Shortly after midnight on 13 May 1945, the British 5th Corps Headquarters in Austria estimated that there were "approximately 30,000 POWs, surrendered personnel, and refugees in Corps area. A further 60,000 reported moving north to Austria from Yugoslavia". The columns had fled to southern Austria ahead of the advance of the Yugoslav forces, the Partisans, hoping to surrender to the British army. The British refused to accept the Axis surrender and directed them to surrender to the Yugoslav military. Most of the captured POWs in the columns were subjected to forced marches over long distances.
The main fighting force against the Axis occupation of Yugoslavia (1941-45), in terms of numbers involved and campaigns undertaken, was the Partisan movement. The Axis-appointed Ustaše government in Zagreb headed the Nazi puppet state the Independent State of Croatia and had its own lethal agenda for Serbs, Jews, Roma and anti-fascist Croats.
This was manifested in the atrocities at Jasenovac concentration camp and elsewhere, the scale of which even shocked German and Italian occupying forces. As early as July 10, 1941, Wehrmacht General Edmund Glaise von Horstenau reported the following to the German High Command, the Oberkommando der Wehrmacht (OKW):
Our troops have to be mute witnesses of such events; it does not reflect well on their otherwise high reputation... I am frequently told that German occupation troops would finally have to intervene against Ustaše crimes. This may happen eventually. Right now, with the available forces, I could not ask for such action. Ad hoc intervention in individual cases could make the German Army look responsible for countless crimes which it could not prevent in the past.—General Edmund Glaise von Horstenau, German military attaché in Zagreb
Increased activity of the bands is chiefly due to atrocities carried out by Ustaše units in Croatia against the Orthodox population. The Ustaše committed their deeds in a bestial manner not only against males of conscript age, but especially against helpless old people, women and children. The number of the Orthodox that the Croats have massacred and sadistically tortured to death is about three hundred thousand.
The Yugoslav Partisan movement grew rapidly from these atrocities. Eventually, units of the Ustaše military began defecting to the Partisans. By 1945, the Yugoslav Partisans numbered over 800,000 men organized into four field armies, and were in pursuit of the remnant of the defeated German and Ustaše forces.
A large-scale exodus of people took place. On May 6, 1945, the Ustaše collaborationist government fled Zagreb, as the Wehrmacht was in retreat and about to surrender.  The remnants of the Ustaše military and the Chetniks (a collaborating Serbian nationalist movement) began to withdraw to the Austrian border on May 12, traveling to Bleiburg where the 38th British Infantry Brigade was stationed.
The Army of the Independent State of Croatia was reorganized in November 1944 to combine the units of the Ustaše and Croatian Home Guard into eighteen divisions, comprising 13 infantry, two mountain, two assault and one replacement Croatian Divisions, each with its own organic artillery and other support units. There were also several armoured units. From early 1945, the Croatian Divisions were allocated to various German Corps and by March 1945 were holding the Southern Front. Securing the rear areas were some 32,000 men of the Croatian Gendarmerie (Hrvatsko Oruznistvo), organised into 5 Police Volunteer Regiments plus 15 independent battalions, equipped with standard light infantry weapons, including mortars. Among the remnants of these forces were numerous Ustaše dignitaries along with the ruling fascist elite, but also a number of civilians, inextricably mixed with the others in the confusion of the retreat. To the pursuing Partisans, the appearance was that the civilians within the retreating column were for the most part collaborationists, as they abandoned their homes and businesses to flee with Ustaše leaders. Retreating alongside the Ustaše military and the Chetniks were the remaining units of the Slovene Home Guard (a Slovene collaborationist militia).
Stipulations of unconditional German surrender also applied to the armed forces of the puppet Independent Croatia. This meant they too had to cease their activities on May 8 and stay where they found themselves. As late as 14 May 1945, however, when the war in Europe had ended, the quisling troops fought battles to keep escape routes open. The quisling troops refused to obey the stipulations of surrender and give up their arms. The Yugoslav commander-in-chief Marshal Josip Broz Tito repeatedly issued calls for surrender, and on May 14 dispatched a telegram to the supreme headquarters Slovene Partisan Army prohibiting "in the sternest language" the execution of prisoners of war and commanding the transfer of the possible suspects to a military court.
