US General Raymond Odierno has taken charge of US-led forces in Iraq, warning that security gains in the country were fragile and reversible.
General David Petraeus, who is credited with pulling Iraq back from all-out civil war, handed over command of the 146,000-strong US force at a ceremony at a former Saddam Hussein-era palace turned US base near Baghdad airport.
But General Odierno said he was aware of the tough task ahead despite a dramatic fall in violence attributed to a military surge strategy.
'Iraq is now a different country from the one I had seen first. However, we must realise that these gains are fragile and reversible,' said General Odierno.
On the eve of the transfer, General Odierno was given a powerful reminder of the violence when a series of bomb blasts killed at least 34 people.
'He knows we are at a pivotal moment - where progress remains fragile and caution should be the order of the day,' US Defence Secretary Robert Gates said as he handed the multinational force Iraq flag from General Petraeus to General Odierno.
'And as we proceed further into the endgame here, I am sure he will make tough but necessary decisions to protect our national interest.'
General 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 all-out civil war when General Petraeus took over as commander in February 2007, almost four years after Saddam was toppled by US-led invading forces.
Petraeus credited for fall in violence
But since late last year violence has fallen to a four-year low, and much of the credit has gone to the counter-insurgency strategies of the 55-year-old general.
Mr Gates said General Petraeus took charge when 'darkness had descended on this land'.
'Merchants of chaos were gaining strength. Death was commonplace. Around the world, questions mounted about whether a new strategy - or any strategy, for that matter - could make a real difference.'
'You have dealt enemies of the United States and Iraq a tremendous, if not mortal blow. History will regard you as one of our nation's great battle captains.'
General Petraeus oversaw the surge, but it was his former deputy General 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.
General 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 Mr Gates said made him the right person to replace General Petraeus.
General 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 Shia extremists.
'Just as important as the surge was the change in our tactics, techniques and procedures that got us back out in the neighbourhoods,' General Odierno told reporters at the end of his previous tour in March.
The imposing general will have his hands full as he takes command of the 146,000 troops at a time of what Gates said was a mission in transition as troop numbers shrink with 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,' Mr 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.
General Odierno has argued consistently against sharp cutbacks in troop levels in Iraq, which is negotiating a controversial security pact with Washington to determine trooop levels after a UN mandate expires at the end of the year.
But the US force will shrink by about 8,000 troops by January, when US President George W Bush leaves office and pressure for further reductions is intensifying as attention shifts to Afghanistan.
