BAGHDAD, Iraq — Thousands of American troops were conducting house-to-house searches in Baghdad yesterday as the city’s descent into chaos forced them to abandon plans to entrust security to Iraqi forces.

Their mission, as described by the commanding general, laid bare the reality of what has taken place since Saddam Hussein was driven from Iraq’s capital more than three years ago. Implicitly accepting previous failures, Lieutenant General Pete Chiarelli ordered his forces to “take back Baghdad.”

America had aimed to become a less visible presence, with its troops increasingly staying in their bases.

But as the world’s attention has been focused on Lebanon, the situation in Baghdad has deteriorated to the point where America has abandoned this stance and sent thousands of troops back on to the streets.

Sectarian fighting pitting Shiite against Sunni — with at least 19 people killed in bomb attacks across the capital yesterday — has raised the specter of total unrest. This internecine warfare has raged since the destruction by Sunni insurgents in February of the Golden Mosque in Samarra, a revered Shiite shrine.

Escalating ethnic cleansing has ensued as combatants in Sunni neighborhoods force out minority Shiite families, and Shiites do the same to Sunnis in their strongholds, resulting in battle lines being drawn across the city.

The situation is so grave that the government is discussing a formal partitioning of the warring communities, a move that could lead to as many as 2 million of the 7 million residents being driven from their homes.

Last month, according to the Iraqi government, 3,438 Iraqis were killed, a 9% rise from June. The real figure was probably far higher as most deaths are never reported, and the bodies are dumped in rivers or sewage works.

After such monumental loss of life, the American bid to regain control of Baghdad with Operation Together Forward is “the defining battle of this particular campaign,” General Chiarelli said. The Iraqi security forces, either due to incompetence, fear, or collusion with the killers, had failed to cope.

A week after the launch of the new campaign some of the most violent southern and western neighborhoods were transformed into mini-prisons, sealed off with blast walls and concertina wire.

Inside these cordons, American soldiers accompanied by Iraqi troops conducted house-to-house searches. Around 15,000 houses are to be searched in the Sunni insurgent center of Dora alone. In the coming months, Zafraniya, Adhamiya, Kadhimiya, and Mansour, the most violent neighborhoods, will all be searched.

In Dora, locals described how the troops poked inside cushions and dug in gardens for hidden weapons.

“They were polite,” one said, “but went through everything, even my wife’s clothes.

“They did not speak Arabic but had a machine on which they would type words, and then a voice would say them in Arabic. I had never seen such a thing before.”

Another, a former translator for a western firm, described how the Americans called for their commanding officer when they discovered that he could speak English.

“He asked me for help to find the bad guys,” he said. “I refused.I am not stupid.”

“Me and my family would be killed most horrifically if they learned I helped the Americans, but the officer was very sad. He said he understood, but he also said, ‘It is so difficult. We do not know where to look. All we want to do is help, but we do not know who to target.'”

This lack of assistance and detailed intelligence may — like an earlier Iraqi security push six weeks ago — doom the American military’s mission to failure, even if restoring order is possible in a city where sectarian groups are now so polarized by mutual fear and hatred.

Not only are the locals proving unhelpful, but the Shiite-dominated government of Prime Minister al-Maliki criticized the American military last week when it tried to spread its security plan to the massive Shiite neighborhood of Sadr City, dominated by the militia of Moqtada al-Sadr, which is blamed by Sunnis for most of the killing.

The blast walls were taken down, and the searches halted even though Washington and London warn that Mr. Sadr is modeling his anti-western movement on Hezbollah.