By the end of March, 1945, it was obvious to the Croatian Army Command that, although the front remained intact, they would eventually be defeated by sheer lack of ammunition. For this reason, the decision was made to retreat into Austria, in order to surrender to the British forces advancing north from Italy. When Ante Pavelić left Zagreb on May 6, he intended to join his regime in Austria. The first and last order that he gave was for his troops not to surrender to the Partisans, but to escape to Austria, in order to implement the Croatian Government's decision of May 3 to flee to Austria. The main column traveled through Celje, Šoštanj, and Slovenj Gradec on its way to Dravograd. On May 11 and 12, generals Vjekoslav Servatzy and Vladimir Metikoš entered discussions with Bulgarian generals to allow the Croatian column to pass into Austria. The discussions were inconclusive, but the Bulgarians suggested they head in the direction of Prevalje and Bleiburg which the column did. They began surrendering to the British on May 15, and this continued until the May 17, making these remnants of the NDH military the last Axis force in Europe to surrender. During this time Ustaše generals Ivo Herenčić of the V. Corps, and Vjekoslav Servatzy as well as a translator, Professor Danijel Crljen, began surrender negotiations with the British and the Partisans, represented by Milan Basta.
Ustaše (Yugoslav collaborationist) representatives attempted to negotiate a surrender to the British under the terms of the Geneva Convention, but were directed to surrender to the Yugoslav military, in accordance with Article 20 of the Hague Convention: After the conclusion of peace, the repatriation of prisoners of war shall be carried out as quickly as possible. General Robertson gave British troops the order, "All surrendered personnel of established Yugoslav nationality who were serving in German Forces should be disarmed and handed over to Yugoslav forces".
The Independent State of Croatia had joined the Geneva Convention on January 20, 1943, and was recognised by it as a "belligerent", that is, as a national state with armed forces in the field. All the signatories of the Convention, including Great Britain and the United States, were informed that this recognition had been given. However, this did not in any way nullify the requirement to immediately repatriate foreign nationals per the Hague Convention, but merely guaranteed the Yugoslav Axis soldiers prisoner of war status upon their surrender, as opposed to that of civilians.
During the retreat across Slovenia and in their time in Austria, the military conflicts between the Partisans and the retreating collaborationist forces continued. Of these, the biggest confrontation was the Battle of Poljana, ending in Partisan victory. The vast majority of the refugees were returned to Yugoslavia and were repatriated as Yugoslav citizens via forced marches under inhumane conditions over long distances.
The exact number of those who met their death in Bleiburg is almost impossible to ascertain. Unlike many other operations of the Yugoslav National Liberation Army, which have been described by the Yugoslav Communists in the minutest detail, very little has been written on operations in Slovenia near the Austrian Border during the week of May 7-15, 1945. This in itself would indicate that things occurred that official and pro-Communist historians consider best not discussed. Generally, there are essentially three schools that have tried to do this:
The first school whose estimates are based mainly on the historiographic and demographic investigations of scientists:
Historians made estimates, based mainly on the historiographic and demographic investigations:
The second school based its findings on accumulated eyewitness accounts.
This school bases its estimates on archeological evidence mostly consisting of mass graves found in Slovenia. Investigations are, however, at an early stage and therefore cannot be definitively linked with these incidents. The total number of potential locations that the Slovenian Commission on Concealed Mass Graves now intends to investigate is around 570. The first excavations in a trench in Tezno Woods at Maribor uncovered 1,179 skeletons, believed to be of Croatians. The trench is 1 kilometer long, 4 to 6 meters wide and the layer of human remains in the section excavated so far measures 1.5 to 2 meters deep.
... Tolstoy reconstructed what happened when, on May 31, the commandant of the military camp at Viktirig, 'Lieutenant Ames', reported that he had received orders for 2,700 of the civilian refugees in Major Barre's camp to be taken to Rosenbach and Bleiburg the following day, to be handed over to Tito's partisans.
