BAGHDAD — American forces on today handed over security responsibility to the Iraqis in a province that America once feared was lost — a sign of the stunning reversal of fortunes since local Sunnis turned against Al Qaeda in Iraq.

But a Sunni Arab leader criticized the Shiite-led government of Prime Minister al-Maliki for failing to embrace its newfound allies, underlining the threat that sectarian tensions still pose to a lasting peace.

Nevertheless, the transfer of Anbar province, the cradle of the Sunni insurgency and the birthplace of Al Qaeda in Iraq, marked a dramatic milestone in America’s plan to eventually hand over all 18 provinces to Iraqi control so American troops can go home.

The 25,000 American troops remaining in Anbar will focus on training Iraq’s military and police forces and standing by to help if the Iraqis are unable to cope with any surge in violence.

The ceremony was held under tight security in the center of Ramadi, the provincial capital where American troops fought ferocious battles with Al Qaeda and other Sunni insurgents until the tide turned in 2007.

“This war is not quite over, but it’s being won and primarily by the people of Anbar. Al Qaeda has not been entirely defeated in Anbar, but their end is near and they know it,” Marine Major General John Kelly, the senior American commander in Anbar, said during the handover ceremony.

President Bush hailed the handover as a major achievement, saying the once-violent province had been “transformed and reclaimed by the Iraqi people.”

“Iraqi forces will now take the lead in security operations in Anbar, with American troops moving into an overwatch role,” Mr. Bush said in a statement. “This achievement is a credit to the courage of our troops, the Iraqi security forces, and the brave tribes and other civilians from Anbar who worked alongside them.”

Anbar became the 11th province to revert to Iraqi security control, but it is the most significant because it borders Baghdad. The others have been in the peaceful Kurdish north or in the heavily Shiite south, which has proven less difficult for the Shiite-led government to control.

Anbar, a predominantly Sunni Arab expanse stretching from the western edge of the capital to the borders of Syria, Jordan, and Saudi Arabia, was long center stage of the war and a springboard for attacks inside Baghdad.

Al Qaeda used the Euphrates River valley as a corridor for smuggling weapons, fighters, and ammunition into the Sunni heartland and on to Baghdad from Syria.

The Anbar city of Fallujah fell under the sway of Al Qaida and other Sunni extremist groups and became the symbol of resistance until U.S. Marines stormed the city in November 2004 in the fiercest urban combat of the Iraq war.

But the loss of Fallujah did not deter the insurgents, who quickly rallied in Ramadi and other cities. In August 2006, an American intelligence report widely leaked to journalists concluded that American forces were powerless to curb the rising power of Al Qaeda in Anbar.

All that reversed dramatically months later when Sunni tribesmen, fed up with Al Qaeda’s brutality, turned against the movement and joined forces with the U.S. to drive the extremists from the province.

Those Sunni groups, known as “awakening councils,” became the model for similar grass-roots movements elsewhere in Iraq credited by American officials with helping curb the bloodshed that had pushed Iraq to the brink of all-out civil war.

Late today, a Sunni awakening leader in a nearby province, Imad al-Mashhdani, was wounded when a suicide bomber blew himself up near the sheik’s home 30 miles north of Baghdad, police said.

Nevertheless, the Shiite-led national government never fully embraced the Sunni turncoats, fearing they might turn their guns on Shiites some day. The government has moved in recent weeks to crack down on such groups, who drew their members from the ranks of former insurgents and veterans of Saddam Hussein’s security services.

During the handover ceremony, Ahmed Abu Risha, whose late brother spearheaded the Sunni revolt against Al Qaeda, alluded to strains over the crackdown, saying the government should appreciate the Sunnis’ role in curbing violence and not judge them because of “their positions” in the Saddam regime.

American troops had planned to hand over Anbar in late June but postponed the ceremony because of sandstorms and a suicide attack that killed three Marines and 20 Iraqis, including locally prominent sheiks.

Sunni politicians in Anbar had asked America to delay the transfer until next year because of a continuing power struggle between the awakening councils and the main Sunni political party over control of the province.

The No. 2 American commander in Iraq, Lieutenant General Lloyd J. Austin III, alluded to the power conflict, expressing hope that Al Qaeda would not exploit political rivalries to attempt a comeback.

“We know that while Al Qaeda may be in disarray, it is not yet completely defeated,” General Austin said during the ceremony. “And our common enemy is both patient and resilient. But I know the people of Anbar province will not allow Al Qaeda the opportunity to destabilize the security progress that has been made here.”