Okay, lets looks at this logically....
South Carolina signed a compact with the US government called a Constitution. The Constitution is a contract between the people who inhabit each state and the collective states as a whole aka union, republic, nation.
The land for the building of Fort Sumter was ceded to the US government by S. Carolina in 1838. It was a legal contract between two sovereign governments, the state and the federal, which gave the federal government exclusive rights over the property "provided" that S. Carolina would still have jurisdiction to serve summons and supeonas on the federal property.
Are we in agreement so far?
Twenty two later in 1861, and after great expense and effort to bring in landfill and material to build the island and the fort, which was still under construction, South Carolina decides to reneg on it's compact with the union, and reneg on it's contract that ceded property to the US government.
Are we in agreement so far?
So after S. Carolina renegs on two legal contracts you think the US government had no right to stay on Fort Sumter? Do you really believe that two binding legal contracts were null and void without the US governments consent, especially ones that they were party to? There were two legal entities that signed binding legal contracts and just because one side decides they don't like the terms years after the fact does not make those contracts automatically null and void. In fact if you think about it, the purpose of signing of legal contracts is to protect both parties from one side arbitrarily renegging on the agreement. No sir, the US government had a binding legal claim to Fort Sumter no matter how much of a hissy fit S. Carolina threw.
I really don't expect you to be in agreement at this point because this same exact arguement has been going on since 1861 (see Lincoln's last speech) and no amount of reason or legaleze is going to convince you otherwise because you are obviously very vested in your version of the truth. But at least we seem to be in agreement that the Civil War put an end to the states right to secede.....aren't we?