A mortar round exploded nearby as the US Marine stepped on to the street in Basra, forcing him to dive back inside his combat vehicle for cover.

“That was the first time that I thought, OK, this is serious, we are not playing games any more,” said Lieuten-ant-Colonel Chuck Western, one of the first Marines to venture into the city in late March to support an Iraqi-led offensive against gangs of well-armed militia.

Holed up at an old police station, the 400-strong battalion of Iraqi soldiers was taking a pounding, but the men cheered at the sight of their team of seven military advisers – embedded officers and soldiers who help to train Iraq’s fledgeling forces.

The Basra offensive, started unexpectedly by Nouri al-Maliki, the Iraqi Prime Minister, on March 25 to rid the oil-rich port city of armed gangs, was the first real test of the Government’s ability to impose its authority on one of the most lawless parts of the country. It also demonstrated a growing distrust of the British military, which was kept unaware of the plan until the last moment after Mr al-Maliki discovered that Britain had been negotiating with the very militia he was trying to expel.

Even after the offensive had started, the 4,000 British troops based at Basra airport were unable to join the fight because of a deal with al-Mahdi Army not to enter the city. It would take six days before the permission was granted by Des Browne, the Defence Secretary.

Details of the “accommodation” between British intelligence officers and elements of al-Mahdi Army, which has been blamed for murders and other atrocities in Basra for the past four years, shocked US and Iraqi officers, who have expressed a sense of betrayal. All parties involved agree that Britain’s reputation in Iraq has been badly, possibly irrevocably, damaged by the episode.

The Charge of the Knights, as the operation was called, got off to a shaky start as Iraqi police and soldiers – trained by the British but lacking much experience – met fierce resistance from the Shia militiamen, threatening a humiliating early defeat for the Iraqi leader.

Demoralised and outgunned, some 3,000 men surrendered their weapons and fled, their vehicles left burning in the street. Mr al-Maliki, who flew down to Basra to take command of the operation, knew that the outcome would probably decide his fate as well.

“There was a lot of confusion at that time,” Colonel Western told The Times from his base in the city of Baquba, northeast of Baghdad. “Units were evaporating from the 14th division. Orders were not necessarily clear. The battalion was being sent all over the place. There was not a coherent plan at the time.”

Corporal Hussein Abid Hamza is a soldier in the 2nd Battalion, 1st Brigade, 1st Division of the Iraqi Army. It is Iraq’s best-trained division and is seen as a rapid reaction unit. Colonel Western and his team helped to train the troops.

Arriving at the northern gates of Basra as part of the first wave of additional manpower, Corporal Hamza admitted that he had an attack of nerves as he saw two colleagues give up and go home. “I told myself that if I gave up too, then someone else would, and someone else would until no one is left,” said the 23-year-old, who opted to stay and fight, holding out for three or four days at the nearby police station in what the Marine advisers refer to as the Battle of the Gates.

Sergeant Joshua Stone, 23, remembers that battle all too well, particularly one night when militiamen started attacking the police station where the Iraqis and their US advisers were based with 120mm rounds.

Times Recommends

• Over the Wall
• Why Africa welcomes the ‘new colonialism’
• Which will come out on top: paper or gold?