Along with an ITN film crew, Observer reporter Ed Vulliamy uncovered the terrifying truth of Serbian-run concentration camps in the Bosnian war. While former Serb leader Radovan Karadzic stands trial at The Hague, Vulliamy is called as a witness – and finds himself cross-examined in a private, close encounter with the man accused of masterminding genocide

The white curtain behind the pane of reinforced glass is raised, and there he is on the other side, not four feet away: wearing a grey jacket and purple tie with a pin attached showing the crest of a double-headed eagle and crossed Cyrillic Cs that stand for "Samo sloga Srbina spasava" – "Only unity saves the Serbs".

It is a tight fit, in the depths of the war crimes tribunal building in The Hague, in the tiny holding cell and visitors' room. On the other side of the thick pane of bulletproof glass is Radovan Karadzic, leader of the Bosnian Serbs during the worst slaughter to blight Europe since the Third Reich, thereafter the world's most wanted fugitive – and now on trial in The Hague. We speak through holes in the glass that he is squeezed against. His American lawyer, Peter Robinson, sits next to him.

On my side of the glass, I share a table with Ann Sutherland, a prosecuting trial attorney for the International Criminal Tribunal for former Yugoslavia (ICTY), due to lead my evidence against Karadzic the next day before the judges, as well as another member of Karadzic's defence team.

This is an interview requested by Karadzic before I give official testimony the following day in open court. Ironically, when the witness unit's call came out of the blue in August 2011, saying that "the defence" had requested an interview, I was driving through pluvial mist up a mountain track in Bosnia to attend the consecration of a small monument to mark a remote mass grave: a crevice into which the bodies of 124 men had been dropped and concealed – a secret well kept by the Serbs for years. The men had been prisoners in concentration camps at Omarska and Keraterm in north-west Bosnia. They had been moved on the very day I arrived, and uncovered the camps along with an ITN crew – 5 August 1992 – to the forest above a hamlet called Hrastova Glavica. Once there, they were taken off buses in groups of three. They were given a last cigarette and shot one by one, their corpses dropped down the cranny in the rock and into the void, to be found and exhumed 15 years later.

I was in The Hague primarily to testify against the man on whose authority I had visited those camps that day: Dr Karadzic. I had also agreed to be interviewed by him – partly out of confusion at the witness unit's phone call that misty day, and partly on the basis that a prosecution witness should be seen by the court to oblige the defence in its requests. And, of course, I was as curious as I was nervous. Somewhere in the back of my mind was the sheer surreality of this encounter.

Karadzic's lawyer, Robinson, began proceedings in the holding cell by saying that, as Karadzic was tired after a day in court, he would ask the initial questions, and have me recall the details of a meeting between myself, the ITN crew and Karadzic two days before we walked through the gates of the Omarska camp. I recounted the strange road to Karadzic's doorstep that summer, four months into Bosnia's carnage, which had begun in April 1992 when the Bosnian Serbs unleashed a hurricane of violence against non-Serbs, carving out an ethnically "pure" swath of territory. In late July 1992, Karadzic appeared on ITN's evening news during yet another fruitless "peace conference" in London, to discuss the slaughter in Bosnia. Karadzic had been questioned about reports of atrocities in concentration camps published in that morning's Guardian. He retorted that they were false, and challenged the paper and ITN to come and see for themselves. I left for Belgrade the next day.

After a delay of several days (while, I now know, the camps were prepared for evacuation and the murder of many inmates), I met Karadzic, outside his headquarters in the Bosnian Serb capital, Pale, at lunchtime on 3 August. He had a weak handshake for someone so reportedly fearsome. Karadzic assured us we would see Omarska. It was, he said, "an investigation centre", while another camp, Trnopolje, was a place where people had come of their own accord – "displaced because their villages had been burned down". We spoke, too, about the camps where Serbs were being held on the other side by Muslim and Croat authorities. There was talk, too, of Serbian history, and its people's long and "celestial" struggle.

