Another Sunni attack

This is the third major Sunni attack in three days. The internet cafe bombing killing 26, the market attack yesterday killing at least 40, and now this....

A suicide car bomber detonated explosives in a crowd of laborers gathered across the street from a major Shiite shrine in southern Iraq Tuesday, killing at least 53 people and wounding 105, officials and witnesses said.

And from the Reuters version

Police in the scene were pelted with rocks by angry crowds. Many appeared to be followers of radical Shi'ite cleric Moqtada al-Sadr, who has many supporters in the town. Kufa is near the holy city of Najaf, 160 km (100 miles) south of Baghdad.

The protesters chanted to the police: "You are traitors!" "You are not doing your job!" "American agents!."

And a new detail from yesterday's market attack.

The assault occurred a few hundred yards from Iraqi army and police positions, but the troops did not intervene until the attackers were fleeing, the witnesses said. They spoke on condition of anonymity because of fear of reprisals.....

The events also raised doubts about the effectiveness of the U.S. strategy of handing over large areas of the country to Iraqi control, while keeping U.S. troops in reserve.

U.S. troops of the 101st Airborne Division reported hearing detonations and gunfire, the U.S. command said. But Iraqi troops are responsible for security in Mahmoudiya, and American soldiers do not intervene unless asked by the Iraqis.

So, I'll go back to the question, how is the US supposed to stop this? How do you stop a man from pulling up in a minivan loaded with explosives? How do respond to a market attack when the Iraqi forces don't intervene? What is the mission? What is the mission on a tactical level?