Well, let's set a benchmark then. My native Tennessee passed a secession ordinance in June, 1861. Union forces siezed Nashville less than a year later. I don't know, but I would think that Tennessee, as a sovereign nation, had at least some Union troops somewhere within its borders for pretty much the whole war.
So how long, exactly, was Tennessee a sovereign nation? Can you be a sovereign nation if foreign troops are all over your territory?
I think it just as easy to accept another interpretation: the states were never out of the Union for an instant. That IS part of history that the winners get to write.
Losers just get to carp and hope other losers will buy off on it.
Walt