In order to withdraw the deployment Bush is going to select Sadr City for a punitive offensive, or possibly a Sunni town like Ramadi or Tikrit. This will provide the political cover for talks with Sadr and Sunni tribes. and deliver the final parting shot before negotiating with the Sunnis Shia and the retreat to Kurdistan and the empty Western Desert.
there is a strong chance that any new offensive will be bloodily repulsed by the defending population.
Privately knowing that the US will have to pull garrisons out of Iraq the administration decides to hammer the capabilities of the resistance to 1) prevent attacks on evacuating US troops 2) punish Iraqi groups willing to fight 3) Allow the US to meet in talks from an apparent position of strength.
Sadr City and (for the sake of example) Ramadi are selected as the focal point of the offensive effort. Sadr is the indigenous leader of nationalistic shia, and Ramadi the hub of Sunni fighters. Attacking both at the same time seems insane but, any independent nationalists in Iraq are a threat to American post war planning.
Marines and Soldiers mass around the slums of Baghdad and the Ramadi and start pounding away at the perimeter...
Students of war will recognize this event in couple of examples. At the end of the Anglo-American War of 1812-14 The Battle of New Orleans took place after the parties knew the war was supposed to wound up. It was fought by the British, after terms had been tacitly agreed between the Americans and the British. The British reduced Americna defences around the Mississippi Delta and then proceeded to move in for the kill against Jackson's force guarding New Orleans. The British were bloodily repulsed. The battle was essentially a strategic waste of effort. The British were not going to reclaim any part of the Gulf Coast nor did they seriously wish to. The expedition was only punitive.
Another example: The destruction of Warsaw in 1944. With the Soviet army nearby the Polish home army decided to stage a revolt against the Germans, predictably the Germans savagely responded and the Home Army was slaughtered. The Russians sat by and waited for the Germans to exhaust themselves killing Poles, who probably would have killed by the Soviets later on. The Germans retook Warsaw and then promptly lost it to the Russians anyway. All along The Germans knew the Soviets would push them out. It was a campaign of pure spite and waste.
What then are the Slum dwellers of Baghdad likely to do when the Americans arrive to re-establish their military credibility before the talking and evacuation begins?
We can see the answer in Hezbullah's fight with Israel. Even though the Generals know that the effort will be a loss they will go ahead and fight the last battle. America will lose a few hundred troops and the Mahdi army will lose thousands. and the result will still be the same either way.