The problem is not so much that the US is printing money when they want ... the Fed isn't as freewheeling as that.
The problem is that as a fiat currency, only the underlying strength of the country's economy gives the dollar any remaining value. With the debt levels the US is building up, along with the US hardly making anything of its own anymore but is now simply giving the paper money to poor (cheap labor) countries to do everything for them, means that over time other countries will care less and less about the US dollar. With the US producing less and less that the rest of the world wants, they are making themselves increasingly irrelevant to the rest of the world's economy.