BAQUBA, Iraq -- When U.S. forces killed Abu Musab al-Zarqawi, the leader of al-Qaida in Iraq, six months ago in a village near here, they hoped security would improve in this strategic province just north of Baghdad.

Instead, security has collapsed in Diyala province, which ranks as one of Iraq's most troubled regions. Insurgent attacks have more than doubled in the past year. Violence has devastated the provincial police force and brought reconstruction to a virtual standstill.

Assassinations have claimed the lives of mayors, tribal chieftains, police officials and judges, including a Shiite member of the provincial council who was killed Tuesday. Many government officials sleep on cots in their offices because driving home is too dangerous.

And Iraqi security forces have been implicated in so many abuses that the U.S. commander here recently gave his Iraqi counterpart an angry lecture, likening the Iraqi troops to an "undisciplined rabble."

U.S. and Iraqi officials blamed the sharp downturn on U.S. neglect and abuses by the Iraqi army. U.S. troops largely disengaged from security for weeks at a time, they say, handing the reins to Iraqi forces who proved to be abusive and ineffective.

U.S. commanders are attempting a sharp change in strategy, hoping that a classic counterinsurgency campaign, combining reconstruction aid with a more active U.S. presence, can turn the situation around.

For now, insurgents appear to have gained the upper hand. They demonstrated their freedom of movement in late December by barreling a dozen trucks through the streets of Baquba's Amin neighborhood, shouting militant slogans and brandishing machines guns and shoulder-fired rocket launchers.

The show of force was similar to another insurgent parade caught on video by a U.S. aerial drone in November. Insurgents were seen hauling Shiite families out of their homes and executing them in the streets, U.S. military officials who reviewed the footage said.

Diyala is an area of fertile farmland, abundant water and untapped oil wells stretching north of Baghdad's suburbs and east to the Iranian border. Its population includes all three of Iraq's main religious and ethnic groups.

Of its roughly 1.8 million people, about 55 percent are Sunni Muslims. But because Sunnis boycotted elections two years ago, Shiites, who make up about one-third of Diyala's population, hold the majority of provincial council seats and control the local security forces. Kurds, mostly in northeastern Diyala, make up about 15 percent.

Until October, the main U.S. force in the province was the 4th Infantry Division. It largely followed the strategy laid down by top U.S. commanders in Iraq last year: Pull American forces back as much as possible and allow Iraqi troops to take the lead in fighting insurgents. U.S. officers here say that approach did not work.

Iraq's Shiite-dominated government appointed a provincial commander who U.S. military officials say was handpicked by the Badr organization, a Shiite militia implicated in hundreds of death-squad killings in Baghdad. The Badr militia is linked to Iraq's largest Shiite political party, the Supreme Council for the Islamic Revolution in Iraq.

Under orders from the Iraqi Ground Forces Command in Baghdad this fall, the commander, Brig. Gen. Shakir Hulail Hussein Kaabi, and his 5th Iraqi Division started a campaign of what U.S. officials describe as abusive raids and detentions.

The problems were so serious that Col. David W. Sutherland, the commander of the 3rd Heavy Brigade Combat Team, took the unusual step of lecturing his Iraqi counterpart during a mid-December briefing.

"Bullying an innocent person is unacceptable. Taking things from houses is unacceptable. Taking cars or things from cars is unacceptable," he said. "We are soldiers, not barbarians."

Since Col. Sutherland took command of Diyala in October, he has increased the number of embedded U.S. advisers with Iraqi Army units and required U.S. approval for any Iraqi operations, effectively rescinding Iraqi control of the 5th Division. Col. Sutherland said several joint raids convinced him that Gen. Shakir was willing to change his tactics and adopt a counterinsurgency doctrine of proportional force.

"In this culture, the more you kill, the more enemies you make. The more you treat with disrespect, the more enemies you make," Col. Sutherland said. "And we were able to show [Gen. Shakir], not subjectively, but objectively, how that happened and what it created."

American commanders won at least a partial victory in late December when the government agreed to replace Diyala's police chief. The chief, Ghassan Bawi, has been accused of tacitly or directly supporting death squads in the province, according to U.S. officials.

Detainees reported kidnappings and torture at the hands of Iraqi policemen, U.S. and Iraqi officials said.

Gen. Shakir said he had changed his tactics and now used more focused operations. But he clung to the view that his main targets were Sunni Arabs, not Shiites.

"The nature of the target is that they are all Sunnis," Gen. Shakir said. "All these problem areas are all Sunni, so our operations are all in Sunni areas. There are actually no Shiites left, because 8,000 Shiites have been killed or displaced."