USA's banking laws and regulations allowed banks to go absolutely crazy and invent reams of ridiculous, fraudulent, dangerous, and null financial products (despite being incredibly profitable, they were basically making money out of thin air, just by valuing shuffling debt across the world.)
The US was THE FIRST COUNTRY to engineer these kinds of financial instruments, and the government (Chosen by YOU, the American people) set up there financial system to allow these kinds of horrible things to take place.
Obviously, because the US was generating so much damn money, every other country followed suit and altered their economies to allow for the reciprocation of these so called "toxic assets".
The US set an example for the rest of the world. A bad one. And now, and for the next half century (that's if we can swing things around [doubtful]) the entire world will be paying for it. This is just the beginning.