Just as has been the case for nations and empires since the creation of the world, the USA will crack up, split up, and either have parts taken over by other countries, or form "new countries" from the pieces.

It may be the Polish model, where all the neighbors dice and slice and digest the various chunks, usually a small part at a time.  Or it may be the Roman model where the big chunks fight each other for the juicy bits in easy reach of them all, while the jackals nibble on the fringes.  Or it may be the British model where the various pieces are "voluntarily" allowed to go their own way, while the core gets smaller and smaller (and more and more arrogant, all too often).  Or the Soviet model of the core imploding and everyone abandoning the sinking ship.  Or the Spanish model of larger groups of colonies rebelling and then shattering again and again.