The Serb leaders charged with ordering the killing of thousands of men at Srebrenica remain free, ignored by Nato troops
The worst single war crime in Europe since 1945 happened two years ago today, when the Serb army in Bosnia launched the attack which led to the massacre of more than 7,000 people, mostly men, who were fleeing from the UN "safe area" of Srebrenica.

The Serb attack began before dawn on 6 July 1995. Five days later the enclave, housing around 40,000 Bosnian Muslim refugees "cleansed" from eastern Bosnia by their Serb enemies, had fallen into the hands of General Ratko Mladic, and the executions of thousands of Muslims and a handful of women were completed within a week.

So far, only 900 bodies have been located, either in mass graves or lying where they died in the hills around Srebrenica, but the International Red Cross lists in a thick volume the names of 7,300 people missing and now presumed dead.

General Mladic and the Serb leader Radovan Kradzic were indicted by the International Criminal Tribunal in The Hague for having ordered the massacre. Both roam freely in Republika Srbska despite the fact that it is patrolled by Nato troops. General Mladic apparently tends a flock of goats which he has named after UN commanders and Western politicians. Their continuing freedom is described by the UN spokesman in Sarajevo, Kris Janowski, as "an insult to the memory of the victims and an embarrassment to the international community".

Srebrenica was one of six Bosnian towns designated as "safe areas" to be protected by UN peace-keepers - although the Western powers declined to provide the soldiers needed to enforce the areas and deter Serb attacks. The major powers knew the policy was a fiction; but many Muslims believed in it.

The town's fall in 1995 wasswift. Some 450 Dutch soldiers whose job was to ensure the safety of the area and the delivery of humanitarian aid, did nothing to halt the advance. They claimed they were powerless to act, and Dutch requests for the use of Nato air power against the attackers were rejected until it was too late.

Nobody, Bosnian or foreign, had expected the Serbs - who had no taste for the kind of street fighting required to capture towns - to take Srebrenica so quickly, but it fell on 11 July and Gen Mladic lost no time in consolidating the victory that would free up his troops for operations against Sarajevo and other enclaves.

On 12 July, he was filmed patting heads, chucking chins and assuring the refugees they would be given safe passage to government territory. But the Muslims were clearly terrified of him; they had reason to be because Mladic ordered that the men were to be separated from the women and children and screened for evidence of "war crimes".

Within hours, the Serbs had lined up buses to carry their victims away. Most of the old men, women and children made it across the line, though many were robbed by the Serbs. But the men simply disappeared. Meanwhile, a column of up to 15,000 Muslim men, about one-third of them soldiers, had gathered on a hill near the town intending to walk across-country and through enemy territory to safety.

Roughly half survived the 50-mile journey; exhausted and starving, they were saved by comrades from Tuzla, who broke the Serb lines and created a rescue corridor. But thousands more were killed en route - many of their bodies still lie in the hills and woods around Srebrenica - or captured by Serb soldiers. Video footage shows Mladic's troops herding groups of Muslim men, unarmed and in civilian clothes, into fields beside a main road where a mechanical digger was digging at least one large grave.

When Dutch peace-keepers taken hostage by the Serbs were driven along that same road on 16 July, their stomachs churned at the stench of rotting flesh. But what they witnessed was not made public at the time.

On 15 July, Carl Bildt and Thorvald Stoltenberg, the EU and UN negotiators, accompanied by Lt-General Rupert Smith, the UN commander in Bosnia, and Yasushi Akashi, the civilian UN boss, met Gen Mladic and his mentor, President Slobodan Milosevic of Serbia, in Belgrade. Gen Mladic promised that the International Committee of the Red Cross would have access to the men held by his troops, and, as usual, he was lying.

The ICRC was denied access to the supposed prisoners for reasons of "security". Their role was to interview relatives of the missing Muslims and compile the best estimate of the number still missing from Srebrenica. The 7,300 the ICRC have identified are almost certainly all dead.

