It was an occupying force inside South Carolina's waters.
Had the North tried to force a resupply of the Fort, they would have been see as the aggressors, even if the resupply was only provisions. Davis screwed the pooch. He decided to reduce the fort by force, rather than allowing Anderson to surrender in a couple of days. His forces were probably adequate thwart any attempt to resupply Sumter. If he had waited, allowed the North to try a forced resupply, He would have been in the right, to fire on that force and the fort.?
Um, the South didn't fire on the fleet, only the fort. And US General Scott had let the cat our of the bag in his communications about the Sumter expedition. Its objective was to reinforce Sumter, not just resupply it. Hence the large number or troops and a large amount of provisions and supplies in the expedition's ships.
Who is the true aggressor in this case? If you look at the history of those times more deeply, you may come to realize that Lincoln fully wanted and intended war. His cabinet had advised him that the Sumter expedition would result in a shooting war. After the Sumter expedition failed, Lincoln consoled Gustavus Fox (the man who proposed the Sumter expedition) saying basically that the mission accomplished what we wanted [Link].
You and I both anticipated that the cause of the country would be advanced by making the attempt to provision Fort-Sumpter [sic], even if it should fail; and it is no small consolation now to feel that our anticipation is justified by the result.
The result, of course, was war. Lincoln had accomplished his goal.
War and the blockade of Southern forts were ways that Lincoln could use to stop the Southern tariff from ruining the Northern economy and the US government's revenue. He was faced with disaster because the North had almost doubled the Northern tariff (something Lincoln and the Republican Party had been in favor of) when Buchanan signed the Morrill Tariff bill on March 2, 1861. The South had made their tariff slightly lower than what had been the tariff in the US before the Morrill Tariff.
The immediate impact of the two tariffs was that many businesses started failing in Northern ports and US tariff revenue started plunging rapidly. Northern businessmen started demanding that the government drop or change the Morrill Tariff or do something rather than inaction. The lower Confederate tariff meant that a lot of the import business would eventually move to Southern ports like Charleston, Savannah, Mobile and New Orleans, and Northern goods would now be charged a tariff by the Confederacy when they were imported into the South. This would mean that Northern manufacturers would have to lower their prices to compete in the South with less expensive European imports. The North would be deprived of much of its tariff revenue and its manufacturing businesses would be hurt.
A delegation from Baltimore met with Lincoln and Lincoln famously said, "... what is to become of the revenue? I shall have no government -- no resources."
Abraham Lincoln accomplished his goal, because Jefferson Davis acted on impulse, not logic. He could have set it out for two days and Sumter would have had to surrender. He was a fool that played into Lincoln’s hand. There was no militarily sound reason for firing on Sumter. Jefferson Davis was outsmarted by Abraham Lincoln and the war that could not be won by the Confederacy was started, by the Confederacy as far as the World community saw it. JMO