We were then passed seamlessly down the chain of command: delivered first into the hands of Karadzic's deputy president, Nikola Koljevic, an Anglophile professor who kept quoting Shakespeare. Koljevic escorted us as far as the largest Bosnian Serb city of Banja Luka, where we were passed on to a Major Milutinovic, who drove us past the incinerated and deserted Bosnian Muslim town of Kozarac to Prijedor, from where the camps were administered. There, we met with the "crisis staff", led by Milomir Stakic and his deputy Milan Kovacevic. And from there we proceeded with the Prijedor police chief and camp commander Zeljko Mejakic through the gates of Omarska, to behold men in various states of shocking decay emerging from a great hangar, being drilled across a yard and into a canteen, where they wolfed down watery bean stew like famished dogs. "I don't want to tell any lies," said a man called Dzemal Paratusic, "but I cannot tell the truth. Thank you for coming." (Paratusic survived, and now lives in Borehamwood, Hertfordshire.)

We were denied access to the rest of the camp despite Karadzic's guarantee, because, explained our hosts: "We have our orders … you can do this and this and that, but not that." And we were bundled out of Omarska and taken to Trnopolje camp, where we found, behind barbed wire, the remarkable sight of men, some skeletal, who had arrived from yet another camp – Keraterm – that morning. There, they said, there had been a terrible massacre one night, of 150 men in a hangar. One prisoner, Fikret Alic, said he had been assigned to loading the bodies on trucks, but had been unable to do so. We left having seen little, but enough to know that a dark horror of vast but inestimable dimensions was unfolding around Prijedor.

Television news footage recorded by ITN on 5 August 1992 of emaciated Bosnian Muslim prisoners at Trnopolje camp in Serb-held Bosnia. Photograph: Reuters/ICTY

The war dragged on another three years, Karadzic's hand eagerly clasped by British and other diplomats beneath the chandeliers of London, Paris and Geneva as he outmanoeuvred them, basked in their friendship and played with their impotence and cynicism, from one abortive peace plan to the next, while the killing on the ground continued. As war ended, in 1995, Karadzic was indicted for genocide and several counts of persecution and crimes against humanity; those same diplomats now baying for his capture.

With time, the awful truth about the camps emerged. Mass graves were uncovered, the bereaved located, and testimony at this tribunal laid bare Omarska's and Trnopolje's secrets: mass murder, and torture, beating, rape, prior to enforced deportation (I had accompanied one of the convoys). The trials at the Hague followed that chain of command down which we had been passed, in reverse: first, Dusko Tadic, a parish-pump killer and torturer who roamed the camps at large; then groups of guards, then Kovacevic, then Stakic – among many others. Koljevic shot himself in 1997. Now here was Karadzic.

For 13 years Karadzic was variously protected by both Serbia and his own Bosnian Serb fiefdom, and by sections of the same international community that were supposedly hunting him. The European Union made his delivery to The Hague a condition for Serbia's consideration for membership and he was arrested in the summer of 2008 – a wild-haired practitioner of alternative herbal medicine hiding behind a false name and a beard, among friends in Belgrade. During my own search for him for the Observer, I had met and drunk with his entourage, a wild and eccentric bunch who compared his writing to Joyce and Dostoyevsky. Nerma Jelacic, now spokesperson for the tribunal in The Hague, and I had been harangued in my rental car as we reached the mountains in which Karadzic had been sheltered above the town of Foca.

But now I sit opposite him – a man charged with "personal" and "superior" criminal responsibility for genocide, extermination, persecution, murder, deportation, unlawful attacks on civilians, violence "the primary purpose of which is to spread terror". In short, he is – allegedly – one of the most proficient mass-murderers of the second half of the 20th century. The prosecutions are roughly divided into three sections: the siege of the Bosnian capital, Sarajevo, between 1992 and 1995; atrocities and ethnic cleansing across the municipalities of Bosnia in that same period, and the massacre of 8,000 men and boys at Srebrenica in 1995.

