Does the U.S. Have a Moral Obligation in Iraq?

Let's pretend the U.S. pulled out of Iraq in the coming months because the President had a huge change of heart. 90% of our troops are gone, by, let's say July 2009.

What happens then?

Let's say, for the sake of discussion, all out civil war breaks out in Iraq, but is limited to that country.

Hundreds and thousands of Iraqis are dying every day, every week, every month. Does the U.S. have a moral obligation to step in and try to resolve the fighting?

Or instead, do we sit back and watch, and wait for the rest of the region to potentially join in?