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A company of Iraqi soldiers abandoned their positions on Tuesday night in Baghdad’s Sadr City, defying American soldiers who implored them to hold the line against the Shiite militias.

The retreat left a crucial stretch of road on the front lines undefended for hours and led to a tense series of exchanges between American soldiers and about 50 Iraqi troops who were fleeing.

Capt. Logan Veath, commander of Company B, the U.S. unit on the scene, pleaded with the Iraqi major who was leading his troops away from the Sadr City fight, urging him to return to the front.

“If you turn around and go back up the street those soldiers will follow you,” Veath said. “If you tuck tail and cowardly run away they will follow up that way too.”

Veath’s pleas failed, and senior American and Iraqi commanders mounted an urgent effort to regain the lost ground. An elite Iraqi unit was rushed in and, with the support of the Americans, began to fight its way north.

This episode was a blow to the American effort to push the Iraqis into the lead in the struggle to wrest control of parts of Sadr City from the Mahdi Army militia and what Americans and Iraqis say are Iranian-backed groups.

That approach was intended to build up the Iraqi military’s fighting capacity and put an Iraqi face on the operation in Sadr City, a Baghdad bastion of support for Moqtada Sadr, the anti-American cleric.

Tuesday’s desertions in Sadr City, although involving a particularly hesitant Iraqi unit, left many of the American soldiers wondering about the tenacity of their Iraqi allies.

“It bugs the hell out of me,” said Sgt. George Lewis, the platoon sergeant for Company B’s 3rd Platoon. “We don’t see any progress being made at all. We hear these guys in firefights. We know if we are not up there helping these guys out we are making very little progress.”

One big problem in Sadr City is that the Iraqi troops have responded to militia gunfire with such intense fusillades that the soldiers have endangered civilians, U.S. soldiers and even their own forces. It has become such a regular occurrence that some U.S. troops are worried that militia fighters have tried to insert themselves between nearby Iraqi units to induce the Iraqi soldiers to fire on one another.