They will fill the VIP stands at Srebrenica next weekend to mark the 20th anniversary of the worst massacre on European soil since the Third Reich; heads of state, politicians, the great and good.

There will be speeches and tributes at the town’s memorial site, Potocari, but the least likely homily would be one that answered the question: how did Srebrenica happen? Why were Bosnian Serb death squads able, unfettered, to murder more than 8,000 men and boys in a few days, under the noses of United Nations troops legally bound to protect the victims? Who delivered the UN-declared “safe area” of Srebrenica to the death squads, and why?

Over two decades, 14 of the murderers have been convicted at the war crimes tribunal in The Hague. The Bosnian Serb political leader Radovan Karadžic and his military counterpart, General Ratko Mladic, await verdicts in trials for genocide. Blame among the “international community”charged with protecting Srebrenica has piled, not without reason, on the head of UN forces in the area, General Bernard Janvier, for opposing intervention – notably air strikes – that might have repelled the Serb advance, and Dutch soldiers who not only failed in their duty to protect Srebrenica but evicted terrified civilians seeking shelter in their headquarters, and watched the Serbs separate women and young children from their male quarry.

Now a survey of the mass of evidence reveals that the fall of Srebrenica formed part of a policy by the three “great powers” – Britain, France and the US – and by the UN leadership, in pursuit of peace at any price; peace at the terrible expense of Srebrenica, which gathered critical mass from 1994 onwards, and reached its bloody denouement in July 1995.

Until now, it has always been asserted that the so-called “endgame strategy” that forged a peace settlement for – and postwar map of – Bosnia followed the “reality on the ground” after the fall, and ceding, of Srebrenica. What can now be revealed is that the “endgame” preceded that fall, and was – as it turned out – conditional upon it.

The western powers whose negotiations led to Srebrenica’s downfall cannot be said to have known the extent of the massacre that would follow, but the evidence demonstrates they were aware – or should have been – of Mladic’s declared intention to have the Bosniak Muslim population of the entire region “vanish completely”. In the history of eastern Bosnia over the three years that preceded the massacre, that can only have meant one thing.

Srebrenica nestles in a verdant valley, among mountains that rise from the banks of the river Drina. It is the location of a famous silver mine – srebro means silver. But by July 1995, Srebrenica had been a living hell for three years.

In the spring of 1992, Bosnian Serb troops had launched a hurricane of violence in pursuit of a racially pure “statelet”, after multi-ethnic Bosnia voted for independence from disintegrating Yugoslavia. And nowhere more savagely than in eastern Bosnia, where entire villages were eradicated, towns torched, their populations killed or put to flight by what Karadžic called “ethnic cleansing”.

Survivors fled into three eastern enclaves where the Bosnian republican army had resisted: Goražde, Žepa and Srebrenica, their populations swelled by displaced deportees, cowering, bombarded relentlessly and largely cut off from supplies of food and medicine. The population of Srebrenica swelled from 9,000 to 42,000, and by March 1993 the situation was sufficiently horrific for a French general, Philippe Morillon, to lead a convoy into the battered pocket and, appalled, promised: “You are now under the protection of the UN. I will never abandon you.” The UN duly proclaimed Srebrenica as one of six “safe areas” to be defended by the United Nations Protection Force (our emphasis), or Unprofor.

The following month, April 1993, the UN security council passed a resolution whereby any peace in Bosnia must “be based on withdrawal from territories seized by the use of force and ‘ethnic cleansing’.” And in the same month, a report from that same security council warned specifically of a “potential massacre in which there could be 25,000 victims if Serb forces were to enter Srebrenica”.

Its fears were justified: Karadžic promised the Bosnian Serb assembly the following July that if his army entered Srebrenica there would be “blood up to the knees”.

Two years later, Srebrenica remained under relentless siege, while the UN, European Union and Contact Group of five nations dealt for peace. Bosnia’s carnage had confounded the world’s most experienced diplomats; ineffective talks and plans had played out and failed for three bloody years. All the while, Karadžic’s hand was eagerly clasped beneath the chandeliers of London and Geneva; diplomats also courted the Serbian president, Slobodan Miloševic, while Mladic dined and exchanged gifts with the UN’s military commanders, soldier to soldier, as they ineffectively sought his cooperation.

