The hulking blast walls that the Americans have set up around many neighborhoods have only intensified the city’s sense of balkanization. Merchants must now hire a different driver for individual areas, lest gunmen kill a stranger from another sect to steal a truckload of T-shirts.

To study the full effects of the troop increase at ground level, reporters for The New York Times repeatedly visited at least 20 neighborhoods in Baghdad and its surrounding belts, interviewing more than 150 residents, in addition to members of sectarian militias, Americans patrolling the city and Iraqi officials.

They found that the additional troops had slowed, but far from stopped, Iraq’s still-burning civil war. Baghdad remains a city where sectarian violence can flare at any moment, and where the central government is becoming less reliable and relevant as Shiite or Sunni vigilantes demand submission to their own brand of law. “These improvements in the face of the general devastation look small and insignificant because the devastation is so much bigger,” said Haidar Minathar, an Iraqi author, actor and director. He added that the security gains “have no great influence.”

The troop increase was meant to create conditions that could lead from improved security in Baghdad to national reconciliation to a strong central government to American military withdrawal. In recent weeks, President Bush and his commanders have shifted their emphasis to new alliances with tribal leaders that have improved security in Diyala Province, the Sunni Triangle and other Sunni areas, most notably Anbar Province.

That area, not Baghdad, was the one Mr. Bush conspicuously chose to visit this week.

But when he announced on Jan. 10 his plan to add 20,000 to 30,000 troops to Iraq, Mr. Bush emphasized that Baghdad was the linchpin for creating a stable Iraq. With less fear of death in the capital, “Iraqis will gain confidence in their leaders and the government will have the breathing space it needs to make progress in other critical areas,” he said.