This investigation at The Hague – the cases against Karadzic and his military counterpart General Ratko Mladic – has been ongoing for 18 years.

On the other side of the bulletproof glass, Karadzic rouses himself. He is courteous, almost jovial, though not quite endearing.

He asks: "Did you get the impression I was accessible" during the war? On that day, yes, certainly. But after finding the camps, I had not been granted permission to travel in his territory. I tell him that "someone dear to you" had withheld authorisation – referring to his daughter Sonja, who ran the press office in Pale.

His initial line of questioning concerns the Omarska camp itself. Did I know it was a "temporary investigation centre" for suspected Muslim fighters? Yes, I know of this claim, I reply. Did I know that 59% of the prisoners in Omarska were sent to a camp for prisoners of war, and 41% were "released to Trnopolje"? No I didn't, until we found the camp.

Did I investigate camps in which Serbian prisoners had been detained? Yes, I did, I reply. Within days of finding Omarska, I was heading for the town of Capljina, and revealed the camp nearby, called Dretelj, run by a Croat-Muslim militia called HOS.

Then, after an hour and a quarter, the "interview" reaches its intended climax. Karadzic produces an old revisionist chestnut of an argument, which claimed that ITN and I had fabricated our reports about the camp at Trnopolje, and that the pictures of prisoners behind barbed wire were those of refugees free to come and go. There was no point in going through it all again: this tired notion advanced by a "media expert", Thomas Deichmann, five years after we found the camp, had been attempted and quashed by successive defendants convicted at successive trials, and had been the subject of a civil court action in London between ITN and the theory's British champion, Living Marxism magazine, in 2000, with the jury finding soundly for ITN.

This revisionist accusation was also endorsed in the late 1990s by British "intellectuals", and has been raised again recently by the distinguished linguistics professor Noam Chomsky. Now Karadzic gives it a whirl: he plays a video of recut Bosnian Serb TV material to make his point. I reply that I was convinced then, and remain convinced, that the men in those pictures were prisoners arrived from Omarska and Keraterm, under guard, and that the camps were real.

I don't sleep that night before my appearance as a witness for the prosecution. I hate doing this; it is disturbing, tremulous, humbling and formidable in its way. As I enter the courtroom the next day I exchange a nod of greeting with Karadzic, who puts on his headphones, raises his eyebrows and makes a facial gesture towards his computer screen, as though to say, "Let's get to it", with gladiatorial fraternity.

On the bench are four judges, with Korean Judge O-Gon Kwon presiding. Ann Sutherland submits evidence from a previous trial, that of Milomir Stakic – sentenced to life, reduced to 40 years on appeal – and outlines the meeting with Karadzic and the discovery of the camps, illustrated with ITN's footage. Of Omarska, in an interview after our discovery of the camp, Karadzic says: "We have 13 prisons and the prison in Omarska is the worst one." Karadzic boasts he could close Omarska "even in two days" if the Muslim side agreed to a prisoner exchange.

In Omarska, there is the film of us trying to see the camp properly – quarters in which we now know thousands of men were crammed, and from which they were called for torture and mass execution – on Karadzic's authority; and being denied access. And now the judges turn to the man who allegedly gave those orders, that he might begin his cross-examination of the witness. Karadzic cuts to the quick: "Do you think that you managed to retain your objectivity?" I try to explain something to the judges: that in the past I have misused the word "objectivity" when I mean "neutrality". "When something is fact-specific, I remain objective," I say, but "I do not attempt to try to be neutral. I'm not neutral between the camp guards and the prisoners, between the raped women and the rapists … I can't in all honesty sit here in court and say I am or want to be neutral over this kind of violence."

Karadzic challenges my use of the word "racialist" to describe his programme – the Muslims of Bosnia are "Serbs who converted to Islam, and that is what Lord [David] Owen thinks as well,", he says. I reply that "the inmates in the camps were either Bosnian Muslims or Croats, and the people running them were Bosnian Serbs … and where I come from, if one self-defined ethnicity seeks to obliterate or clear the territory of all members of another ethnicity and to obliterate any memory of them, that is racialism."

