Bush knows that an Iraq Victory is impossible with Iranian support of the bad guys
The US hasn't withdrawn troops, merely moved them around the country, particularly as the new Iraqi troops and police have been deployed. There is one possible reason for this. I think the liberation of Iran will be much more intense, that is, there will be actual and coherent formations of enemy troops, and for a longer time. And of course, it will bring about wider and open civil war in Pakistan, fomented by groups supported and recruited by Iran. Also, I'd be surprised to see Kurdistan remain as quiet as it is now.