BAGHDAD — Violence in Iraq is at its lowest level in four years, but ask Capt. Mike Forbes, and he will tell you his job as a troop commander in Baghdad has gotten harder, not easier.

He spends less time worrying about roadside bombs and battling armed extremists than on his previous two tours to Iraq, and he and his soldiers are happy about that.

But now they are digging into less violent, albeit more complex problems that still hamper Baghdad's western Mansour district: working closely with local Iraqi officials to fix sewer systems and electricity and keeping corruption down in hopes of making the calm last.

"A quiet environment doesn't necessarily mean a less challenging one. It just gets more complicated," a boyish but weary looking Forbes said on Thursday, standing inside the small patrol base in Mansour that houses 30 members of B Troop, 4th Squadron, 10th Cavalry Regiment.

"The hotter an area is, the simpler," said Forbes. "You just go find the bad guys and kill them."

For many soldiers across Iraq, the recent drop in violence has meant a renewed focus on the rebuilding role the U.S. military first took on years ago, but often had to put aside, as it battled years of severe violence.

The chairman of the Joint Chiefs of Staff, Adm. Mike Mullen, last week called it the next urgent task across much of Iraq: helping the government turn recent security improvements into a longer-lasting stability by getting services to people and the economy moving.

Much of the effort is concentrated in small urban outposts like patrol base Washash in Mansour, a former Sunni insurgent stronghold, where B Troop operates. The soldiers serve as a quick-reaction force to back up local Iraqi soldiers and work closely with city officials to get basic services out.

For Forbes' soldiers, the task is complicated by the end of the U.S. troop surge. As American units pull out and head home, those left behind have to take on wider geographic responsibilities.

They also must, usually with just a few weeks' handover, manage the key relationships with Iraqi officials that have helped lead to the drop in violence.

That came sharply home Thursday during Forbes' first meeting with a local leader of the "Sons of Iraq," the former Sunni militants who once fought the Americans but switched sides and began to fight against al-Qaida.

The "Sons of Iraq" leader, sitting with Forbes in patrol station Washash's tight quarters, raged for several minutes about a Shiite police commander in Mansour who he said had threatened him and was extorting money from people in the area.

"You're my No. 1 commander, and if you can't back me up, I should quit," the former Sunni militant said to Forbes.

Then, gesturing to the pistol at his side, he threatened to kill the Shiite police officer himself if the officer is not replaced — exactly the type of violence U.S. forces want to prevent.

After the man had calmed down and left, Forbes said his predecessor had not briefed him on the man's gripes, so he had no idea whether they were true.

But maintaining good ties with "Sons of Iraq" leaders is key to keeping al-Qaida at bay so Iraqi and U.S. officials can focus on reconstruction.

"The transfer of relationships is the most difficult and also the most important," said Forbes. "In a week or two transition period (with the previous U.S. unit), things are going to get missed."

Neither the Sons of Iraq leader nor the military would reveal the man's name, to protect him from reprisals from al-Qaida.

In all, there are now about 145,000 U.S. troops in Iraq, down from a peak last year of nearly 170,000.

As if to remind the soldiers of the stakes, a car bomb exploded on a busy street not far from the patrol base as Forbes wrapped up his meeting. The blast outside a mosque killed two people and injured 10 others.

As Iraqi and U.S. soldiers converged at the scene, men in a nearby shop with shattered glass stared at the car's twisted hulk.

"It keeps you on your toes because it could all go south at any time," said 1st Sgt. Richard Shopp, the senior enlisted man at the patrol station.

Despite the challenges, Forbes remains optimistic that he can help the Iraqis rebuild their lives in his corner of Baghdad. Working with local officials to get small projects completed is starting to pay off, he said.

"They now have solar-powered street lights in Mansour," he said. "A year ago, they didn't even have street lights."