When we meet in London he is dressed in rather idiosyncratic civvies: pinstripe jacket over sunshine-yellow sweater and faded denims accessorised with rather sophisticated desert boots, leather briefcase and a long, meticulously rolled umbrella of the kind carried by City gents when City gents were gentlemen. On one wrist he wears the chunky, multi-function watch of a man keen to know height above sea level and his blood pressure count at the press of a button. On the other, his green wristband is inscribed with the words, Save Our Troops – the first signal of hard-edge anger beneath Collins's compelling loquacity and charm.

Experience has left him with scant time for modesty, and, as he reflects on the potent impact of his speech, he declares: "If my message was taken up by the rest of the world then that was because there was a failure among politicians who weren't giving that sort of guidance." In our hour of conversation Collins's criticism of the Bush and Blair war pact is unequivocal, but it is scarcely more searing than his contempt for the present top brass in the Army.

Ten weeks after delivering that address, the "soldier-thinker", known with some pride in his regiment as Nails, was hit by the rapid fire of war-crimes accusations. An American army reservist, Re Biastre, had accused the Royal Irish commanding officer of mistreating Iraqi prisoners, of shooting at the tyres of a lorry driven by looters and of pistol-whipping a Ba'ath party official. No sooner had the news broken that Collins was now under investigation by the Royal Military Police than the Ministry of Defence confirmed it was also examining the wider military environment within the regiment.

In short, the implication was that the hero was a bully, and Collins found himself on gardening leave. But, by the end of both inquiries, the official statement pronounced he had no case to answer and no stain or blemish on his character. Biastre, it emerged, had been motivated by revenge, having been disciplined by Collins for insubordination while under his leadership. Similarly, the other accusations were found to have been born out of malice. And so the episode should have ended, except that, during the course of these bitter events, Collins came to realise he was fighting a much more invisible battle than the one which had preoccupied him in Iraq.

Today, in Rules of Engagement, A Life In Conflict, the hero bites back. His memoir is incendiary in its condemnation of what he believes was a smear campaign waged against him by senior figures in the Army itself. "It simply stood by, not lifting a finger to help," he says, that action, or rather inaction, supporting his view that "any honour within the Army, and any vestige of a duty of care as an employer to me and my family no longer existed". Even after his name had been cleared Collins still felt that his enemies, "eaten by envy and motivated by professional jealousies", were many and powerful. "I realised that the prospect of advancement was limited," he writes.

A native of Belfast, he is a graduate of Queen's University there, and Sandhurst, and now lives in Kent with his wife, Caroline, a maths teacher, and their five children. After several tours of duty in Northern Ireland and the Falklands, Collins volunteered for the SAS for a two-year stint before assuming command of the 1st Royal Irish in 2001. He is the recipient of the Queen's Commendation for Valuable Service and received the OBE for the liberation of Iraq in 2003. After the war crimes allegations were dropped he was promoted to colonel.

But, before calamity struck, Collins had anyway incurred the wrath of senior staff by speaking out against the "societal forces and trendy opinions" which, in his view, were running the risk of reducing the Army to no more than a "Home Guard". In his memoir he writes: "I left the Forces convinced there was a need for a cull at the highest level to cut the destructive dead wood stifling initiative, pandering to civil-service assaults and failing to give leadership to the Services."

In conversation he is equally aggressive in his criticisms, and it is not difficult to imagine how such objections could trigger a "Get Collins" campaign. "We have a generation of leaders in the Army who were brought up during the Cold War in the seventies and eighties," he says. "They have never tasted actual combat, although some have been in low-intensity conflicts. But many of them haven't been to a fight in a playground. They live along very regulated, predictable lines and they have no comprehension of the realities of war, so they feel very threatened around strong personalities."

Collins argues that if the Army is now to be run like a business it could find "smaller, fatter, more ginger" businessmen to do the job. "But the fact is that the people at the top aren't businessmen. They've got there by dint of being the shortest pencil in the class." And if he hadn't left the Army but today was a highly-decorated brigadier, what would Collins feel? "I'd still be furious and — if these people dared to show their faces to me, which they never have and never will — what I would say to them is that when they join the Army they swear an oath of allegiance to the sovereign to defend the nation.

"But when they find themselves defending Labour Party policy, they're on the wrong bus. That's what they're doing now. By disbanding Scottish regiments they're contemplating cuts to deliver money to the treasury, and to hell with the defence of the nation." Collins says that even before the disbandment of four battalions, the Army complement was down by 17,500, and yet, given current commitments, it was being stretched as never before.