By spring 1995, the Contact Group – the US, UK, France, Germany and Russia – appeared to abandon the 1993 resolution against rewarding ethnic cleansing, as it sought to partition Bosnia between a Serb statelet and a Muslim-Croat federation. Then the French foreign minister, Alain Juppé, had privately confided the working map in mid-1994: it showed the three eastern “safe areas” to be contiguous with one another and part of the federation.

But Miloševic complained to the Contact Group’s negotiator, an American, Robert Frasure, that the safe areas constituted “a monstrous excrescence” within Serbian territory. Frasure reported to the national security council in Washington that Miloševic would not agree to peace unless he had a “modified” map that ceded the safe areas.

America’s national security adviser, Anthony Lake, told Frasure in a memo that he favoured revising the map. The former Dutch defence minister Joris Voorhoeve recalls a meeting with Lake at which the American appeared to be “one of a number of persons – who might not like to be reminded of the fact – who then thought the enclaves were indefensible anyway … They considered the enclaves to be very complicated situations which did not fit into a future map.”

Lake, who is now head of the UN Children’s Fund, Unicef, said last week: “While holding the position of executive director of Unicef, whose humanitarian mission depends on its non-political character, I have had to decline, often regretfully, to speak publicly about events in my previous career as a government official. I apologise and wish it were otherwise, for there is no doubt about the importance of the war in Bosnia. There was no issue about which I cared more deeply.”

A CIA memo, since declassified, described the eastern safe areas as “fish bones in the throat of the Serbs”. Frasure later told a meeting that he saw “one last card. To make a deal with a Chicago mafia boss, one must be ready to give enough ground to ensure he will fulfil his part of the contract. It’s the same with Miloševic.”

A counsellor to President Clinton, Alexander Vershbow, would recall in 1998 that by June 1995, “Srebrenica’s future seemed pretty gloomy. We were already then considering that some kind of swap for at least the smallest of the eastern enclaves for more territory would be wise.”

France and Britain agreed: General Bertrand de La Presle, adviser to the president, Jacques Chirac, would later visit Mladic on 29 May, with a message “from the French president and the French government”. According to Mladic’s notebooks, found in his flat while he was on the run, it said: “France clearly understands your concern, that you do not want the Contact Group map. Since last fall [1994] three amendments to the Contact Group proposal have been adopted on the initiative of France and Great Britain … The map can change through negotiations.”

On 3 June, at a meeting in Paris, Britain’s defence secretary, Malcolm Rifkind, would urge that the enclaves were “untenable”. Rifkind said last week: “The UN declared safe areas with, in their judgment, a minimum troop requirement to make them so. Britain increased its numbers in Bosnia and so did France, but not others. They can call them safe areas, but you have to put enough troops there to make them safe, otherwise they are untenable.”

Pressure was put on the Bosnian president, Alija Izetbegovic, to concede Srebrenica and the other safe areas. “The message was clear: the enclaves have no future,” recalls the Bosnian government’s chef de cabinet, Mirza Hajric. Izetbegovic had told civilian authorities in Srebrenica back in September 1993 that surrender of their town might be the price of peace; they refused to discuss it. In April 1995, the presidency summoned 15 military commanders from Srebrenica to the government-controlled town of Tuzla, forbidding them to return. The protection of the safe area was, the government argued, the duty of the international community.

Meanwhile, on 8 March, the Bosnian Serb military command had issued “Directive 7”, which escalated what had, until then, been called the “slow suffocation of the enclaves” and now ordered “combat operations to create an unbearable situation of total insecurity of life with no hope of survival or life for inhabitants of Srebrenica and Žepa”. The directive demanded the “permanent removal” of Bosnian Muslims to “liberate definitively the entire Drina valley region”.

Mladic told the Bosnian Serb assembly of his plans for the Bosniak population of the enclaves: “My concern is to have them vanish completely.” Both the directive and Mladic’s speech were known to western governments.

On the same day, 8 March, Mladic met the British general Rupert Smith, the head of UN peacekeeping in Bosnia, at the Hotel Panorama in “cleansed” Vlasenica. According to Smith’s military adviser, Lt Colonel James Baxter, “Mladic took out the map and drew a scratch over each of the enclaves”.

During March, says the then head of military planning at the UN peacekeeping department, General Manfred Eisele, the department and the Netherlands pushed for reinforcements for Dutch troops in Srebrenica. The proposal was overruled by the US, he says, on grounds that the enclaves were “untenable” and US helicopters would be used to transport the reinforcements.

