Baghdad, Iraq ? A new and potentially worrisome fight for power and control has broken out in Baghdad as the United States prepares to pull combat troops out of Iraq next year.

The struggle, which played out in fierce weekend clashes, pits two vital American allies against each other. On Sunday, Iraqi soldiers backed by U.S. combat helicopters and American troops swept into a central Baghdad neighborhood, arresting U.S.-backed Sunni fighters in an effort to clamp down on a two-day uprising that challenged the Iraqi government’s authority and its efforts to pacify the capital.

But the fallout from the operation is already rippling far beyond the city’s boundaries. Both the Iraqi security forces and the Sunni fighters, known as the Awakening, are cornerstones in the American strategy to bring stability. The Awakening, in particular, is widely viewed as a key reason violence has dramatically dropped across Iraq.

Many leaders of the Awakening, mostly former Sunni insurgents who joined hands with U.S. forces to fight the insurgent group al-Qaida in Iraq, have long had a contentious relationship with Iraq’s Shiite-led government. But the weekend battles have sparked fresh frustration and mistrust of both the U.S. military and Iraq’s mostly Shiite security forces, according to interviews with Awakening leaders across the country.

“The situation is now very fragile, and no Awakening member would remain silent over this injustice,” said Saad Abbas al-Luhaibi, leader of an Awakening group in Anbar province. The tensions raise concerns that uprisings could erupt in other Awakening-controlled areas — or that many Awakening fighters could return to the insurgency, allowing al-Qaida in Iraq to fill the vacuum in Sunni areas.

The clashes also opened a window onto the new military relationship emerging between the United States and Iraq, as well as the struggles Iraq’s government will likely face as it takes more control over security.

The violence erupted Saturday minutes after Iraqi and U.S. troops arrested Adil Mashadani, the Awakening leader in Baghdad’s Fadhil neighborhood, on charges of committing sectarian crimes and terrorist acts.

The U.S. military said in a statement Sunday that Mashadani was suspected of extorting more than $160,000 from Fadhil residents, orchestrating roadside bomb attacks against Iraqi security forces and having ties to al-Qaida in Iraq. The military stressed that Mashadani was not arrested because of his role in the Awakening. Mashadani’s deputies have denied the allegations.