Ashdown sits back and draws breath before describing his three and a half years in office in Bosnia: "It's been knackering, carpet-chewing, frustrating, depressing, wonderful and a huge privilege," he says. "There's been nothing like it in my long life - as a soldier, politician, diplomat or businessman. I can't imagine what the pattern of my life would have been without it."

It's a sunny autumn day in Sarajevo as Ashdown gives the Guardian his valedictory interview. When he talks over his time here, he speaks with a raw emotional vigour and direct honesty for which politicians are not generally known, and for which one remembers him during the days of war in this region. "But I shall go home now," he says. "You have to know when your time us up. I knew that with the Liberal Democrats, and I know that now, in Bosnia."

It is exactly a decade since the hurricane of violence that engulfed this country abated, uneasily - exactly a decade since organised mass rape, the burning of millions from their homes and the enforced deportations that came to be called "ethnic cleansing" finally ceased.

Unlike his three predecessors in this office, Ashdown's history in Bosnia goes back to the opening months of that war, and the night I first met him, on August 7 1992. The ITN journalists Penny Marshall and Ian Williams and I had just forged a way into the infamous concentration camps at Omarska and Trnopolje, and Ashdown, then Liberal Democrat leader, had flown out as soon as he heard about it. He summoned us to his room at the Hyatt Hotel in Belgrade late at night; next morning, he went to the camps, to see for himself.

For all the outrage of those days, the war dragged on for another three bloody years, and when Ashdown took on the role as high representative, he said: "I am here because I think it was a terrible sin of the west to allow those years of war." Indeed, with the Bosnian Serb architects of mass murder - Radovan Karadzic and General Ratko Mladic - on the run, wanted for genocide, it is easy to forget that during those three years of savagery that he unleashed, Karadzic's hand was eagerly clasped by western diplomats beneath the chandeliers of London, Paris and Geneva.

Throughout that time of international complicity, there were few - very few - voices calling for cogent intervention. And there was only one British politician who kept on coming back to Bosnia, each time returning to London to lambast the government for its resistance to intervention. Ashdown accused foreign secretary Douglas Hurd of "using humanitarian aid to blackmail the victims of aggression into capitulation", and prime minister John Major of calculated inaction over the Srebrenica massacre.

"We who came here saw what was happening - you did too - that this was far more than a war in a faraway place. This was a moral imperative, a terrible vision of the future," he says. "The generous way of putting it is that we were not ready for this. The less generous way is to say: 'How was it possible to return to the politics of appeasement of the 1930s?'"

Finally, after limited bombing of the Serbs, the war was halted by the international community just as the Bosnian and Croatian armies were turning the tide, and a peace forged at Dayton, partitioning Bosnia into two "entities", the "Republika Srpska" and a precarious Muslim-Croat "Federation". "It was a superb agreement to end a war, but a very bad agreement to make a state," says Ashdown. "From now on, we have to part company with Dayton and try to build a modern democratic state, for which I have tried to lay the foundations."

Ashdown was appointed high representative to Bosnia by the Peace Implementation Council (PIC), a consortium of powers and signatories to Dayton, in May 2002.

He came, he now says, "because Bosnia is under my skin, and still is. It's the place you cannot leave behind. I was obsessed by the nightmare of it all; there was this sense of guilt, and an anger that has become something much deeper over these last years. I love this country, I love these people, though I can't say I love their politicians. People are always nicer than politicians, but here, you can mark that difference up a hundredfold."

Ashdown assumed an authority which has been compared to that of a medieval pope. He enjoys - and has used - sweeping powers to issue decrees, sack politicians, judges and whomsoever he wants. "This is in a sense an anachronism," he says, "power that should make a liberal blush. And looked at from the outside, I suppose it is legitimate to see it that way; it is frightening to have so much power. But actually that is not what my job has been like, and it would be a foolish high representative who worked that way." Ashdown has been criticised for "absolutism", but the reality, he says, is that he operates under tight diplomatic constraints and accountability to that capricious body he represents, the so-called "international community". "I am formally accountable to the steering board of the PIC," he says - principally the governments of the USA, UK, France, Germany and Russia - "and I meet with nine ambassadors from the PIC every week. I have to have the (national) capitals' broad agreement with what I do. Sometimes, if I have 70% of them behind me, I'll go ahead with a decision. Once, I even had an embassy threaten to cut off diplomatic relations with me. I am also," he adds, "responsible to the Bosnian people. If I pass a decree that is refused, my authority is gone like the morning dew."

Ashdown's first break with his three predecessors - competent, professional bureaucrats - was one of style. He left the stockade of the "international presence" in Bosnia to look local politicians in the eye, hold "town hall" meetings all over Bosnia and sleep on the floors of refugee camps with Jane at his side.

