if you bought, owned and operated a piece of property, and was asked just to leave because someone else felt they owned it, would you???
perfect example, guantanimo bay..
cuba asked us to leave, we did not..
we are still there...
are you saying we should give up our base there because cuba is an independent nation?????
I eagerly await your reply..
Assuming Country B began preparations to use that base as a jump off point for an invasion of Country A, all other matters being neutral, Country B would clearly have the right to consider the buildup a hostile act and respond with force, even preemptively.
Particular to Fort Sumter, why would you assume that it belonged to the Northern States? Common property is generally dissolved between the partners when a partnership is dissolved. Clearly there is a strong argument that, at a minimum, all federal property within a seceding state would automatically revert to that state. I don't recall the CSA claiming ownership of any territory outside of their borders, although they would have had the same claim to federal lands in the North, as the North had on federal lands in the South. The peaceful dissolution of Czechoslovakia is a good example. Did all of the governmental lands in Slovakia continue to be held by the Czech Republic? No, they did not.
South Carolina spent several months requesting that Fort Sumter be placed back into their possession. But alas, not all divorces are amiable.