Seriously, like when Portugal went bankrupt at the turn of last century. Buffett and Peterson are touting a new documentary highlighting the looming REAL financial crisis in America.
http://money.cnn.com/2008/08/21/news/economy/buffett_town_hall.ap/index.htm?postversion=2008082105
So, what would happen nowadays if a sovereign nation went completely belly up, without assistance from the IMF or World Bank?
There is no formalized banktruptcy law or procedure for the bankrupt of nations the way there is for private persons. The nation simply stops paying debts. This will of course make creditors very angry, which can lead to military consequences - one of the reasons of the French intervention in Mexico in the 1860s which resulted in the proclamation of an Austrian nobleman as Emperor of Mexico was Mexico’s suspension of interest payment to foreign creditors.
Sometimes, debtors and creditors will reach an agreement. During its economic crisis around the turn of the millennium, Argentina threatened to unilaterally suspend interest payments, but finally Argentina, the IMF and a conference of creditor representatives set up a new payment plan. See Paris Club for details.