Eastern Slavonia front

Many intensive fights occurred in the area during the main conflict but not as much as in Kninska Krajina, where fighting was at its finest for most of the war.

One of the first battles in the area as well the entire war, was the Battle of Borovo Selo. The clash was precipitated by months of rising ethnic tensions, violence, and armed combat in Pakrac and at the Plitvice Lakes in March.

The immediate cause for the confrontation in the heavily ethnic Serb village of Borovo Selo, just north of Vukovar, was a failed attempt to replace the Yugoslav flag in the village with the flag of Croatia.^[9] The unauthorised effort by four Croatian policemen resulted in the capture of two by a Croatian Serb militia in the village.^[10]

To retrieve the captives, the Croatian authorities deployed additional police, who drove into an ambush. Twelve Croatian policemen and one Serb paramilitary were killed,^[11] before the Yugoslav People's Army (JNA) intervened and put an end to the clashes. The two captured policemen were ferried across the Danube and transported to Novi Sad, but were released and returned to Osijek by the evening of 2 May.^[10] Vojislav Milić, a paramilitary from Valjevo, was the only fatality among the Serb militia.^[12] Four other paramilitaries were wounded.^[13]

Some of the police killed at Borovo Selo were found to have been mutilated, their ears cut, their eyes gouged out and their throats slit.^[14] These acts were meant to inflame ethnic hatred.^[15]

The next major battle was the Battle of Osijek, also known as Siege of Osjek. It involved the artillery bombardment of the Croatian city of Osijek by the Yugoslav People's Army (JNA), occurring from August 1991 to June 1992 during the Croatian War of Independence. Shelling peaked in late November and December 1991, then diminished in 1992 after the Vance plan was accepted by the combatants.^[16] Airstrikes and attacks by JNA infantry and armored units against targets in the city accompanied the bombardment, which caused approximately 800 deaths and resulted in a large portion of the city's population leaving.^[16][17]

Croatian sources estimated that 6,000 artillery shells were fired against Osijek over the period.^[18] After the JNA captured Vukovar on 18 November 1991, Osijek was the next target for its campaign in Croatia. The JNA units subordinated to the 12th (Novi Sad) Corps, supported by the Serb Volunteer Guard, achieved modest advances in late November and early December, capturing several villages south of Osijek, but the Croatian Army maintained its defensive front and limited the JNA's advances.

In the aftermath of the Battle of Osijek, Croatian authorities charged thirteen JNA officers with war crimes against civilians, but no arrests have been made to date.^[19] Croatian authorities also charged the wartime commander of Osijek's defence, Branimir Glavaš, and five others with war crimes committed in the city in 1991. The five were convicted and received sentences ranging between eight and ten years.^[20] On 20 January 2015, after serving five years in prison, he was released from prison, after Croatia's Constitutional Court rescinded his war crimes conviction on procedural grounds. His case was sent back to the Supreme Court for retrial.^[21]

Then, before main fighting in Osijek, Town of Vukovar was besieged by the Yugoslav Peoples Army and the Serbian Forces, The JNA began to intervene in favour of the rebellion, and conflict broke out in the eastern Croatian region of Slavonia in May 1991. In August, the JNA launched a full-scale attack against Croatian-held territory in eastern Slavonia, including Vukovar. Vukovar was defended by around 1,800 lightly armed soldiers of the Croatian National Guard (ZNG) and civilian volunteers, against as many as 36,000 JNA soldiers and Serb paramilitaries equipped with heavy armour and artillery.^[22][23][24] During the battle, shells and rockets were fired into the town at a rate of up to 12,000 a day. At the time, it was the fiercest and most protracted battle seen in Europe since 1945, and Vukovar was the first major European town to be entirely destroyed since the Second World War.^[25][26]

When Vukovar fell on 18 November 1991, several hundred soldiers and civilians were massacred by Serb forces and at least 20,000 inhabitants were expelled.^[27] Overall, around 3,000 people died during the battle. Most of Vukovar was ethnically cleansed of its non-Serb population and became part of the self-declared proto-state known as the Republic of Serbian Krajina. This Battle was a Phyrric Victory for the Yugoslav-Serbian troops, but the town of Vukovar, fully destroyed, was now in the hands of the Serb rebels.

