BAGHDAD -- First Lt. Justin John, 6 feet 4 and built like a linebacker, plopped down on a sofa in front of Ibrahim Suleiman al-Zoubaidi, one of the leaders of the mainly Sunni armed groups that have helped the U.S. military quell violence in Iraq since last year.

Mr. Zoubaidi had one thing on his mind: This week, Iraq's Shiite-led government will assume authority over the groups, which have been backed by the United States. "They will kill us," he declared. "One by one."

Across Baghdad, leaders of the groups speak about the transition in similarly apocalyptic terms. Some have left Baghdad, saying they fear that the Iraqi government will conduct mass arrests after the handover. Others are obtaining passports and say they will flee to Syria.

Lt. John, a 24-year-old platoon leader, tried to reassure the Iraqi. "It's a new thing," the lieutenant said. "It's going to take some time to get used to."

Recognizing that the government has been wary from the outset about the creation of armed, mainly Sunni groups under U.S. control, U.S. military officials are taking several steps to prevent their sudden disintegration. U.S. officials see the Sons of Iraq as a central factor in the reduction in violence, along with the temporary increase in U.S. forces, a year-long cease-fire imposed by a Shiite militia leader and the stepped-up assassinations of key insurgents.

Lt. John's unit -- 2nd Battalion of the 4th Infantry Regiment, attached to the 1st Brigade Combat Team, 4th Infantry Division -- has set aside funds to pay Sons of Iraq guards for 90 days, in case the Iraqi government does not.

U.S. soldiers say they will sit in as Iraqi officials hand out salary payments during the first few months. And the Americans have demanded that the Iraqi government refrain from arresting any of the Sunni fighters, many of whom are former insurgents, unless authorities have arrest warrants issued within the past six months. That will make it harder for the Shiite government to arrest Sons of Iraq leaders for acts committed before they joined forces with the Americans.

In recent weeks, U.S. military officials began shrinking the Sons of Iraq ranks by offering members micro-grants that amount to early-retirement packages. This month alone, Lt. John's company has handed out more than 30 grants, totaling more than $60,000.

"The big issue that concerns us is what happens if the government drops the ball and stops paying these guys," said Capt. Parsana Deoki, 32, of New York. "You'd have up to 400 SOI without jobs, without an income. That presents a problem. They have military training and access to weapons -- unemployed, with weapons, young men with an established chain of command. You can fill in the blanks."

The Sons of Iraq, also known as Awakening Councils, began in Anbar province in western Iraq in 2006, when tribal leaders joined forces with U.S. troops to fight the growing influence of al-Qaida in Iraq, a largely homegrown insurgent group that imposed a dogmatic brand of Islam.

Desperate for solutions to curtail the endemic violence that gripped Baghdad and several Iraqi provinces in early 2007, the U.S. military sought to replicate the Anbar model across the country. Awakening groups sometimes formed overnight, especially in places where networks of insurgents had weapons and a chain of command.

Mohamed Abdul Hussein al-Kurtani, a Sons of Iraq leader in Dora, had been running a local cell of the 1920 Revolution Brigades, an insurgent group that opposed the presence of U.S. troops in Iraq and had been dueling with the local cell of al-Qaida in Iraq.

"He decided it was a fruitless battle to work against us," said Sgt. Brian Bailey, 31, of Holbrook, Mass. "Instead of hiding in the shadows, they came out."

Mr. Kurtani, like most leaders, wears civilian clothes. Guards under his command wear tan uniforms and carry AK-47 assault rifles. Most Sons of Iraq are assigned to neighborhoods where they live and earn $300 to $500 a month.