There follows questioning that amounts almost to a general chat about politics: how both Serbs and Croats were, says Karadzic "in favour of a decentralised Bosnia consisting of three entities whereas the Muslim side wanted to have a unitary Bosnia". I agreed with his analysis, but couldn't resist an observation that "there's a jump between the policy and mass murder". Judge Kwon kindly puts an end to this meandering discussion; time for the first break. Then back into the arena. There is no gladiatorial camaraderie from Karadzic this time, as we re-enter the court; his face has hardened, his eyes steeled. And his voice too. Do I remember that Karadzic accepted some of the peace plans? Yes, I remember "endless plans, treaties, none of which amounted to very much on the ground. The killing carried on." Do I know about the "fighting" around Prijedor? My initial article from the camps quotes a prisoner who had been involved. I say that what resistance there was had been subjugated by the time we arrived – this discourse continues a good while.

Then he asks about Omarska, quoting my article: "There was no visible evidence of serious violence, let alone systematic extermination." I reply that we were trying to get into the hangar "where we had suspicions that appalling things were taking place. Hindsight has shown that they were". "How do you know?" asked Karadzic. "I've heard from scores of people who were in Omarska that there was widespread and systematic killing... The tribunal's own record over the years would, I think, suffice."

Karadzic questions the veracity of a quote from a boy talking about a massacre of 200 men in the Keraterm camp. I reply that: "He got the number wrong, but the massacre did take place." Then Karadzic insists: "If I told you, Mr Vulliamy, that none of this is true, and that all those who said anything about killings saw a single killing of a person who was mentally disturbed, would you believe me or would you believe them? … It seems you choose to believe things which are detrimental to the Serbs quite easily."

A single killing? I have to let this sink in. Does he really believe this? "I don't choose to believe things that are detrimental to one side or the other. I don't believe that only one person was killed in Omarska and Keraterm put together … I do believe that very many more than one single mentally disturbed person was killed … Sorry, with respect, I have to say that if you tell me it is only one, I don't believe you, sir. Nothing personal … And the detriment to the Serbs is irrelevant. That's not how I measure these things."

"With all due respect," retorts Karadzic, "it would be relevant if it were true. However, I told you that they all saw a single killing. They all discussed killings, but only saw one." Then we move on to Trnopolje. In my initial report, says Karadzic rightly, I said that Trnopolje could not be called a concentration camp, but I have since changed my mind. Judge Baird, sitting on the end of the bench, asks for clarification.

I try to explain that in the immediate aftermath of our discovery, I thought the invocation of the Holocaust by much of the mass media was not useful to our coverage, and use of the term "concentration camp" encouraged it. But that on reflection "I have decided," I told the bench, "after consultation with people at the Holocaust museum and survivors [of the Holocaust] to use the term very much with reference to its proper definition which comes from the Boer war in South Africa. It's fair to say that Trnopolje was exactly that [a concentration camp], where thousands of civilians were concentrated prior to enforced deportation and death."

Karadzic pushes his theme. Did I know civilians had been "evacuated from a combat zone" to Trnopolje? "That was not deportation … this was evacuation … based on requests made by these persons". I reply that I had been on a deportation convoy "of people who [had] told me something different … that soldiers and policemen had come around to their houses and given them ultimata to leave … The people on the convoy that I travelled with were leaving anything but voluntarily." On the same route four nights later, "large numbers of people were taken off the buses and executed on Mount Vlasic, known to this tribunal as the Vlasic massacre".

By now Karadzic's tone is harsh, combative. He refers again to the accusation that ITN and I somehow "staged" the camp at Trnopolje. Karadzic plays a section of Bosnian Serb TV making a film about us. "Our thesis [is]," he says, "that the fence around the building tools is what we saw … You, in your turn, contest that, right?" "Yes I do. This thesis, as you call it, was advanced in 1996 or 1997, we heard nothing about it between 1992 and that year from you or anyone else … Those men were detained and under guard." And on we go: "Do you see the wheelbarrows?" "I didn't notice them at the time, there were other things to look at … I'm saying that my description of them as prisoners had been proved accurate over and over again."