Next bigger armed presentation was Operation Baranja, conducted by the Croat Forces the offensive quickly gained ground after the HV advanced north of the Drava River into Baranja. The defending force of the Croatian Serb Territorial Defence Force supported by the Yugoslav People's Army (Jugoslovenska Narodna Armija – JNA) artillery were caught unprepared and offered light resistance. However, the offensive was not authorised and it was cancelled six hours after it had been launched. An investigation was launched by the HV, and two senior officers were relieved of their duty, before being reassigned to new positions. According to General Martin Špegelj, the decision to abort the offensive was wrong as it meant a missed opportunity to recapture Baranja which had been lost to the JNA in 1991.^[28] Admiral Davor Domazet-Lošo pointed out that the offensive presented a significant political problem to Croatian leadership as it meant that the political leaders of Croatia had poor control over its military, or that Croatia was violating its international obligations towards the United Nations (UN) in respect of the Vance plan at the time the country sought to become a member of the UN.^[29]

During the fighting, and in its aftermath, the JNA shelled several Croatian settlements and three Croatian civilians living in Croatian Serb-controlled Baranja were killed in retribution.^[30] The battle is commemorated annually in the towns of Belišće and Valpovo.^[31] The battle had no Croatian success later as they withdrew, and it could be called a Serbian victory, since they retained Osijek and Belišće.

Last attempt by the Croat forces was Operation Vukovar '95, a planned military-police operation of the Croatian National Guard in early November 1995 and in mid 1996.^[32] The operation was intended to militarily liberate the last remaining occupied part of Croatia after Operation Storm, including the region of Eastern Slavonia.^[32] The operation was abandoned due to intervention of UN peacekeepers and the signing of the Erdut Agreement. Immediately after Operation Storm, the Operation Manjinjorgo was carried out as a preparation for the Vukovar military operation,^[32] with the goal to mobilize around 30,000 Croatian soldiers from five guard brigades in Eastern Slavonia.

About 100,000 more soldiers were deployed in reserve, in case the Yugoslav Army was involved in the conflict.^{[citation needed]} The Serb forces, however, were well entrenched, protected by minefields. In the occupied territory, the Serbs had about 25,000 soldiers and civilian volunteers at their disposal.^[33]

By putting the signatures of representatives of the Croatian government and Serbs on the Erdut Agreement on 12 November 1995 and the adoption of the UN Security Council Resolution no. 1037 on 15 January 1996, which established the United Nations Transitional Administration for Eastern Slavonia, Baranja and Western Sirmium (UNTAES), the process of returning the region of Eastern Slavonia to Croatian rule began. Even after the adoption of the Erdut Agreement, Yugoslavia and the Serbs believed that there was a real, although relatively small, possibility that Croatia would launch a military operation. Although the operation was abandoned, according to the memories of Mate Granić, parts of the Croatian leadership influenced President Tuđman to launch the military operation.^[34] Granić does not specify the exact period when this proposal was made, but it can be assumed that it was the second half of 1996.^[33]

According to Granić, he then stated in an interview for the Croatian press that a military takeover of Eastern Slavonia for Croatia would have severe international consequences, and that after that the mentioned operation was aborted Tuđman was allegedly welcomed by Granić's statement to get rid of the pressure from the Ministry of Defense to carry out the mentioned operation.^[34]

• Central Intelligence Agency, Office of Russian and European Analysis (2002). Balkan Battlegrounds: A Military History of the Yugoslav Conflict, 1990–1995. Washington, D.C.: Central Intelligence Agency. ISBN 978-0-16-066472-4. OCLC 50396958.
• Central Intelligence Agency, Office of Russian and European Analysis (2002). Balkan battlegrounds: a military history of the Yugoslav conflict, 1990–1995, Volume 2. Washington, D.C.: Central Intelligence Agency. ISBN 978-0-16-066472-4.