BAGHDAD—American forces on Monday handed over security responsibility to the Iraqis in a province that the U.S. once feared was lost — a sign of the stunning reversal of fortunes since local Sunnis turned against al-Qaida in Iraq.

But a Sunni Arab leader criticized the Shiite-led government of Prime Minister Nouri 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-Qaida in Iraq, marked a dramatic milestone in America’s plan to eventually hand over all 18 provinces to Iraqi control so U.S. 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-Qaida 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-Qaida has not been entirely defeated in Anbar, but their end is near and they know it,” Marine Maj. Gen. John Kelly, the senior U.S. 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,” 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-Qaida used the Euphrates River valley as a corridor for smuggling weapons, fighters and ammunition from Syria into the Sunni heartland and on to Baghdad.

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, a U.S. intelligence report widely leaked to journalists concluded that American forces were powerless to curb the rising power of al-Qaida in Anbar.

All that reversed dramatically months later when Sunni tribesmen, fed up with al-Qaida’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 U.S. officials with helping curb the bloodshed that had pushed Iraq to the brink of all-out civil war.