The Federals had been invited to leave by the sovereign state of South Carolina.
South Carolina had no sovereignty over Fort Sumter.
And still doesn’t by the way. Interesting how that all worked out, huh?
http://en.wikipedia.org/wiki/File:Flag-raising_Fort_Sumter_Charleston_Harbor_1865.jpg
Don’t you know that once a state agreed to be in the union it had to bow down to the feds whenever ordered to do so? sarc