Karadzic produces the famous picture of the skeletal Fikret Alic behind the barbed-wire fence. "How can you be so certain that this is not just the way he normally looks?" "I know that's not how he normally looks … I met him in Slovenia the following spring, and he was of normal build." "Are you saying that within two months his condition deteriorated so much that he was on the verge of extinction?" "Yes … perhaps the conditions in Keraterm were so appalling that his condition had deteriorated in two months." "Did you see him half naked when you saw him in Ljubljana?" "No, he was clothed".

Karadzic questions my use of the term "mass murder". "Did you establish it yourself, or did you hear it from others and believe it?" "I had met hundreds if not scores of people who have survived the camps, and hundreds if not scores of people bereaved by the camps." "Do you believe that people were also killed in combat?" "Yes, I do, without doubt."

Ed Vulliamy appears as a witness at the Karadzic trial. Photograph: ICTY

Karadzic, justifiably, finds some of the sillier things I have written about him. The first is a headline in a Bosnian magazine: "I live for the day when I'm going to take the stand in The Hague against Karadzic". He asks whether this makes me an impartial witness against him. I don't recall if I had said that or not, but I answer: "No disrespect, I have not lived for this day."

There's another article, even more embarrassing, in which I called Karadzic a "tin-pot tyrant" with a "cocksure swagger". "Do you have any proof that I was a tyrant?" he asks. I concede that he was, indeed, elected on his own territory, though not across Bosnia. And: "Forgive the cocksure swagger," I reply, "You did have one at the time. The language is a little strong, I'll admit." Throughout the exchange, Karadzic pursues his theme of my being "anti-Serb". "The Serbs consider you highly partial, most partial, isn't that right?" To which I reply: "Well if so, that's unfortunate. I am, as I tried to explain when we were talking about neutrality, highly partial about extreme violence. I'm not highly partial about any race of people or ethnicity or whatever. In fact, I'm highly partial against racialism. So I'm not anti-Serb, I'm anti what was done in the name, tragically, of Serbia".

Later, I stress that I took "this allegation of anti-Serbian sentiment extremely seriously" and had "proceeded immediately to investigate camps with Serbian prisoners … and I made it my business to do so in the interests of impartiality, and partiality over the practice of putting people into camps". Judge Morrison intervened: "As you know, Dr Karadzic … it isn't the Serbian people who are indicted in this case, nor the Serbian state. It's you, and you need to concentrate on that reality." To which Karadzic replies: "Thank you, Excellency. However, as things stand, I have been indicted … for everything that every crook did on the ground. I am trying to prove that I had nothing to do with the system whatsoever."

In his parting remarks, Karadzic insists that my descriptions of the terrible state of prisoners in Omarska were made only after President George H Bush had expressed his horror at our discovery. I reply that my original story described the inmates as "horribly thin, raw-boned, some almost cadaverous…"

I can see what Karadzic is driving at: I was glory-hunting, and cranked it up in order to give interviews on radio and win awards. This hurts, and I explain that I care not a damn about giving interviews or winning prizes, and: "Do I wish history had never had Omarska in it? Yes." Complimenting my initial report from the camps, Karadzic adds, at an intense pitch, that "the rest is nothing but a big story, and I'm really sorry that you put yourself in that position and that you were finally proclaimed an anti-Serb". This is searing stuff, and Judge Kwon rules it "necessary comment. Unless [he turns to me] you wish to comment on that." Which I do: "Just to say that I have nothing against the Serbian people whatsoever, my complaint is against what was done in their name."

