China.
But after they conquer it, ISIS will fight them dearly for it. Of course there will be very few current Americans still alive to observe the outcome. The entire continent will be "cleansed" by the process.
Possible breakaway southern region? We can round up the Rats and hand them over to the Chinese for sweatshop labor. Boy, would it be fun to see Lena Dunham groveling for a thimble in manacles in front of her yellow master.