Initially, the territory of the SAO Western Slavonia was relatively large and the rebel forces could theoretically advance north to the Hungarian border and sever the link between Zagreb and Osijek. In such case the Virovitica-Sisak-Karlovac-Karlobag line would have been established.
In reality, the territory of the SAO Western Slavonia was mostly hills and forests with relatively poor communication infrastructures. Establishing and maintaining a base for such ambitious operations required more resources than the Serbian government was willing or able to invest. As a result, the SAO Western Slavonia didn't receive as much support from the Yugoslav People's Army as other parts of the RSK did. Most of its forces were local militias, poorly trained, lightly armed and unable to match their Croatian counterparts.
This situation became apparent in late 1991 when local Croatian forces, armed and equipped from the captured JNA garrison in nearby Varaždin, conducted a series of offensives which ultimately reduced SAO Western Slavonia to a relatively small pocket centered around Okučani. It is believed that the Sarajevo armistice in January 1992 saved the forces of the Serb Krajina from being pushed across the River Sava and the SAO Western Slavonia from disappearing altogether.
The position of the rebel area in Western Slavonia became even more precarious with the escalation of Bosnian War. It became quite clear that the position of the rebels in Western Slavonia in such circumstances was untenable in the long run. Veljko Džakula and a group of other local rebel Serb politicians in 1992 and 1993 started secret negotiations with the Croatian government about eventual peaceful transfer of the rebel areas in Western Slavonia under Croatian authority.
On February 18, 1993 Croatian and local Serb leaders (led by Džakula) signed the Daruvar Agreement. The Agreement was kept secret and was working towards normalizing life for the locals on the battlefield line. However, the Knin authorities learned of the deal - and Džakula was arrested by the authorities of the RSK and replaced by hardliners.
The UN negotiators saw this development as an encouraging sign that the conflicts in the former Yugoslavia could be ended peacefully. The authorities of Croatia and the rebel areas in Western Slavonia were encouraged to take part in various confidence building programs like family reunions. Some of those programs began to bear fruit in late 1994. The most important was the opening of the Zagreb-Županja Motorway for Croatian traffic.
In late April 1995 there was an inter-ethnic incident which resulted in the deaths of a number of Croats and Serbs and in the disruption of the motorway. Hrvoje Sarinic, Franjo Tudjman's former Cabinet Chief, confirmed under cross-examination at the Milosevic trial a transcript detailing the then Croatian leadership's plan to stage an incident as a pretext for the offensive on May 1st, although no linkage was shown to the abovementioned incident. Hrvoje Sarinic, as a witness at the same trial, downplayed the significance of such a plan, pointing out there were many incidents daily, and to his own exepriences at being refused entry on the motorway, problems with reopening the oil pipeline, and finally stating that he was "very astonished that any incident was needed, because it was quite legitimate to free part of the country that had been occupied".
On the early morning of May 1st, Croatian Army forces (which included the elite 3rd and 5th Guards Brigades) and Special Police units began their advance from three directions. Over 7,200 soldiers and policemen participated in the operation.
Rebel forces were quickly overwhelmed and in a few hours time local commanders and civilian authorities issued orders for evacuation across the River Sava into Bosnia’s Republika Srpska.
By the afternoon of May 2nd all rebel forces were evacuated and the Croatian Army had achieved all of its initial aims. One large group of rebel soldiers and civilians, including Džakula, failed to evacuate and had to surrender near Pakrac. The operation produced a total of around 1,500 Serb POWs, the largest capturing of an enemy force to date in the war.
The forces of the RSK also launched a retaliatory action against civilian targets by launching a number of cluster shells on Zagreb on May 2nd and 3rd. The attack killed 7 and injured at least 175 people. On that very same day the leader of the RSK, Milan Martić publicly took responsibility for the shelling, and that statement was used against him at his ICTY trial.
Initially, Croatian authorities reported that Croatian Army and police units had 55 fatalities during the course of the operation. In 2000, the official quote was that 33 members of the army and 9 members of the special police units were killed, and that 162 were injured. The government refused to publish a list of these casualties and only spoke in general numbers, causing some public outcry.
The outcome of the operation was not surprising, considering the huge disparity in numbers, equipment, training and, ultimately, morale between two opposing sides. However, it proved to be major boost for Croatian Army and it tested procedures and tactics that would be employed in Operation Storm with even more wide-ranging results.
The UNPROFOR also removed itself from the former UNPA Sector West, but did not make any effort to oppose the Croatians since the October 1993 Security Council resolution affirmed that the United Nations Protected Areas were an integral part of the Republic of Croatia.
Among Serbs in the RSK and the Republic of Srpska, Operation Flash caused huge demoralisation. The 500 km² of the SAO Western Slavonia was not only the first major part of the RSK to be erased from the maps; it also proved the untenability of the Greater Serbian project under the new circumstances. Rebel Serbs from Western Slavonia not only failed to get support from Serbia proper, but also from Republika Srpska.
Operation Flash is also notable for the professional conduct of Croatian military and police. There were very few reports about reprisals and/or atrocities against POWs or Serb civilians, which later represented a sharp contrast to the more controversial aftermath of Operation Storm. The U.N. Secretary General´s Personal Representative, Yasushi Akashi said that "massive" human right abuses have taken place during the offensive. A subsequent and in-depth Human Rights Watch investigation showed there were only isolated incidents and criticized the Secretary General for his preemature and counterproductive statement.
Gojko Sušak had claimed that 454 people were killed in the Operation on the Serbian side. The Croatian Helsinki estimated that 83 Serb civilians died, while Veritas came to a total figure of 283 killed Serbs (both military and civilian).
The Croatian Government's commission on missing and captured persons composed a list of 168 fatalities together with places of burial, of which 79 bodies were identified. Veritas, a Serbian organization, run by former rebel Serb intelligence officer, composed its own list which has another 113 people listed as missing as a result of the Operation Flash.