As Concisely As I Can Put It...

The reasons for the introduction of foreign forces in Iraq (WMDs, connections to al Qaeda) have been completely debunked.

The forces in Iraq have not been given the resources to achieve the mission that they have been tasked with. The administration has displayed very little ability to provide these resources. One might even go so far as to say that these resources are being withheld so as to facilitate a situation which can be used to justify the continued presence of these forces indefinitely, and even the expansion of them ("surges").

The mission in Iraq is a flawed one that has not been defined, neither initially or over the course of four years of redefinition and renaming.

The continued presence of foreign forces in Iraq is only further intensifying frustration on the part of the Iraqis. All they see is an occupation that has no foreseeable end and policies being put into place that will strip the Iraqi people of any significant claim to the natural resources of the country.

The answer to the question of whether more terrorists will be created in Iraq if the US withdraws now has to be closely compared to the answer to the question of how many terrorists and insurgents are being created and emboldened by the presence of the US right now. At best, it's a push. More likely, it's worse now than it would be if and when the occupying forces are withdrawn.