You're recalling a scene that never existed.
That's a massively huge mis-recollection you've got. You seriously thought a ten year old kid would walk all the way from Mississippi so he can get operated in Dr. O'Doull's tent?
No, it was "impenetrable" because the United States didn't bother penetrating it, because there was no reason to head into Mississippi in the first place in this war.
Yeah. Truth is, even Vicksburg and Jackson weren't real important in the '40s, and they were two of the state's larger cities(with the latter being the state capital, of course). Turtledove had the right idea with the U.S. Army going thru Georgia and Alabama, I think, given the higher proportion of industry in both those states(especially for AL with the iron works in Birmingham and the rocket factory in Huntsville).
By the way Dave, if you're out there, I would like to apologize again for the poorly thought-out comment I wrote out last month.....I hope I didn't offend you on accident.