This explains it about as well as one can in a single paragraph:

“I recognize that Americans want our troops to come home as quickly as possible. So do I. Some contend that we should set a deadline for withdrawing U.S. forces. Let me explain why that would be a serious mistake. Setting an artificial timetable would send the wrong message to the Iraqis – who need to know that America will not leave before the job is done. It would send the wrong message to our troops – who need to know that we are serious about completing the mission they are risking their lives to achieve. And it would send the wrong message to the enemy – who would know that all they have to do is to wait us out. We will stay in Iraq as long as we are needed – and not a day longer.