The violence in Iraq mounts by the minute. Media reports paint a grim picture, but on-the-ground it's even worse. A sunni blogger at Healing Iraq writes that the Sunni neighborhoods have erupted into street clashes between Interior Ministry troops and armed Sadrite mobs. over 100 mosques have been destroyed in reprisal attacks, and many Sunni Imams have been killed. Iraq the Model essentially corroborates it, discussing the role of Sadr in the attacks and the political fallout it's causing by dismantling the nascent Iraqi government. Whoever pulled this off knew exactly what they were doing, and they suceeded. The goal of pitting the Shiites against the Sunnis and undermining the new government is achieved, without a doubt. In this kind of environment, the U.S. has a real Sophie's choice. It can:
1) Try to stop the armed Shiite mobs. If US forces fire on these groups, who are acting with help from the Interior ministry police and others in carrying out their attacks on Sunnis, it risks inflaming the situation further. Instead of directing their anger at the Sunnis, Shiites will likely turn on U.S. soldiers as well. It also further undermines the Iraqi government by putting it between its "constituents" and the U.S. forces it is relying on for security.
2) Let the Shiites commit their violence. This one is equally bad. First because who knows how long this will go on. Second because this will be the last straw to the Sunnis. If they see that the new political order and the U.S. doesn't care while armed mobs roam about killing them and destroying their mosques, joining the insurgency doesn't become as bad as it may have seemed before. If the Sunnis lose all hope in the new political order, the insurgency will probably get a further lease on life and stronger support.
So which one? There's really no hope of a peaceful solution unless the Shiites decide to calm down, but they're not likely too. Then there's the idea that this could get worse and mushroom into a full-scale war beyond the armed skirmishes now taking place. Again, whoever did this knew exactly what they were doing, and they accomplished it.