On the surface, here's what is going on: Markets fear that nothing will be accomplished at this week's EU Summit and that the eurozone may collapse.
Drilling deeper down: The European Central Bank appears to have a policy of maintaining near panic, most likely in an attempt to provide cover for EZ members to move closer to a more centrally planned fiscal and banking EZ sector. If panic heats up, the ECB will move in and attempt to douse the fears and prop up the markets--a very dangerous game. One misstep and the EZdoes collapse.
As for the U.S., though the immediate downturn is a result of fears of U.S. problems as a result of collapse in the EZ, Bernanke would very likely step in to prop up U.S. banks, if there is a collapse in the EZ.
The real problem in the U.S. right now is that Bernanke has slowed money growth dramatically and that the previously created manipulated mini-boom is about to crash.