The following week, I watch another witness facing Karadzic – a doctor whom I had met the day we entered the camps in Trnopolje. Idriz Merdzanic had tried his best to run a "medical centre" in the camp, treating beaten prisoners and raped girls with whatever medicines he could scavenge from surrounding houses. He had been transported to Trnopolje after attempting to treat the wounded, included a badly injured girl of 13, as the Serbs "cleansed" the town of Kozarac, near Prijedor. On the day we visited the camps, he gave ITN an extraordinary interview, on a knife-edge between what he wanted to say and what he felt he could say and live – much of it with a roll of the eyes.

The doctor was ITN's only inmate witness when it sued and defeated Living Marxism at the high court in London over its thesis that Trnopolje was a lie. When I asked the doctor how he felt about those who followed Karadzic's cue in saying reports of the camp were fabricated, he replied: "It's hard to explain my feelings. I have no words for this behaviour. On one hand, we are trying to survive what happened to us, on the other we have these people telling us that it is a lie, that it did not happen. It is hard enough to find words to describe the camps and what happened, but there are no words to describe what these people do."

For a book I am writing, I had visited Merdzanic this summer at home with his family in Kiel, northern Germany. Now working as a surgeon, he said: "I report what I have seen to The Hague, but I never relive it. We do not talk about it, it's a defence mechanism, we lock it away. Everyone has their way of coping, and the experiences are different. Everyone in their own way tries to deal with their own experience of their contact with that hell."

"It is with us all the time," his wife, Amira, added (both her parents were murdered in Prijedor), "and it will be with us all the time until the end of the line. What we do to survive is to keep the door closed."

When the tribunal was established by the UN security council in 1993, its mandate was "to bring to justice those responsible for serious violations of international humanitarian law committed in the former Yugoslavia since 1991". There was an additional charge: "And thus contribute to the restoration and maintenance of peace in the region." This second is an ambitious claim for a court of law, and begs the questions: what has been achieved, and what next, when the trials of Karadzic and Mladic are over?

The mandate is a statement of contrition as well as ambition. For three long, bloody years, Bosnia's war was arguably one of the worst failures of diplomacy the UN has ever endured, along with its mishandling of the genocide in Rwanda, where it also established a tribunal. In its diplomacy, the UN did little more than appease – and often encourage – the pogrom Karadzic is accused of masterminding. UN "protection force" troops stood haplessly by as the slaughter continued, and their commander, General Bernard Janvier, took lunch with Mladic three days before the Srebrenica massacre, which Mladic and Karadzic are accused of ordering; 8,000 men and boys were executed after Dutch UN troops evicted much of the UN-declared "safe area's" population from their compound and looked on as the Serbs separated out males from females, for brazenly obvious motives.

And there is a thread between these origins and what has become a weariness with the tribunal's work on the ground, and among the victims themselves. After Karadzic's arrest in 2008, the streets of Bosnian cities were lined with honking cars, but after that of Ratko Mladic last year, there was no such celebration. The chief prosecutor at The Hague, Serge Brammertz, echoed the wider brief when he said: "These victims have endured unimaginable horrors – including the genocide in Srebrenica – and redress for their suffering is long overdue … We believe that it can have a positive impact on reconciliation in the region." While Sabaheta Fejzic, who lost her son and husband in the Srebrenica massacre, says: "I am not that happy. I was disappointed so many times by the work of the Hague tribunal."

Certainly, the International Criminal Tribunal for the former Yugoslavia has become part of a burgeoning industry of war crimes trials – and a boon to those who would defend war criminals. One British defence lawyer, who had worked on two of the trials, was reported to me as making up to $100,000 a month advising and defending those accused of war crimes around the world. The practice of "fee-splitting" between lavishly paid defence counsel and their criminal clients became so widespread and lucrative by 2002 that it provoked a protest from the US state department. But also groundbreaking achievements are plain to see. Even apart from landmark legal successes, the narrative of Bosnia's catastrophe has been told for history's record by its victims from those blue chairs at the witness stands – even if only to empty press and public galleries. Leaders have been made accountable, international law developed, strengthened, clarified and made applicable to internal conflict.

