From an economics perspective, bank collapse was ultimately brought on by Senate Democrats. While Bush did have a Republican Congress for most of his administration (until 2006 when the House flipped) he didn't have the veto-proof majority that Obama had until 2010. There were several cases starting in 2001, before 9/11 where the administration testified before Congress in an attempt to get them to more closely regulate the GSEs. This was repeated in 2003 and again in 2005. Finally in 2005, McCain drafted legislation but never got enough votes to even try for cloture so it could go for a vote. After 2006 there was no chance.
If I could point to one single problem that resulted in the US mortgage collapse it would be the allowance of giving adjustable rate mortgages to "non-traditional" borrowers. If I could point to the one thing that made it worse, it was a misuse of TARP and the quantitative easing policy of the federal reserve in QE1.
But we also have to put this in a global context. The financial collapse in Europe has nothing to do with US mortgage markets. That collapse is due to plain old financial irresponsibility. If you look at the economies of Europe, the countries that are doing best are the ones did NOT go for the "stimulus" model and instead chose the so-called "austerity" model. The ones that scaled back benefits and the size of government, got their budgets under control, endured a little shrinkage for a period of time, now see their economies expanding at a sustainable rate. Those that chose the "stimulus" model see their problems getting even worse as their debt levels have skyrocketed and the cost of servicing that debt is now through the roof and eating up their tax revenues. France has since their recent elections decided to DECREASE their retirement age to 60. They are going to pay a heavy price for that decision. 60 is too young and looking at European population demographics, they are looking at a shrinking working age population and an increasing pensioner population and this reduction just made that problem even worse. Hollande has made a decision that is going to absolutely destroy the French economy, but that is what Socialists do. There is not a single documented case of socialism improving the well-being of a country in an overall sense. There are many documented cases where throwing off socialism has greatly improved that well-being.
But has the US simply disallowed adjustable rate mortgages for government guaranteed loans, we would not be in the mess we are currently in. Had government purchased those loans when the banks started to collapse, converted them to fixed rate loans, and sold them back on the open market, we wouldn't be in trouble today.
Pabst Blue Ribbon - The beer that made Gerlach famous.