For the most part the war is nearly over. American and Iraqi casualties have been the lowest last two years.
The Iraqi government itself has asked the US military for a date in which to leave.
With things becoming increasingly stable in the area the US has no further reasons to stay. Obama knows wars are expensive and has been talking with our military commanders for the best timing to end it. *should he be elected* I'd expect half our troops or more will be home in a year.