More relevant to the complaints of the secessionists in 1860, consider the situation of the British fortress at Gibraltar in Spain, guarding the strait and potentially cutting off sea traffic from one half of Spain to the other at any time. Spain certainly would like it back -- but trying to seize it would definitely be an act of war.Honestly, I don't think those questions are all that relevant. I'm certainly not the first to make this comparison, but I think the whole situation is closely paralleled by the situation at Guantanamo Bay.
We signed a lease with Cuba at a time when they were our friend. They've been trying to get us to leave since the 1950's but there is nothing they can do about it short of attacking the base and starting a war with the United States. Do you think they would be justified in doing this?
It's exactly the same case with Fort Sumter. South Carolina ceded the title to Fort Sumter and Fort Moultrie in the 1840's at a time when they were a friend of the Federal Government. If secession was constitutional—which IMO it was not—then South Carolina was a sovereign state when they ceded the title to the Federal Government. They had no legal right to take it back just because the relationship between them and the Federal government changed, just as Cuba has no legal right to take back Guantanamo Bay just because the relationship between them and the United States has changed.