Well, I am not expert–but why the hell not try?
My scenario for the US going down under is a financial collapse that starts to pick up steam anywhere from 6 months to 5 years from now. The economy will really crash this time, unemployment will soar, and such. The government in the first stages will try everything it can to control the situation: Mainly welfare, and in highly disturbed areas, martial law.
But our federal government, being an incompetent monstrosity, will either help little, or actually make things worse. As things continue to spiral out of control, and we even hear of some deaths from malnutrition, the federal government will just start losing any sort of legitimacy. State and local governments, along with private groups (including organized crime) will start stepping in. These entities will rather quickly start replacing the federal government in practice. Most likely the federal government might just quit the game entirely, or be relegated to a mostly ceremonial role, maybe something like the Holy Roman Empire in the early 19th century. There would likely be some secession movements.
But the logistics situation will be desperate. While I somehow doubt that there will be a majority die off (50+ % of population), still millions will die for various reasons. Food will be hard to come by, though I doubt impossible. Rural areas may be somewhat better off the most urban areas. As situations break down in many areas, riots, localized civil wars, and insurgencies will pop up. While race relations will likely be piss poor across America, I would imagine that in the mid Atlantic states (Maryland and such places) along with the Southwest would descend into a full blown “race war.”
All things considered, I somehow get a feeling that history likes decades, so maybe 10-15 years for society to reestablish. If America does come together again, I imagine the Northern part of the country will be “America,” while a lot of the Southwest will become part of Mexico, and the “South” will become it’s own nation again, never to rejoin the rest.
Then again, I have no qualifications, so please don’t base any planning onto this. This is just some thoughts I have.