Treading on shards, then, between the "international community" and three wary ethnic populations, Ashdown set out to create a unified state which could find its way into the European Union - something the Muslim-Croat Federation was desperate to do (poignantly, they included the EU's starry flag in theirs), while the instinctively separatist Serbs - who doggedly identify with Serbia, not Bosnia - were ready to contest all efforts. "My first job," says Ashdown, "has been to do the best thing for the Bosnian people. Not for the Serbs, the Bosniaks [Muslims] or Croats, but the people as a whole. My second job has been to try to use my power to create institutions of a modern state that could enter the European Union, and there was very little time. The door was closing, and I wanted to get Bosnia through before it shut."

To get Bosnia in, he had to establish a state-wide military, and a state-wide unified police command (to which the Bosnian Serbs have only recently agreed). There was more; the list of what are called "benchmarks set for integration into Euro-Atlantic structures" is a long one. Since May 2002, Ashdown has supervised the establishment of a Bosnian judicial system, including a new chamber to try those newly accused war crimes, and referrals from The Hague. He has welded together a single-state intelligence structure under parliamentary oversight, a unified customs service, and an expanded council of ministers. There have been dictates on education, public services, and the terrain Ashdown calls "civil society" - the people as a social organism, able to express itself democratically. And yet: "The greatest failure," he says, "is that although we have created institutions, we have not created a civil society."

Ashdown's reforms have ploughed on - "very fast, perhaps too fast" - but Bosnia remains a scarred, divided country, its wounds far from healed.

The word "reconciliation" has been bandied about by outsiders since almost the first days after the war; the idea that those whose lives have been shattered by persecution should somehow "forgive and forget" and move on. But before reconciliation, you need reckoning. And reckoning means the perpetrators of crime admitting and coming to terms with what they have done - with all the judicial implications of that. This is why Ashdown announced, when he took the post, that he would put "justice first".

Since then he has assailed the rampant criminal and political networks that entwine racketeering with protection of those indicted by The Hague - markedly the still-fugitive Karadzic and Mladic. Ashdown's sacking of tainted figures in high office began within weeks of taking over, proceeding to the dismissal, last year, of 59 senior officials and politicians of the Bosnian Serb structure, for their involvement in the protection of war criminals, and Karadzic in particular. "I was told there would be riots in the streets," Ashdown says, "but there were no riots. People do not want politicians they know to be corrupt."

The purge is not complete, however. In many municipalities, the murderous authorities remain intact. There are people employed in the state security apparatus about whom serious wartime allegations have been made, in one instance involving a former interrogator in the Omarska camp itself who now holds high office. "Politics is compromise," says Ashdown. "What one has to do is respect legal procedure. If The Hague says to me they have evidence against someone, I will remove them. But I cannot be governed by innuendo, by what someone says to me about person X or person Y."

Another attempt at reckoning came when Ashdown tasked the Republika Srpska to establish a commission into the Srebrenica massacre, which duly repeated the well-worn lies that only hundreds had died, mostly fighting one another. Ashdown was furious, and ordered the commission to go away and try again. Accordingly, last year, in an unprecedented and unrepeated document, the Bosnian Serbs admitted "that between July 10 and July 19 1995, several thousands of Bosniaks were liquidated in a manner that represents a serious violation of International Humanitarian law".

But Srebrenica, however infamous, was merely iconic of so much other unspeakable violence over those three years, especially in 1992, in places whose names the world has forgotten, if it ever knew them. Srebrenica was but a coda to three years' carnage, deportation and rape from the Drina valley in the east to the camps to the west. "If I have regrets," says Ashdown, "they are that we could not do more like the Srebrenica commission. This country is about history, and unless the Serbs in particular - although terrible things were done by the Bosniaks and Croats too - come to some understanding of this history, we cannot build a stable state. The major burden of guilt is on them, and they have to acknowledge it, just as the Germans acknowledged it."

For all Ashdown's attempts to bring about reckoning at official level, it progresses slowly on the ground. The worst crimes - even when there have been convictions at The Hague - remain widely denied or justified, or some non-sensical blend of the two. A pivotal moment of Ashdown's tenure was to rebuild - physically, symbolically - a replica of the iconic old bridge at Mostar, destroyed by Croats during the siege of Muslims in 1993. But the Croats have mocked Ashdown's school reforms, with what amounts to apartheid - Bosniak Muslim children segregated from their Croatian counterparts in both learning and play.

"I can create institutions," says Ashdown, "But I can't rewrite the chips in people's heads. It works both ways: there are victims of tragedy who come to me who have experienced grief of such magnitude that they cannot reconcile, however much people say to them 'forgive and forget' - they can't, and if I were them, I don't think I could either. Likewise, I cannot change the mentality of those who committed the crimes, or the fools who followed them.