Mark Harmon is a former public defender in California, who recently retired as senior prosecutor for the ICTY – having been with the tribunal from the start. He has worked on the cases that climbed the pyramids of crime and power in Bosnia, from the days he first muddied his boots on the soil of mass graves in Srebrenica to his work on the Karadzic case. Harmon knows better than anyone how the war Karadzic and Mladic are accused of masterminding was ordered and executed, and how they came to arrive at The Hague.

Harmon recalls the very first trial in 1996 – that of Dusko Tadic, who toured the Omarska and Keraterm camps, killing and beating. There was much criticism at the time about the expense of trying a minnow in the war, and disbelief that Karadzic or Mladic would ever grace the same dock. "Tadic was one of the most important cases," reflects Harmon. "It established the existence of a large crime base, it confirmed the jurisdiction of the tribunal and it established that the violations applied to an internal armed conflict. Tadic shifted the paradigm of protections in international armed conflict to internal armed conflict. The law was set, the platform established that we were capable of trying the cases we were charged to try."

As the crime base was established, and the tribunal scaled the ladders of command towards Karadzic and Mladic, the cases became more dependent, says Harmon, on "access to relevant documents, rather than blood and guts". In September this year, the tribunal convicted Momcilo Perisic, former chief of general staff of the Yugoslav army in Belgrade, a case on which Harmon worked, "which showed a man directing the war from his desk in Serbia – no direct contact with victims at all. Building up the pyramid, the work was based less on the victim testimony of earlier trials than facing down the difficulties of direct government obstruction of our efforts...the trials become more sterile and lose the victims' voice, because the trials at the top, with the likes of Karadzic, are all about proving linkages, with the atrocities already established".

In his most remarkable case, Harmon led the investigation, prosecution and conviction of General Radoslav Krstic, General Mladic's senior officer in command of the Srebrenica massacre. Krstic was one of the very few cases in which the prosecution had a penitent witness from the perpetrating side, a soldier in the Bosnian Serb army called Drazen Erdemovic, who came to The Hague remorseful at what he had done, pleaded guilty and was given a lenient sentence. Thereafter, he testified in numerous Srebrenica cases as a prosecution witness. Erdemovic told the court about unrelenting execution after the fall at Srebrenica, so that the death squads had to mass-murder in shifts. He testified to his wish that he be relieved of his execution duties. Most importantly Erdemovic gave information leading Harmon's chief investigator on the case, Jean Rene Ruez, to an execution site about which the world knew nothing, at the Cultural Centre in the town of Pilica.

"Erdemovic, and the Krstic case, had a huge impact", says Harmon. "This was at a time of total Srebrenica denial by the Serbs. And there was Erdemovic, saying he couldn't kill any more, sitting in a café having a cup of coffee while over the road – closer than the wall of this café here – 500 people were being killed. We would never have known if Erdemovic hadn't told us. As it is, Jean Rene Ruez went to the Pilica Cultural Centre and discovered a grisly massacre scene. Blood smeared the walls, and under the stage of the cultural centre, there were stalactites of coagulated blood". At the same time, Harmon and the investigating teams began to trace the mass graves where the 8,000 executed around Srebrenica were buried, after US Secretary of State Madeleine Albright made the apposite satellite images available. "We were able to see the freshly dug holes – and trace how the Serbs had moved body parts from one mass grave to another to try and conceal the evidence, and lay the ground for exhumations."

Harmon says the wider legacy of the tribunal, as a deterrent for future war crimes and criminals, "is hard to measure. You can't measure deterrence, and we must not overclaim. But it was a pioneering institution; it took some baby steps towards holding people who commit war crimes to account. It developed and refined international law and criminal procedure. The international criminal court down the road is here today because of the success of the International Criminal Tribunal for the former Yugoslavia. If we had failed, it would probably still be in the laboratory. Out of that experiment, people have been trained – inoculated if you will – to become major players in these other tribunals, for the prosecution and the defence – because these cases are about doing justice.

"And I don't think it ever occurred to Karadzic and Mladic, when they were doing these things, that they would be where they are today".

