Perhaps I can help. With our dependence on foreign oil over the years we have sent literally trillions of dollars over seas. In either cash or US debt obligations.
Remember the Dubai port deal and the stink that arose over that?
Now a couple of months later Dubai coughs up $7.5B to bail out Citibank and seldom is heard a discouraging word.
Central Banks / governments around the world have vast sums of US cash and or debt obligations that they have converted into "sovereign funds".
It's conceivable that any one of them could make a run on the bank/Fed and our economy would crash in a heart beat.
The only thing preventing it is the so called status quo. Or lets not upset the apple cart.