As a loyal officer under Saddam Hussein, the Iraqi major never imagined that one day he would become an insurgent, but when Iraq fell five years ago he was left bitter, jobless and desperate to drive the invading forces out.

“I saw my country collapse right in front of my eyes,” said Abu Abdullah, who has since orchestrated countless attacks against the US military, spent time in the notorious Abu Ghraib detention centre and briefly joined forces with al-Qaeda.

Recalling the invasion, he told The Times: “I felt as though my freedom was being snatched from me. It was one of the darkest moments of my life.” In many ways Mr Abdullah’s story is the story of the insurgency in Iraq, where the changing allegiances of Sunni Arab fighters has dictated the pace of a conflict that has claimed tens of thousands of lives since 2003.

He, like many Sunni Arab officers and other Saddam supporters, resorted to guerrilla warfare to kill better-equipped US soldiers but gradually found that his nationalistic resistance had fallen under the control of the militant Islamists of al-Qaeda.

Appalled at the cruelty of attacks sponsored by al-Qaeda, Mr Abdullah switched sides recently and is cooperating, albeit reluctantly, with the US military as part of a grassroots security drive that has spread across Iraq.

Five years ago, as Major Abdullah, he was holed up in the Iraqi city of al-Kut, south of Baghdad, listening to the sound of American combat aircraft dropping bombs on buildings and the thunder of invading tanks. “When the infantry entered al-Kut most of my soldiers stopped fighting. They realised that the US Army was much more powerful than ours,” he said. “We pulled out and returned to Baghdad. All my soldiers vanished. It was over.”

Mr Abdullah, a married father of one, drove his family to his parents’ house in Samarra, a predominantly Sunni Arab city north of the Iraqi capital, which eventually became a haven for al-Qaeda.

With the former army disbanded, he spent the next year at home defeated and with nothing to do – until he started meeting other former army officers at coffee shops in town.

“We started to discuss things and develop serious ideas. Eventually we agreed to form groups and start fighting,” Mr Abdullah said in a late-night interview at a Baghdad hotel, dressed in a maroon and blue tracksuit.

They were well prepared to begin an insurgency because, three months before the invasion, Iraqi military commanders had instructed all soldiers and officers to receive specific training in street fighting. Recruiting young men locally from April 2004, he started a branch of al-Tawhid wal Jihad, one of four main Sunni Arab insurgent groups that ultimately combined to become al-Qaeda in Iraq.

They had an abundance of rockets, guns, ammunition and bomb-making material, thanks to the many old Iraqi army warehouses dotted around the country that had been abandoned.

“Our objective was clear: to remove the occupying forces. We did not launch attacks in urban areas, just the outskirts of towns and on the main highways,” Mr Abdullah said, emphasising that his group also never targeted Iraqis.

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