You're Bradydamn right that it was a mistake.
There are certain formal processes that would have to take place if you were going to allow South Carolina (and others) to secede in a way that would at least be acceptable to all parties involved---and attacking United States Government holdings is sure as shit not the way to do it.
You can't even call it an illegal occupation because the U.S. Government already had a fort there. It's not as if South Carolina seceded and the United States sent troops to a fort that had never housed U.S. troops before.
In fact, let's look into that in the light most favorable to South Carolina, from the Declaration of the Immediate Causes Which Induce and Justify the Secession of South Carolina from the Federal Union:
Okay, so we start with the fact that their logic is that electing a POTUS who did not want to expand slavery into the new territories is tantamount to the Government becoming destructive of the ends for which it was established. That ignores, of course, the fact that South Carolina was NOT one of the new territories, so for the time being, what happened in the new territories had nothing directly to do with them.
In the present case, that fact is established with certainty. We assert that fourteen of the States have deliberately refused, for years past, to fulfill their constitutional obligations, and we refer to their own Statutes for the proof.
The Constitution of the United States, in its fourth Article, provides as follows: "No person held to service or labor in one State, under the laws thereof, escaping into another, shall, in consequence of any law or regulation therein, be discharged from such service or labor, but shall be delivered up, on claim of the party to whom such service or labor may be due."
Okay, so they are pissed off that slavery is not expanding into the new territories AND because the north has become hostile to the Fourth Article that called, effectively, for slaves to be returned to the state that they escaped from was not being done.
The following paragraph states:
This stipulation was so material to the compact, that without it that compact would not have been made. The greater number of the contracting parties held slaves, and they had previously evinced their estimate of the value of such a stipulation by making it a condition in the Ordinance for the government of the territory ceded by Virginia, which now composes the States north of the Ohio River.
Further down:
We affirm that these ends for which this Government was instituted have been defeated, and the Government itself has been made destructive of them by the action of the non-slaveholding States. Those States have assume the right of deciding upon the propriety of our domestic institutions; and have denied the rights of property established in fifteen of the States and recognized by the Constitution; they have denounced as sinful the institution of slavery; they have permitted open establishment among them of societies, whose avowed object is to disturb the peace and to eloign the property of the citizens of other States. They have encouraged and assisted thousands of our slaves to leave their homes; and those who remain, have been incited by emissaries, books and pictures to servile insurrection.
AND:
A geographical line has been drawn across the Union, and all the States north of that line have united in the election of a man to the high office of President of the United States, whose opinions and purposes are hostile to slavery.
AND:
Sectional interest and animosity will deepen the irritation, and all hope of remedy is rendered vain, by the fact that public opinion at the North has invested a great political error with the sanction of more erroneous religious belief.
In an official document, not only did South Carolina use the institution of religion to defend slavery, but further defined the notion that slaves should be freed as, "Erroneous religious belief!" I honestly wouldn't have believed even this if I hadn't went looking into the deeper history of South Carolina, specifically. South Carolina, as a would-be sovereign, maintained that, pursuant to Christianity...freeing the slaves was religiously wrong!
That stands on its own. I don't even need to offer additional commentary.
Would you like to tell me more about how the Civil War was NOT about slavery, or should we go ahead and dust off the Confederate Constitution for examples that I already know are there?