American commanders are trumpeting security gains in places such as the western Anbar province as a sign that their partnership with Iraqi security forces is working, and that the local troops can keep the country safe.
But fears are growing about a possible resurgence in sectarian tensions - fed by the Shiite-dominated government's plans to blacklist more than 500 parliamentary candidates over suspected links to Saddam Hussein's regime.
In Baghdad, U.S. Vice President Joe Biden met with Iraq's leaders Saturday to try to alleviate the pressures. While he kept expectations of a breakthrough low - telling reporters after a meeting with President Jalal Talabani it was up to the Iraqis, not him, to resolve the issue - his visit alone underscored Washington's concern.
The White House worries the bans could raise questions over the fairness of the March 7 parliamentary election, which is seen as an important step in the American pullout timetable and a way to break political stalemates over key issues such as dividing Iraq's oil revenue.
"I am confident that Iraq's leaders are seized with this problem and are working to find a just solution," Biden said during his visit.
The Marines formally handed over U.S. responsibility for Sunni-dominated Anbar, Iraq's largest province, to the Army during a ceremony at a base in Ramadi, the scene of some of the war's most intense fighting. Overall control of the province shifted from the U.S. military to Iraq in September 2008, but the U.S. continues to provide support for Iraqi forces.
Iraqi and American color guards stood together at attention as both countries' national anthems were played by a U.S. military band.
As many as 25,000 Marines were in Iraq at the peak of the fighting, mostly in Anbar province. Fewer than 3,000 remain. All but a handful of those will ship out in a matter of weeks.
The Marines' extended stay in Anbar went against the grain of the Corps' usual role as a fighting force designed to quickly seize territory and then turn it over to the Army to maintain control from fixed bases.
Sharing the front row at the handover ceremony with American Army and Marine generals were some of Anbar's influential tribal sheiks in traditional checkered headdresses and gold-embroidered robes. Their decision to shift support to the Americans is credited with sapping the Sunni insurgency - including al-Qaida in Iraq - of much of its strength in areas near Baghdad.
Maj. Gen. Terry Wolff, the Army commander who assumed responsibility for the province, said he hoped security gains cemented by U.S. troops and their Iraqi counterparts would ensure a smooth transfer despite the overall drawdown in American forces.
"The goal that we all seek is the Iraqis securing their own election, and that the election is fair and the election is free," he told reporters after the handover.
If all goes as planned, the last remaining Marines will be followed out by tens of thousands of soldiers in the coming months. President Barack Obama has ordered all but 50,000 troops out of the country by Aug. 31, with most to depart after the parliamentary election in March.
The remaining troops will leave by the end of 2011 under a U.S.-Iraqi security pact.
The changeover at Ramadi, 70 miles (115 kilometers) west of Baghdad, leaves the U.S. Army's 1st Armored Division with responsibility over both Baghdad and Anbar, the vast desert province that stretches from western Baghdad to the borders of Syria, Jordan and Saudi Arabia.
The province was once the heart of the deadly Sunni insurgency that erupted after the U.S.-led invasion in 2003. In the battles for control of the cities of Fallujah and Ramadi, the Marines saw some of the most brutal and deadliest fighting of the war.
Violence began dropping off in the province in late 2006 when Sunni fighters - known as Awakening Councils - turned against al-Qaida and sided with the Marines to fight the insurgency.
The upcoming parliamentary election is considered an important step toward speeding the U.S. troop pullout and seeking progress on stalled political initiatives. Among them: passing laws clarifying the rules for foreign oil investment and dividing the revenue among Iraq's main groups.
But plans to ban hundreds of candidates have raised deep concerns in Washington that the voting could widen rifts between the majority Shiites who gained power after Saddam's fall and Sunnis who are struggling to regain influence.
Biden, who arrived late Friday, had a full agenda of meetings with Iraqi leaders, including Prime Minister Nouri al-Maliki, who has strongly supported the blacklist and has resisted attempts at possible American mediation.
Some Sunni leaders have accused the Shiite-led government of using the ban as a political tool. But al-Maliki insists that Iraq must purge all ties to Saddam's Sunni-dominated regime. A vetting panel has put 512 names on the blacklist and more are expected.
Iraqi government spokesman Ali al-Dabbagh told The Associated Press that during the meeting with al-Maliki, Biden was careful not to "give the wrong message that America wants to interfere in the Iraqi affairs."
Biden later met with Talabani, who has asked for a legal review on the blacklist. The courts are expected to examine whether the vetting panel has legal grounding because it does not have formal parliamentary approval.
The panel includes two controversial Shiite figures: Ali al-Lami, who was once detained by the U.S. military over a 2008 attack in a Shiite district of Baghdad; and Ahmed Chalabi, who is blamed for supplying U.S. officials with faulty intelligence on Saddam's weapons program prior to the 2003 invasion.
Al-Lami is also a candidate in the March election - raising further complaints from Sunnis about possible political motives behind the list.