The US policy-making Principals Committee, meeting on 19 May, expressed its view that: “The only realistic option is to seek Allied support for an Unprofor pull-back from vulnerable positions” – ergo, the safe areas – “coupled with more robust enforcement of the remaining mandate, including Nato air strikes.”

The French general Bernard Janvier, overall commander of UN troops on the ground, told the security council member states on 24 May that: “The enclaves are indefensible, and the status quo untenable.” He said UN troops were too vulnerable in the safe areas, and should either be reinforced, or withdrawn to make way for air strikes.

The following day, 25 May, any prospect of further air attacks collapsed anyway as 400 UN troops were taken hostage by Serbs, in retaliation for an air strike.

Two days later, presidents Clinton and Chirac and the British prime minister, John Major, spoke by telephone to discuss a response, including halting air strikes. Next day, 28 May, according to the declassified US national security archive, the Principals Committee formalised a decision, apparently made during the phone call, “to suspend the use of Nato air strikes against the Serbs for the foreseeable future”.

Lake, in a memo to the president, outlined the need of secrecy: “Privately we will accept a pause on further air strikes but make no public statement to that effect.’’

During early June, the UN military monitor in Srebrenica, the Kenyan colonel Joseph Kingori, reported to peacekeeping headquarters that the Bosnian Serb “Colonel [Vlatko] Vukovic insisted on trying to find out what would be the reaction of the United Nations if the Bosnian Serb army would capture the enclave and expel the population – literally, all the people living inside that enclave”.

In later testimony at the International Criminal Tribunal for the former Yugoslavia, Kingori testified that he had reported that any “safe passage” granted to those leaving the area “did not apply to those considered as war criminals”, ergo, men of fighting age. Kingori’s reports went apparently unheeded.

On 2 June, Mladic ordered a “destruction of the Muslim forces in these enclaves”. Voorhoeve insists that western leaders knew of this order, but that he and his troops were kept in the dark. “The intelligence services of at least two of the five permanent members of the UN security council knew already at the beginning of June 1995 – a month and a half before the attack – that the Serbs intended to capture, in the coming weeks, the three Eastern enclaves – meaning Srebrenica, Žepa and Goražde,” Voorhoeve says. “These two big countries had advance knowledge of the Serbian battle plans and did not share it with the Netherlands.”

The Observer has independently verified this and the two countries were the US and the UK.

Smith, Janvier and the UN’s special envoy to the Balkans, the Japanese diplomat Yasushi Akashi, met on 9 June in Split, where Janvier pushed for ceding the enclaves, saying: ‘‘Most acceptable to the Serbs would be to leave them the enclaves. It is the more realistic approach and it makes sense from the military point of view.” He added: “But this is unacceptable to the international community.”

Smith was forthright, warning Akashi of a forthcoming “crisis that, short of air attacks, we will have great difficulty responding to”.

A whole month passed while Mladic prepared his assault and, it transpired, the massacre. On 6 July, he ordered his tanks to advance. Two days later, a UN military observer reported: “The Bosnian Serb army is now in a position to overrun the enclave. Since the UN response has been almost nonexistent, they will continue until they achieve their aims.”

On the same day, despite American reconnaissance planes portraying the alarming situation around Srebrenica, a cable from US intelligence in Zagreb informed Janvier’s HQ, also in the Croatian capital, that the Bosnian Serbs had “no interest in occupying Srebrenica given that they have no idea what they would do with all the local Bosnian Muslims”.

Also on 8 July, Akashi and generals Smith and Janvier met at UN headquarters in Geneva. Smith was told to return to his holiday on the Croatian island of Korcula while Akashi, the only man in the Balkans with authority to order air strikes, went to Dubrovnik for a two-day break.

The Bosnian leadership in Sarajevo warned the UN on 8 July that “genocide against the civilian population of Srebrenica may occur” but did not call for evacuation. The populace chose to remain, wrongly believing the world would comply with legally binding obligations to protect them.

The stories of the fall and ensuing massacre are well known. Srebrenica’s inhabitants sought protection at the Dutch HQ, but were expelled. The UN’s envoy, Akashi, sent a cable: “The Bosnian Serb army is likely to separate the military-age men from the rest of the population, an eventuality about which Unprofor will be able to do very little.” Indeed, Dutch soldiers watched Mladic’s troops separate women and young children (for expulsion) from men and boys (for killing). Many of them had been expelled from the compound.