Collins dismisses Biastre as someone who, "like his allegations against me", had little attachment to planet earth. "He told me he was a special-forces soldier but it turned out he was a part-time reservist. He told me he was a New York cop. Well, he was a traffic policeman from Buffalo, upstate New York. But when I was called to the Royal Military Police I discovered I was fighting a much bigger battle. They were now investigating hundreds of sundry little allegations scraped up from dustbins from God knows where. Basically, anyone who had a grudge against Tim Collins felt free to pick up an open phone line to the RMP."

In retrospect does Collins feel his eloquent pugnacity contributed to his misfortune? "Of course, there were people who didn't like my style of command, people I'd upset along the way." Collins, by his own admission, is a complex character. Brother officers who know him well describe him as a "one-off", a highly professional and uncompromising "warrior". So, how would he define his style? "Singularly focused towards achieving the mission." It was put to him during the investigations that he drove men rather than led, commanded rather than taught but he insists that this was not the view of most his soldiers. "I stand on the stage with very few other commanding officers who have led three operations — one of them a war — and brought all their men back alive. Let that be my judgment."

Collins was following a family tradition when he joined the Army. Does he now feel betrayed by the service he loved? "Maybe betrayed is too strong a word. There's been malice, certainly, but perhaps much more incompetence. In its incompetence and enthusiasm to attack me, the military didn't manage to put a glove on me. I put every one of the allegations to bed." What buoyed him against despair, he says, were bags of letters declaring support. "And those members of the Lords and the Commons who told me that if push came to shove they'd be there for me."

But he still wonders who really gained from the whole sorry mess. "Nobody gained," he says, ruefully, and with the continuing slaughter in Iraq, with the revelations about Abu Ghraib and disclosure of prisoner abuse in Afghanistan, it's a question and answer of ever wider relevance for him. "This war on terror that we're waging, what's the plan? Are we just throwing troops and insults around the international community until we all get tired and fall over?" Tim Collins ended his memorable address at the start of the war with the following sentiment: "Let's bring everyone home safely and leave Iraq a better place for us having been there." His men's conduct, he reflects, is testament to the fact that they understood precisely what he was saying. President Bush may have the words framed on his wall but, says Collins, there's scant evidence he's got the message."

Extract from the inspirational eve-of-battle speech delivered on March 19, 2003, by Lieutenant Colonel Tim Collins to his men of the First Battalion, the Royal Irish Regiment at the start of the second Gulf war.

We are going to Iraq to liberate and not to conquer . . . We will not fly our flags in their country. We are entering Iraq to free a people and the only flag that will be flown in that ancient land will be their own. Show respect for them. The enemy know this moment is coming, too. I expect you to rock their world. Wipe them out if that is what they choose. But if you are ferocious in battle, remember to be magnanimous in victory. If there are casualties of war, remember they did not plan to die this day. Bury them with reverence and properly mark their graves. If you harm your regiment or its history by over-enthusiasm in killing, or cowardice, know that it is your family who will suffer. You will be shunned unless your conduct is of the highest order.

Face To Face Rules of Engagement, A Life in Conflict by Tim Collins, Headline, £20.

War desecrates everything. Even heroism is not immune. When Tim Collins addressed his troops on that March day, his impassioned oratory flashed across the world. In Britain it was hailed as his Agincourt moment, a reference to the rallying cry of Henry V of England, preparing his men for battle against the French in 1415. In Washington, President George W Bush instructed that the words be framed and hung on a wall in the Oval office. For Collins that particular distinction would become one of many piercing ironies, given what was to follow. Collins, aged 44, resigned from the Army last year after 22 years and now seems set to become one of the media's military commentators. When we meet in London he is dressed in rather idiosyncratic civvies: pinstripe jacket over sunshine-yellow sweater and faded denims accessorised with rather sophisticated desert boots, leather briefcase and a long, meticulously rolled umbrella of the kind carried by City gents when City gents were gentlemen. On one wrist he wears the chunky, multi-function watch of a man keen to know height above sea level and his blood pressure count at the press of a button. On the other, his green wristband is inscribed with the words, Save Our Troops – the first signal of hard-edge anger beneath Collins's compelling loquacity and charm. Experience has left him with scant time for modesty, and, as he reflects on the potent impact of his speech, he declares: "If my message was taken up by the rest of the world then that was because there was a failure among politicians who

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