General Raymond Odierno will take charge of US-led forces in Iraq on Tuesday from David Petraeus, the commander credited with pulling the violence-wracked country back from all-out civil war.
   
General Petraeus hands over command after heading a controversial US military "surge" strategy by President George W. Bush that helped to stem the daily bloodletting in Iraq which killed tens of thousands of people.
   
The formal handover ceremony will be attended by top Iraqi and US officials, including Defence Secretary Robert Gates who made a surprise visit to Baghdad on Monday.
   
As Odierno takes charge, Petraeus becomes the new chief of Central Command with responsibility for US troops from the Horn of Africa to Central Asia, including the conflicts in both Afghanistan and Iraq.
   
Iraq was spiralling into an all-out civil war when Petraeus took over as commander in February 2007, almost four years after the invasion that toppled executed dictator Saddam Hussein.
   
But since late last year violence has fallen significantly to a four-year low, and much of the credit has gone to the counter-insurgency strategies of Petraeus.
   
On Monday, Gates credited Petraeus's "brilliant strategy" and its implementation by US troops and field commanders for the success of the surge.
   
"I think he's played a historic role. There is just no two ways about it," he said.
   
General Petraeus oversaw the surge, but it was his former deputy Odierno who first proposed it in December 2006 to a resistant Pentagon, setting the stage for what would become a pivotal turn in the unpopular war.
   
Odierno, a hulking artillery man criticised for running roughshod over civilians during his first tour to Iraq in 2003-2004, implemented the "surge" strategy as the corps commander from December 2006 to March 2008, which Gates said made him the right person to replace Petraeus.
   
Odierno carried out the detailed counter-insurgency campaign that poured US troops into Baghdad, cleared Al-Qaeda insurgents from havens in communities surrounding the capital, and targeted Shiite extremists.
   
By the time he left Iraq 15 months later, levels of violence had begun their downward plunge.
   
"Just as important as the surge was the change in our tactics, techniques and procedures that got us back out in the neighbourhoods," Odierno told reporters at the end of his previous tour in March.
   
The imposing, six-foot-five (1.99-metre) Odierno will have his hands full as he takes command of the 146,000 troops on the ground.
   
He takes charge at a time when according to Gates the forces are on a "mission in transition" as troop numbers shrink with more and more provinces being handed back to Iraqi control.
   
"There is no question we will still be engaged as we are, but the areas in which we are seriously engaged will I think continue to narrow," Gates said.
   
"And the challenge for General Odierno is how do we work with the Iraqis to preserve the gains that have already been achieved, and expand upon them even as the number of US forces are shrinking."
   
Iraq currently handles security in 11 of its 18 provinces with plans to take over a couple more by the end of the year. But the country still remains fragile.
   
On Monday, within minutes of Gates' arrival two car bombings in Baghdad killed at least 12 people and wounded dozens more.
   
And later a woman suicide bomber killed 32 people and wounded dozens when she blew herself up in a crowd of people breaking the Ramadan fast in the town of Bala Druz in restive Diyala province.
   
Last week Petraeus told AFP in an interview that he was leaving behind a "significantly improved" Iraq but still one which was vulnerable to "lethal attacks by Al-Qaeda and Shiite militias."
   
Odierno has argued consistently against sharp cutbacks in US troop levels in Iraq.
   
But the 146,000-strong US force in Iraq will shrink by about 8,000 troops by January, when Bush turns over the presidency to his successor. Pressure for further reductions is likely to intensify as attention shifts toward Afghanistan.