date pointed to just the opposite. One of the first acts of the confederate congress had been to vote an army of 100,000 men. An army over 5 times the size of the U.S. Army. An army approved in spite of the fact that not a single hostile act had been committed by the federal government to date. Why the need for such a large army if not to use it? Could it be that the southern leadership felt that a nation born out of the fire of battle would be better equipped to survive? A victorious war with the North would leave ill feelings between the two countries - the North would be the resentful vanquished and the south would be the proud victor. The chances of their ever being a reconciliation between the two countries after that would have been nil. Likewise, the chance of any confederate state deciding that secession had been a mistake and attempting to rejoin the North would also have been nil. The confedate states would have guaranteed northern non-interference as they set about building their nation and, perhaps, expanding it south and west.
So the idea that the south was tricked into war is ludicrous. They got the war that they wanted from the beginning. It just didn't turn out the way that they expected.