sorry the pemail didn't go through.
in the original version of the game Kentucky could be blitzed by whoever invaded; it was such a problem that there was a gentleman's agreement between early players that the blitz would not occur.
the problem was fixed in an early patch.
the simple answer is that whoever invades cannot overrun so the usa can't get to Bowling Green and the CSA can't get to the Ohio River crossings, Count your blessings that the only crossing you need to defend is at Cairo.
as I recall, once either player takes all the population centers they control the state, although if it decided to remain neutral, regardless of who invaded, then it remains neutral.
"L'audace, l'audace, toujours l'audace."