From CSIS, 26 Jul 06: The Gains and Risks in Sending in More US Troops
...The US and Iraqi forces are unlikely to be able to do more than buy time in Baghdad, or anywhere else, unless they can operate in a climate where there is a major move towards a new political bargain between Iraq's factions, where people can see reconciliation working at least at the top, and there is hope that the government is finally becoming effective. The political dimension must move in tandem with the military and security dimensions, and it is not. The fact that Iraqis voted to divide by sect and ethnicity—Arab Sunni, Arab Shi'ite, and Kurd—remains the driving reality. It is being increasingly compounded by intra-Shi'ite tensions, particularly Moqtada al-Sadr's factions but also tensions between Dawa and SCIRI.

This lack of the political dimension necessary to succeed in the military and security sectors is particularly critical in Baghdad and its surrounding areas—although Basra, Kirkuk, and Mosul all have their own growing divisions. The more mixed the city, the greater the tensions, and Baghdad has at least 5 million people, and possible now some 7 million in the greater Baghdad area and surrounding towns. Some 15-20% of Iraq's people are now in major urban areas which daily requires them to divide to survive...