As this Washington Post article notes, U.S. Absence Seems To Make Difference:

Sadr City is a largely impoverished section of Baghdad that is home to about 2 million people, many of whom support the anti-American Shiite cleric Moqtada al-Sadr, a onetime backer of Maliki who has become his chief rival. Sadrist officials negotiated the entry of Iraqi troops, apparently winning agreement that U.S. forces would stay out.

This is not by accident. While the American public is shielded from this, it is clear that American deployment stirs up nationalistic violence, and it is also clear that, notwithstanding US protestations of precision, the carpet bombing of whole neighborhoods that provide any level of threat make the violence a couple of magnitudes worse beyond that.

We need to leave now.