Its all about the CASH and CREDIT. Virtually for decades starting 1990s USA is run on the credit. Credit is the money we borrow from others with incentive of certain interest and payback. At this moment, China , Japan and some middle eastern countries are lending money to USA and Europe. Because of housing crisis , USA now is almost in a state of huge debt. It doesnot have Capital to pay back those trillions of dollars back. But to keep economy running it still needs money. Coincidently, world still believe in the capacity of $. We still feel more freedom to invest in USA then other countries.