Relations between Britain and Iraq suffered “catastrophic failure” after Baghdad bypassed the British military and called in the American “cavalry” to help the recent offensive against Shia militia in Basra, The Times has learnt.

About 550 US troops, including some from the 82nd Airborne Division, were sent from Baghdad to Basra to join up with 150 American soldiers already serving with Iraqi forces in the southern city.

The Ministry of Defence made much of the fact that British troops, based at Basra airport outside the city, were not requested in the early stages of the operation. British officials claimed that the Basra offensive was proof that Iraqi troops could cope on their own.

The Times has learnt, however, that when Britain’s most senior officer in Basra, Brigadier Julian Free, commander of 4 Mechanised Brigade, flew into the city to find out what was going on, Nouri al-Maliki, the Iraqi Prime Minister, who was orchestrating the attacks on militia strongholds, declined to see him.

Brigadier Free flew to Basra city with Lieutenant-General Lloyd Austin, the commander of American and coalition forces in Iraq, on March 27, two days after the operation began. The Iraqi Prime Minister spoke only to the US general.

A source familiar with the sequence of events said that Mr al-Maliki seemed to have it in for the British because of the alleged “deal” struck with the Shia militia last year under which they agreed not to attack Britain’s last battalion as it withdrew from Basra in return for the release of several of their leading members from prison.

According to The New York Times, Baghdad turned to the Americans for help when the Basra operation was launched. Two senior American military officers, Rear Admiral Edward Winters, a former member of the US Navy Seals special forces unit, and Major-General George Flynn, a Marine, were sent to Basra to help to coordinate the operation. Soldiers from the 82nd Airborne Division were drafted in as combat advisers and air controllers were positioned to call in airstrikes.

Ryan Crocker, US Ambassador to Baghdad, told The New York Times that the first he learnt of the Iraqi plan for Basra was on March 21. “The sense we had was that this would be a long-term effort, increased pressure gradually squeezing the special groups [the Iranian-backed Shia militia],” he said. “That is not what emerged. Nothing was in place from our side. It all had to be put together.”

A source told The Times that US forces were in Basra, eating and sleeping alongside their Iraqi counterparts, “basically doing the work that we were supposed to do. It was a catastrophic failure of diplomacy.”

The source described the moment when the American general arrived at the British base from Baghdad: “Suddenly the cavalry appeared.”

The source said that the Americans provided “loads of technical equipment and combat power”. As soon as the Americans arrived and started hitting houses in Basra, the daily attacks of indirect fire on the British base stopped. The source said that during that time the mood among the British forces on the base was “miserable”.

There was even speculation that Mr al-Maliki had refused to talk to Gordon Brown since the operation began. But a Downing Street spokesman said: “Mr Maliki spoke with the Prime Minister on the phone during the Nato summit in Bucharest [last week].”

It was not clear who had initiated the phone call, but Downing Street said that the two leaders had been trying “for a few days” to speak to each other and that when they did the conversation had been “constructive”.

A defence source played down the snub, saying: “Mr al-Maliki only deals with people at a certain level.” The source added that Major-General Barney White-Spunner, the British General Officer Commanding Multinational Division Southeast, had been out of the country at the time.

British troops did become involved in the operation eventually, first with RAF Tornado GR4 aircraft firing warning shots over Shia militia areas of Basra and then supplying troops from two battle groups to help the Iraqis, although not in the city itself.

Artillery was also fired from the British base at Shia militia targets.

However, British troops are now back in Basra serving alongside Iraqi forces for the first time since withdrawing from the city in September. The Ministry of Defence confirmed that 150 British soldiers were now embedded with the Iraqis in Basra, serving as military transition teams.

The 150 British troops now in Basra city have been drawn predominantly from the 1st Battalion The Royal Regiment of Scotland battle group. They have been formed into six military transition teams (Mitts), made up of advisers, force protection units equipped with Mastiff and Warrior armoured vehicles, and medical support.

The presence of British troops in Basra again was the main reason why Des Browne, the Defence Secretary, told the Commons last week that the plan to reduce the force levels in Iraq from 4,000 to 2,500 from the spring had to be postponed.

Both the Americans and the British felt that Operation Charge of the Knights was a hastily conceived offensive without proper planning or consultation. The result is that the American and British military will have to be based in Basra for the foreseeable future, something which neither coalition partner had foreseen or planned for after security for the south was handed over to the Iraqis.

There were reports that Mohammed Waili, the Basra governor, was effectively under house arrest. Sources in the Basra provincial council said that his Fadhil bodyguard had been replaced by Iraqi army soldiers.

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