Early on 12 July, the Dutch commander in Srebrenica, Colonel Ton Karremans, met Mladic, with orders to “let the Serbs organise the transport” of civilians out of Srebrenica. But, says General Onno van der Wind of the Dutch defence ministry, the UN then provided 30,000 litres of petrol which proved necessary for the genocide. “After Unprofor approval,” says Van der Wind, “the fuel was delivered in Bratunac [the Bosnian Serb HQ outside Srebrenica] after the arrival of a logistical convoy.” The UN petrol was used, he says, to fuel transport of men and boys to the killing fields, and bulldozers to plough the 8,000 corpses into mass graves.

The mass murder was later described at The Hague by Judge Fouad Riad as “written on the darkest pages of history”. A sole “executioner” to turn prosecutor’s evidence at the trials, Dražen Erdemovic, described how death squads asked to sit down – they were so tired, killing wave upon wave, busload after busload, of men and boys.

One of the very few men to survive the killing fields, Mevludin Oric, recalled: “I just threw myself on the ground; my nephew shook, and died on top of me.” Mevljudin remained lying, face down, all day. “When they finished shooting, they went to get other groups. They kept bringing new rounds of men. I could hear crying and pleading, but they kept on shooting. It went on all day.”

For a while, Mevludin lost consciousness. “When I came round, it was dark, and there was a little rain. My nephew’s body was still over me; I removed the blindfold. There was light coming from bulldozers that were already digging the graves. By now, [the Serbs] were tired and drunk, still shooting by the light of the bulldozers. They went to those who were wounded and played around with them. ‘Are you alive?’ And if the man said ‘Yes’, they would shoot again. Finally they turned off the lights.

“I started to move a little. I got my nephew off me. I arose and saw a field full of bodies, everywhere, as far as I could see. And I cried; I could not stop myself.” Amazingly, “there was another man on his feet. I thought I was dreaming, seeing things. I walked towards him; I had to step on bodies to get to him – there was no patch of land without bodies. I hugged and kissed him – his name was Hurem Suljic.” Mevludin and Suljic walked through the forests to Tuzla, narrowly escaping discovery and death many times. Their journey to safety took 11 days.

According to declassified US cables details of the killings reached western intelligence and decision makers soon after they began on 13 July; CIA operatives watched almost “live” at a satellite post in Vienna. From that day, spy planes caught what was happening. “Standing men held by armed guard. Later pictures show them lying in the fields, dead,” according to one cable.

A senior state department official insists: “All US partners were immediately informed.” Yet the slaughter was allowed to run its course, no attempt made to deter the killers, or to locate the men and boys, let alone rescue them.

The next day, 14 July, the UN security council said it feared “grave mistreatment and killing of innocent civilians”; it said it had received “reports that 4,000 men and boys have gone missing”. But the diplomats continued business as usual.

That day, the European Union’s special envoy, Carl Bildt, met Mladic and Miloševic while the killing machine was at full throttle, though he seems not to have mentioned the massacre. Bildt says that he urged Mladic that “boys and young adults from Srebrenica who have been taken to Bratunac need to be released”. He said the Red Cross should be allowed to register prisoners. Bildt had foremost in mind, it seems from his memoirs, the release of 30 Dutch hostages, and wrote a report after the encounter saying: “Mladic readily agreed to most of the demands on Srebrenica.”

On 15 July, Bildt met Mladic again – and Miloševic – for lunch with Akashi and Smith. Only Smith raised the issue of “information on mass killing and rape” and threatened force “if UN forces are attacked”. But all the group got in return was an assurance that Dutch soldiers would be free to leave on 21 July, with their equipment and the 30 hostages, and with that the delegation left.

Bildt told the Observer last week: “It was clear that knowledge of what really happened” at Srebrenica, “wasn’t there until considerably later. On the meetings of [July] 14-15, there are also good UN cables,” he said, which “will be released”, after a conference this week. He continued: “There were certainly extensive discussions of, and clear reactions to, Srebrenica also on the 15th. Free and immediate access for ICRC and UNHCR to Srebenica to register and help POWs was among the key points. I see that you have seen the brief Mladic account of what was agreed. Worded differently and more brief than UN account but no difference in substance.”

The war ended after the Dayton peace agreement of December 1995, after the US envoy, Richard Holbrooke, negotiated a map that ceded Srebrenica and Žepa, but kept Goražde in the federation. Holbrooke told Bosnian Hayat TV in 2005, on the 10th anniversary of Dayton: “I was under initial instructions to sacrifice Srebrenica, Goražde and Žepa”.