"But what I can do is establish the expectation of retributive justice. Have we done that? No, in all honesty I can't say that's been done. But we have come close, only 10 years after the war. We are not there yet, but I'd be disappointed if in two years' time there was not some movement towards truth as a precursor to reconciliation. 'Truth and reconciliation' are always combined, but I would split them: I don't think Bosnia is ready for reconciliation, but I do think it is ready for truth."

Ashdown said two years ago: "We have invented a new human right here - the right to return home after a war." The precarious return - mainly by Bosniak Muslims to rebuild the razed towns and villages from which they were "cleansed" is one of the phenomena of present-day Bosnia. "It's a miracle," says Ashdown, "that 10 years after a war in which 250,000 people were killed - one-sixteenth of the population - and two million displaced, that one million of them have gone home. But there are problems: many of them are old and face extreme hardship. What we have to do is to make their livelihoods viable, get them the proper prices for their produce, try and make them stay rather than do what is anyone's right to do, sell their property and leave again."

Ashdown's branch office has been particularly energetic over the return around Srebrenica, and Ashdown himself stayed with some of the returnees - "a way of paying homage to the sheer courage of those people who have come back". One family with whom Ashdown stayed is that of Hasib Huseinovic, who, along with his fellow villagers of Suceska, high on a mountaintop above Srebrenica, rebuilt their incinerated village from nothing. When I went to visit Huseinovic in deep winter, he explained that he had come back in part because he hoped his missing son would come walking back across the newly planted field.

This summer, however, at the Srebrenica anniversary commemorations and burials of hundreds of dead, Ashdown and Huseinovic met again. "I was with Hasib," says Ashdown, "when his son's coffin, number 84, was passed over the heads of the crowd. I helped Hasib fill in the grave ..."

More than a hint of a tear appears in his eyes. "Sorry," he says. "I'm not very good at this bit ... refugees ... God, I've seen enough refugees for one lifetime."

Ashdown's tenure has been one long illustration of John F Kennedy's refrain that you cannot please all the people all of the time. Although he is warmly respected and eagerly greeted wherever he goes, mention of Ashdown's name makes hardline Serbs and Croats bristle with resentment and bile. More surprisingly, though, Ashdown has aroused hostility and disillusionment among the Bosniak intelligentsia whom he had presumed to be his allies. "Paddy Crashdown," reads graffiti near his office; he has been accused of inconsistency, of playing God. Ashdown has the misfortune to be a long way down the line of brazen foreign presences in a city and country weary of comfortably-salaried "internationals" - many of them, unlike Ashdown, emotionally detached, career-hopping to the next blighted destination.

"Maybe it's legitimate criticism," says Ashdown, "though it can be hurtful. Maybe I haven't paid sufficient attention to the people with whom I would have a natural affinity as a liberal, and they feel let down by that. But it's not my job to be popular - I'm goal-driven; my job is to get results. And mind you," he adds, "I've had much nastier things said about me in the British press than in the Bosnian press."

Ashdown says his job "was to try to ensure that the office I have held does not continue to exist. And, by the end of next year, it will cease to exist." His successor, due to be announced within 10 days from a shortlist of four, is intended to be Bosnia's last high representative.

So what next for the prize fighter of British and Balkan politics, still only 64, whose quintessence is restlessness? He could easily make a career fixing international crises. Or he could go back into national politics; the Liberal Democrats are said to await his return with a mixture of great expectation and trepidation. "What my future will not be is active politics in the Liberal Democrat party," he says.

"But I do have huge plans for my garden. I really need a rest." It sounds plausible for a moment, but as one leaves Ashdown's office, into the autumn sunshine and straight across a bridge on which two lovers - one Serb, one Muslim - were famously shot dead by a Serbian sniper during the war, it is hard to imagine this tempest of a man spending the rest of his years pruning English roses.

On guilt Bosnia is under my skin, and still is. There was this sense of guilt, and an anger that has become something much deeper over these last years

On reconciliation I don't think Bosnia is ready for reconciliation, but I do think it is ready for truth

On justice This country is about history, and unless the Serbs in particular - although terrible things were done by the Bosnians and Croats too - come to some understanding of this history, we cannot build a stable state. The major burden of guilt is on them, and they have to acknowledge it, just as the Germans did

On popularity Maybe I haven't paid sufficient attention to the people with whom I would have a natural affinity as a liberal, and they feel let down by that. But it's not my job to be popular - I'm goal driven; my job is to get results

On his reign in Bosnia It's been knackering, carpet-chewing, frustrating, depressing, wonderful and a huge privilege. There's been nothing like it in my long life. I can't imagine what my life would have been without it