The most obvious would be a financial collapse brought on by the rest of the World “waking up” and ridding themselves of the American Dollar as the “gold standard” of currencies. (We’re $30 trillion in debt, and we don’t produce/manufacture squat, that China/Germany/Russia/India couldn’t, as well)

If the middle class has no jobs/savings/investments to worry about, all bets are off. Otherwise, who knows??