Seasoned diplomats insist the massacre came as a surprise. The US assistant secretary for human rights, John Shattuck, said: “We had the Omarska model in mind” – ergo, that Mladic would imprison men in camps, for use as “an extremely valuable bargaining counter to gain territorial exchange or even political concessions” as Richard Butler – the US intelligence officer who worked as the Srebrenica military expert at the International Criminal Tribunal for the former Yugoslavia – put it.

A US briefing paper on Srebrenica reads: “We did not have any information on any Bosnian Serb intent to commit atrocities against the Muslim defenders or population of Srebrenica.”

Pauline Neville-Jones, then political director at the British Foreign Office, argued as late as 2009: “It still remains to be established whether the Serbs had a long-range intention to do just that [massacre men and boys]. Serb forces engaged in an ethnic cleansing campaign to rid Srebrenica of its Muslims [which] eventually became genocide when the decision was made to separate men targeted for extinction.”

Jean-Claude Mallet, the director of strategy at the French defence ministry, says in an interview: “I had no illusion that atrocities would be committed. We had reported that. But never such as the ones that occurred.”

The International Criminal Tribunal for the former Yugoslavia rejects these views, ruling that the killings were premeditated well in advance. In the conviction of the Bosnian Serb general Radislav Krstic for aiding and abetting genocide at Srebrenica, the court ruled: “Without detailed planning, it would have been impossible to kill so many people in such a systematic manner in such a short time, between 13 July and 17 July.”

The International Court of Justice would rule in 2007: “It must have been clear that there was a serious risk of genocide in Srebrenica.”

France’s foreign minister at the time, Alain Juppé, says in an interview: “We all knew the men would be annihilated, or at least that the Serbs were not sparing the lives of prisoners”.

Not a single politician, diplomat or senior soldier saw fit to resign over the betrayal of Srebrenica. It will be interesting to see if anything approaching an apology – let alone a reckoning – by Britain, America or France is spoken next weekend. Most of those involved were promoted or moved on to lucrative positions. After he had left the government, the former British foreign secretary, Douglas Hurd, who had chastised attempts at intervention to help Bosnia, along with Neville-Jones, famously beat a path to Belgrade to engage Miloševic – just before he was indicted for genocide – on behalf of the NatWest markets bank.

The reaction of Akashi to the killing, as it began on 13 July, was to assure that the UN “should not fear an international outcry as at no time have Unprofor drivers or vehicles assisted in the evacuation”.

Toby Gati, the US assistant secretary of state for intelligence, told the current US ambassador, Samantha Power, for a book: “Ethnic cleansing was not a priority of our policy. When you make an original decision you are not going to respond, then I’m sorry, these things are going to happen.”

The then UN secretary general, Boutros Boutros-Ghali, told the BBC on 11 July 1995, when Mladic entered Srebrenica: “We have been humiliated and duped. We will have to live with it. But in several days, it will belong to the past.”

Bildt, in his memoirs, insists that: “They [the Bosnian leadership] knew that the peace settlement would mean the loss of the enclave. So from this point of view what happened made things easier.”

L’affaire Srebrenica: Le Sang De La Realpolitik (The Srebrenica Affair: The Blood of Realpolitik),
by Florence Hartmann, will be published on Tuesday by Don Quichotte Éditions, Paris, as an e-book from
all online book stores, price: €9.90, and later in the year in print

Key players in 1995

Radovan Karadzic

Bosnian Serb political leader, currently detained in The Hague by the tribunal

Slobodan Miloševic

Serbian president, died in prison in The Hague in 2006 while on trial

Alija Izetbegovic

Bosnian president who called a referendum on independence for Bosnia in 1992

Carl Bildt

Swedish politician who became the European Union’s special envoy to the former Yugoslavia

Anthony Lake

Served as America’s National Security Adviser under Bill Clinton, 1993-1997

Malcolm Rifkind

British defence secretary 1992-1995. Said the UN safe areas were untenable

Pauline Neville-Jones

British diplomat who led the UK delegation at the Dayton peace negotiations

General Rupert Smith

British soldier was head of peacekeeping forces in Bosnia.

Colonel Joseph Kingori

Kenyan officer who was the UN’s military monitor in Srebrenica

General Bernard Janvier

The French head of the UN’s peacekeeping force from 1995

This article was amended on 5 July 2015 to clarify General Rupert Smith’s role.