"These people will never be found again," says Michael Kleiner of the ICRC, which has investigated, to no avail, the many claims that the Srebrenica men are hidden away as slave labour. "We have reason to believe they were all killed, although we don't have the proof." The evidence came from the handful of survivors - men who cheated death by hiding under the corpses of their friends and relatives - who bore witness to what happened in the next eight or nine days.

Gen Mladic and his men commandeered bulldozers to dig mass graves and trucks to transport their prisoners to execution sites, in fields and warehouses across the strip of eastern Bosnia that borders Serbia. The Muslim men were lined up and shot dead, the bodies flung into pits. They were victims of Serbian determination to revenge earlier defeats, to prevent Muslim soldiers fighting on, and ultimately to cleanse Bosnia of Muslims.

Last summer, forensic experts working for the International Criminal Tribunal for the former Yugoslavia exhumed more than 450 bodies in four mass graves near Srebrenica. Their investigations continue, says the tribunal spokesman, Christian Chartier. More graves may well be identified this summer: I have seen myself evidence of a huge operation in eastern Bosnia in muddy fields containing human bones, clothing, ID cards, odd shoes. Schools and grain stores are still scarred by bullet holes, blood stains and the desperate graffiti of dying men.

"These crimes demand a legal accounting," says Christian Chartier. "If one believes there will be no peace without justice, justice requires trials, and trials at the tribunal require a defendant in the dock." But only one man has been brought to justice so far, and he was a pawn.

Drazen Erdemovic, a Croat fighting in Gen Mladic's army, shot Muslims during the Srebrenica executions. "I had to do this," he declared, pleading guilty to a charge of crimes against humanity brought by the war crimes tribunal. "If I had refused I would have been killed together with the victims."

Erdemovic told the Hague tribunal that on 16 July - the day after Gen Mladic gave the Red Cross access to Muslim prisoners - his unit shot dead hundreds of Muslim prisoners. He was ordered to serve 10 years in prison, the first person to be sentenced for war crimes since the Nuremberg and Tokyo trials following the Second World War.

But the real perpetrators remain free, and Gen Mladic and Radovan Karadzic - both charged with genocide for ordering the summary executions of "thousands" in Srebrenica - are apparently invisible to the 20,000 Nato troops in Bosnia, 5,000 of whom are British.

NATO officials argue that they were given the right but not the obligation to arrest war criminals under the Dayton peace plan (which was sealed mainly because the failure of Western policy in Srebrenica persuaded Nato that is must bomb the rebel Serbs in order to end the war). Amnesty International and other humanitarian groups say Nato officials are wrong to deny their obligation. Under the Geneva Conventions, to which all Nato members are signatories, soldiers have an absolute duty "to search for persons alleged to have committed ... grave breaches" of the Conventions.

The first sign of a change of policy occurred last week when the War Crime Tribunal made its first arrest, based on a secret indictment, with the help of the UN in Croatia. But Mr Karadzic lives openly at his home in Pale, accompanied by bodyguards; he even registered to vote with foreign officials sent in to oversee local elections. Gen Mladic is more discreet, but he is thought to share a holiday house with his wife inside the main Serb military complex at Han Pijesak, 30 minutes' drive from Srebrenica.

WE cannot change the past, but we can try to administer justice. Arresting Gen Mladic and Mr Karadzic would surely be one way for the British government to act on Robin Cook's fine words about a foreign policy based on principles with action. Meanwhile, the widows of Srebrenica - the daughters, sisters, and aunts of the thousands who died - exist in limbo, reluctant to begin the process of grieving. Most of them will never have a body to bury. And abroad, the massacre is a matter for historical debate. The question is whether any government in the former Yugoslavia, and outside it, could have stopped the slaughter. David Rohde, author of A Safe Area, which tells the story of Srebrenica, says: "It's more a conspiracy of neglect than a smoky room where everyone decided they were going to let Srebrenica fall - which is just as inexcusable. It could have been saved."