U.S. military hits a wall in Sadr City

But the mission ended before it began. Machine gun fire blasted out from the third floor of a building along the route. A Bradley fighting vehicle fired back, sending a thunderous roar through the neighborhood of middle-class homes and businesses. Then, the lead tank hit a roadside bomb.

As gunshots and grenade blasts raged in the night, the two Iraqi construction workers accompanying the troops quit.

Army Capt. Alan Boyes wasn't worried. None of his men were injured, and at $500 a day, he knew that the contractors hired to operate a crane to install 6,000-pound slabs of the wall would be back or that others could be found to replace them. But the violence that night and several attacks since highlight the hurdles American troops face as they try to take on fighters loyal to Shiite cleric Muqtada Sadr without plunging into the heart of his stronghold and sparking an all-out uprising of his heavily armed followers.

"Everyone knows we won't go past Route Gold," Boyes said, referring to the street along which the wall is being built, separating more than two-thirds of Sadr City from a rectangle where U.S. forces occupy a smattering of small bases. "It's a political thing."

It is also the same position the U.S. faced 15 months ago, when the first of 28,500 additional American troops arrived in Baghdad to help quell violence. At the time, commanders opted to not pour troops into Sadr City as they had done in other trouble spots, fearful that it would spark a bloody backlash from Sadr's Mahdi Army militia.

Little has changed in the 11-square-mile corner of northeast Baghdad, but the stakes are higher now. An Iraqi military offensive launched against Shiite militias in late March has drawn in U.S. troops and has led to near-constant fighting in Sadr City. Sadr has threatened "open war" if the offensive does not end. U.S. troop deaths have climbed to their highest level in seven months, mainly because of the clashes in and around Sadr City, and the additional American troops will be gone by July. On Saturday, the Iraqi government said it had struck a deal with Sadr's aides to halt the fighting, but the two sides disagreed on its terms and it was unclear what it would yield.