Among the tribunal's critics are people who have a didactic or political interest in undermining it, or like to jeer pointlessly. But there are others who wish it well and have followed its progress. Among the latter is the expert on the landmark trials at Nuremberg that were the ICTY's inspiration – Peter Maguire, author of Law and War, a book about Nuremberg, and another on the genocide in Cambodia.

"The biggest problem facing all of the UN courts today," he says, "is that they were so grossly oversold by human rights advocates during the 1990s. At best, a war crimes trial can convict the guilty and exonerate the innocent in a timely manner. To ask trials to teach historical lessons or provide some form of therapeutic legalism is asking too much of any trial. The idea that war crimes trials can 're-educate' societies is based upon the assumption that the Nuremberg trials did more than punish the guilty and exonerate the innocent, they also transformed Nazis into law abiding democrats. The fact is that neither assumption stands up to analysis."

Maguire argues that "by the end of the 1990s, 'the legacy of Nuremberg' had become little more than a rhetorical tool used to justify any and all war crimes trials and the long march towards an international criminal court with universal jurisdiction. My former teacher, the late Telford Taylor [a prosecutor at Nuremberg], taught me that war crimes prosecutions – under any circumstance – signified failure: failure to act, failure to deter, and finally failure to prevent. Simply put, trials never can make up for disgraceful inaction in the face of preventable atrocities. Nobody in their right mind opposes the punishment of war crimes perpetrators, but coming after the bloodiest century in the history of man, is it enough to seek salvation in new codes of international criminal law and world courts?"

The woman on whose shoulders much of the tribunal's extra-legal mandate – its legacy on the ground – falls, is its head of outreach, Nerma Jelacic – also head of communications for the ICTY. She is from Visegrad, a town on the Drina river in eastern Bosnia, scene of horrific violence. Jelacic's plans are to impact the tribunal's work in a country more torn than at any time during the war: "They involve entrenching the current outreach offices and moving the operation and the defence lines from The Hague to the Balkans: not just to Sarajevo, Zagreb, Belgrade and Pristina - but to the municipalities, the villages themselves.

"The work of the tribunal," she says, "is still being undermined by elements of society which should and could have a healing effect, but they don't: politicians, media, religious leaders - some still maintain the divisions in society. And that is one big machinery to fight against. These divisions are entrenched now and it will take many years for those societies to emerge even partially healed from the traumas they faced. The truth is that no people or nation in former Yugoslavia is ready to see its own reflection; to accept what they see and come to terms with its own past.

"What has happened at the tribunal," adds Jelacic, "is that an unprecedented amount of work has been done by this tribunal and it has changed history. But if you ask anyone 'Has the tribunal brought reconciliation?' the answer is of course, 'No it hasn't.' By itself, it never could have. But if you ask me whether I am going to get to work on unfertile ground and try to bring recognition of the importance of the enormous amount of work done by this court, especially if you compare it to other conflict countries and the attention they received in the 90s, the answer is, 'Yes'."

"What I want to do is to break down the barriers, on the individual basis that a raped Muslim woman has a lot in common with a raped Serbian woman. If people can one day recognise the commonalities between the people who were reaped, beaten, tortured and had their loved ones killed, something of what has happened here at this tribunal will have contributed to that recognition".

Towards the close of our session in the holding cells it seemed churlish for there not to be a little banter with Karadzic. Talk turned to what a "fantasy" Yugoslav football team would have looked like at the next World Cup, had the country not torn itself apart: Vidic of Serbia in defence, Modric of Croatia and Dzeko of Bosnia in attack. "We'd win it," Karadzic says, a keen football fan who was once a psychiatric consultant to the FK Sarajevo football team which now plays in what he calls "Muslim Sarajevo".

Karadzic's final aside in the holding cells is directed towards his prosecutor, Ann Sutherland: "Ah, you see how hard Miss Sutherland is trying to convict me. It will make my freedom even sweeter!"

The War is Dead, Long Live the War by Ed Vulliamy will be published by The Bodley Head in the spring