Obligation to pay up more than is feasible. Same situation, albeit different cause.
Half of government spending operates by borrowing money. If would-be creditors do not believe the money will be paid back, they won't lend the money. When the money can't be borrowed, the money is printed. This is beginning, hence QEn. If other nations begin to collapse outright (see Greece), creditors will clam up - but the entitlement hordes will still demand their payouts, and bureaucrats will still expect paychecks, and both will threaten riots if not paid, so the money will flow from the printing presses.
There is a huge difference between "it's available, albeit expensive" vs. "it's not available". We can argue all we want about whether hyperinflation will happen, but so long as money can be borrowed it won't happen. The moment there's a bond sale and nobody buys, the presses fire up and the value of the dollar plummets.
Bears repeating. That day will come.....likely soon.