A comprehensive root cause analysis of the inflated numbers is given by the British historian D. B. MacDonald 
By contrast with Jasenovac, however, most impartial historians converged on much lower number of dead, suggesting that Bleiburg was by no means as significant as the largest death-camp in Yugoslavia. ... Jasper Ridley attempts a more precise figure, although there is no way of knowing for sure. ... Of these, he noted that the Allies agreed to surrender 23,000 to the Partisans between 24 and 29 May - a mixture of Slovenians, Serbians, and Croatians. Reports from the time according to Ridley, indicate that not all the 23,00 were killed
MacDonald's final conclusion is:
Inflating the numbers of dead at Bleiburg had several layers of significance. Firstly, it gave the Croats their own massacre at the hands of Serbs and/or Communists, which allowed them to counter the Serbs' Jasenovac genocide with one of their own. Secondly, it allowed Croats to distance themselves from the Serbs and the Communist regime that had carried out the massacres. They could portray Croatia as an unwilling participant in the SFRY, more a prisoner than a constituent nation. Thirdly, by suffering such a massacre, the Croats underwent their own 'way of Cross', as it was frequently dubbed in Croatian writings.
Further, Christopher Booker published a lengthy analysis of the Bleiburg controversy in A Looking Glass Tragedy. The Controversy Over The Repatriations From Austria In 1945 . The leading idea of this book is elaborated in the book overview :
Many "massacres" described in lurid detail never took place. As Booker describes how the story of the repatriations came to be presented in such a distorted fashion, his book turns into a study of people's willingness to cling on to a "make believe" version of history, even when all the facts have proved it wrong.
His research is fully summarized in the Chapter 12. 2. Bleiburg: The Massacre That Never Was (page 188). The main points of his research are:
a) there are only nine documents in the British Army archives related to the Bleiburg, Austria, May 1945. No traces of any massacre ever committed in Bleiburg or its surroundings;
b) Tolstoy's 'impartial' evidence for this massacre having taken place came from three 'eyewitnesses' whom he quoted at length from interviews conducted when he was writing his book  40 years later;
c) all 'evidence' came from narrative stories of those who claimed to be the witnesses.
The first Croats to return to the fields of Bleiburg came in secret in 1952, while regular annual visits began in the early 1960s. The first Croatian religious leader to come to the site was Cardinal Franjo Šeper, then Prefect of the Congregation for the Doctrine of the Faith, who paid a visit in 1977.
Many top-ranking politicians and Catholic and Muslim clerics visit the site annually. Prime Minister Ivica Račan visited the site in 2002. Prime Minister Ivo Sanader visited the site in 2004. For the 60th anniversary commemorations in 2005 a large crowd was in attendance, with speeches by Croatian parliamentary speaker Vladimir Šeks and head of the Muslim Community of Croatia, Mufti Ševko Omerbašić. In 2006, the site was attended by Croatian government officials Đurđa Adlešić and Damir Polančec and Bosnian Croat politician Martin Raguž. Catholic mass was led by bishop Josip Mrzljak, while imam Idriz Bešić represented the Islamic Community of Croatia. In 2007 a new altar was installed at the site. Cardinal Josip Bozanić inaugurated the altar at the 2007 commemorations which drew 10,000 people.
In 2008, the Croatian Parliament was represented by the leader of the Croatian Peasant Party Josip Friščić, while the Croatian Government was represented by minister Berislav Rončević The Croatian and Slovenian governments reached an agreement at this time of cooperation on organizing military cemeteries, similar to earlier agreements Slovenia reached with Italy and Germany. According to the Slovene government, the mass grave site in Tezno is being planned as a memorial park and cemetery.
In 2009, Croatian President Stipe Mesić made a statement declaring that the Bleiburg commemoration has turned into an Ustaše festival funded by the Parliament, whose representatives he criticized for idly standing by while people in the crowd displayed Ustaša markings (which are illegal in Croatia).
The Bleiburg massacre was the subject of a 1999 film, Četverored.