American's financial force

Trade imbalance has nothing to do with housing bubbles. Even china has a housing bubble!
As the 'middle' class has increased in size, and as the world has been stable for a while (no great world wars), most people have let their greeds dominate. No one wants to save cash. There was a wonderful article on NPR, "Big money got hungry", it says there is about 30 trillion dollars running around looking for a place to invest.
Wall Street bankers just milked this situation with Fed's help. A bankers have no ethics, so given an opportunity, they will happily lie to make a buck. So Wall Street started using the age old 'house prices always go up' belief of the world to sell mortgage backed securities to the gullible. This created a demand for mortgage backed securities, which banks gladly satisfied by creating more,, even if it meant creating that mortgage with sub-prime borrowers. The banks were making money off the transaction.
Freddie and Fannie ended up buying 6 trillion dollars of this junk. But they didn't care either. Freddie Fannie officials are very comfy with our Congress, so they just kept on showing profit and giving themselves bonuses.
So, sub-prime loans ended up either in retirement funds or in Freddie/Fannie. Housing bubble continued. Of course, at some point, the 'sub-prime' borrowers got exposed (by failing to pay) and the whole thing collapsed.
The Fed, by keeping interest rates low, forced people to look for such shady investments, and also made such shady investments lucrative. Finally, when the bubble collapsed, Bernanke, by bailing out his overlords at goldmansachs and by flooding the market with money (and hiding 1.2 trillions dollars of Freddie/Fannies junk morgtages) has now re-inflated the bubble, using the same trick, -even lower interest rates.
Bernanke is not a very clever or insightful man. He is popular with Wall Street and Obama because he listens to them. The term paper he wrote on the Great Depression, and got a PhD, only means that, ... well, ..he is a typical PhD. That does not make him intelligent. But he is still in power because the traders (GoldmanSachs...) and home builders are being allowed to flush their garbage holdings in this second bubble.
So yes, Fed's low interest rates definitely help fuel the bubble, and still is.