Baghdad - US Defence Secretary Robert Gates arrived in the Iraqi capital Baghdad on Monday on the eve of a shift in command of the US-led forces.

US General David Petraeus is spending his last day in Iraq on Monday as commander of US-led forces.

In a ceremony that will be held today, and attended by Gates, Petraeus will hand command to his former deputy Lieutenant-General Ray Odierno.

Odierno served as Petraeus right hand for 15 months until February. He will be promoted to full general on Tuesday.

Petraeus led the Army's 101st Airborne Division during the 2003 invasion of Iraq.

He was specially appreciated for his management of the occupation forces in the northern part of the country, including the restive city of Mosul.

He also led efforts to train a new Iraqi army and was a major contributor to the writing the Army's counterinsurgency manual.

Petraeus term, which started in early 2007, was marked by a decline in violence in Iraq.

Backed by 30,000 extra US soldiers, Petraeus and Odierno, implemented a new counter insurgency strategy that, combined with other factors, improved drastically the security situation in the country dubbed as "the most dangerous country in the world."

While political observers believe deadly attacks in Iraq are not yet history, they admit that the country is way beyond the civil-war wreck it had been in the post-invasion era.

According to reports, violence in the Iraqi capital had fallen to its lowest level since 2004.

The spokesman for Baghdad Operations Command, General Qassim Atta, meanwhile said in August that military operations have dropped by 89 per cent in August 2008, compared to the same month of 2007 and 2006.

This drop in violence prompted US President George W Bush to plan to withdraw an additional 8,000 troops from Iraq by February.

The pullout will reduce the US presence to about 138,000 soldiers. Odierno's main task, thus, according to observers, will be how to maintain the gains of his predecessor as the number of US forces declines and Iraqis are handed over more of their country.

In a farewell letter posted on the Web site of the Multi-National Force in Iraq, Petraeus described US troops achievements in Iraq as "the stuff of history."

Petraeus dedicated the bulk of his letter to praising the achievements of his forces in Iraq.

He wrote that when he took command in February 2007 he noted that the situation in Iraq was hard but not hopeless.

"You have proven that assessment to be correct. Your great work, sacrifice, courage, and skill have helped to reverse a downward spiral toward civil war and to wrest the initiative from the enemies of the new Iraq," the letter read.

"You have not just secured the Iraqi people, you have served them, as well," Petraeus writes, describing his forces as "builders and diplomats as well as guardians and warriors," the letter added.

Petraeus, however, acknowledged that with all these achievements challenges remain. "(US-led forces') tasks in Iraq are far from complete and hard work and tough fights lie ahead," he noted in the letter.

In the meantime, Petraeus is being promoted to head of US Central Command, which operates in the Middle East expanding his area of responsibility to 25 countries including Iran, Afghanistan and Pakistan.

His promotion follows the March resignation of Admiral William Fallon following news reports that he was at odds with the White House over how to handle Iran. (dpa)