By Caroline Wyatt
Defence correspondent, BBC News, Basra
The withdrawal of British forces from Iraq will begin on Tuesday as the top British general in southern Iraq hands over his command to an American general in Basra.
Major General Andy Salmon will transfer authority for what will become Multi-National Division South to US Major General Michael Oates. He will now command British and American forces in the south of the country.
The bulk of British forces have to leave Iraq by the end of July
A few British troops will leave Basra on Tuesday - although the majority of the 4,000 British forces still in southern Iraq are not due to go just yet.
Most will leave by 31 May, the official date set for the end of combat operations, with only about 400 remaining after that - either in coalition HQ roles or helping to train and mentor the Iraq navy in the port of Umm Qasr.
Under the current agreement with the Iraqi government, the bulk of British forces have to leave Iraq by the end of July.
In a separate deal, American troops are to stay on until the end of 2011, although this June they will have to finish withdrawing their soldiers from Iraqi cities to bases outside the main centres.
The transfer of authority does mark the beginning of the end of Britain's military presence in Iraq, six years after the US-led coalition invaded the country and deposed Saddam Hussein.
Already, the main British base at the airport just outside Basra - known as the Contingency Operating Base (COB) - is taking on an increasingly American flavour.
Fast food outlets are springing up (Annie's Pretzel Bar, motto: 'Spoiling Dinners since 1988') and convoys of American armoured vehicles can be seen churning up the tracks, scattering clouds of fine desert sand.
US soldiers are also a much more visible presence on the base, with British troops handing over many of the buildings and their duties at the camp, as they start packing up.
It is a major logistical operation, with 4,000 servicemen and women and six years worth of military equipment to move.
The US military role in southern Iraq will be slightly different. American forces are to focus more closely on training and mentoring the Iraqi police, who are still less trusted than their army counterparts.
They will also help train Iraqi forces to maintain border security, as well as keeping open the main supply route between the south and Baghdad.
During Operation Telic 13, the codename for British operations in Iraq since the invasion in March 2003, British troops have focused mainly on mentoring the Iraqi army, in particular the 14th Division.
"I think we can go with our hands on our hearts, holding our heads high, very proud of what people have achieved here over the last six years," says outgoing Major General Andy Salmon.
He cites the recent provincial elections, which passed off mainly peacefully in the south, and the improved security situation, including interest from new investors in Basra.
We are standing at the memorial wall in Basra, with its rows of brass plaques recording the 179 UK service personnel and the MoD civilian who lost their lives over the course of the British campaign.
"We have to remember the lessons - we've always got to learn lessons in any campaign - then get back, have some well-deserved leave and get on with the next job."
A few miles away, at the Iraqi army's main base at Basra Operations Centre in the old Shatt-al-Arab hotel, another transition will be taking place between the US and Britain.
British Colonel Richard Stanford, who advises the head of Iraqi forces General Mohamed, will also hand on to an American, Lieutenant Colonel AJ Johnson.
"It's truly a pleasure for me to come in and dovetail on the accomplishments of British forces here," Col Johnson tells the BBC.
He has recently arrived from Sadr City in Baghdad, one of the heartlands of support for the radical Shia cleric Muqtada al-Sadr, and is finding Basra a striking contrast as he is shown around a shiny new shopping centre in the city.
It stocks everything from women's racy underwear to make-up, jewellery and perfume - all discouraged, sometimes violently, by the Shia militia before the Iraqi army launched its successful operation Charge of the Knights last March.
US, and later UK, forces embedded training and mentoring teams with the Iraqi forces within Basra city to help during the operation.
Feelings are still mixed among the Basra population
"The British here truly have an understanding of an insurgency-rich environment. The steps they have taken and set in place have set American and coalition forces up for success as we partner with the Iraqis here," says Col Stanford.
On the streets of Basra, British soldiers from the Princess of Wales Royal Regiment are performing their last mentoring patrol with their Iraqi charges.
Accompanying them in their American-built Humvees, the patrol was conducted to a distinctly Iraqi soundtrack blaring tinnily from one of the Iraqi soldier's mobile phones.
Feelings in Basra about the transition are mixed. We drive with the patrol to the edge of what was once the most dangerous area of Basra, the Hayaniyah. Col Stanford, an Arabic speaker, chats to market traders.
One man says he didn't bother voting in the most recent provincial elections: "Why should I?" he asks. "Politicians are all the same. The last governor was bad, and the next one will be even worse."
He is still convinced that Britain and America only invaded Iraq to take control of its oil supply, and says foreign armies have no place here.
Others, though, say they are thankful to British forces for helping remove Saddam Hussein - though most are now happy to rely on the Iraqi army to keep the peace in Basra.
Another man tells us that he prefers British forces to American troops.
"They're friendlier, more sociable. I've heard the Americans can be more aggressive."
So, as British forces prepare to leave Iraq, what does Col Stanford think they have learned from their six years here?
"We have re-learned some of our lessons from history - that we need the help of local forces. We trained the Iraqi army to secure Basra province and that's what they have done."
If there have been a few cross-cultural difficulties between the British and the Iraqis, the incoming Americans will have to learn a whole new language as they gradually take over from British soldiers in the coming months.