SAO Eastern Slavonia , Baranja and Western Syrmia The Serbian Autonomous Oblast of Eastern Slavonia , Baranja and Western Syrmia ( ) was a self - proclaimed Serb political entity in eastern Croatia , established during the Yugoslav Wars . It was one of three Serb Autonomous Oblasts ( SAOs ) proclaimed on the territory of Croatia . The region included parts of geographical regions of Slavonia , Baranja and Syrmia . The entity was formed on 25 June 1991 , the same day the Socialist Republic of Croatia decided to withdraw from Yugoslavia , following the Croatian independence referendum , 1991 . In the first phase of the Croatian War of Independence , in 1992 the region joined the breakaway Republic of Serbian Krajina ( RSK ) . History . The SAO Eastern Slavonia , Baranja and Western Syrmia was declared on June 25 , 1991 , and on June 26 its first president was declared to be Goran Hadžić . Like the SAO Krajina , it was developed by the Serbian National Council , but was the one representing Eastern Slavonia , Baranja and Western Syrmia . Initially , it was a separate Serb autonomous region ( oblast ) , but subsequently joined the Republic of Serbian Krajina ( RSK ) in February 1992 . Its borders were established by the end of 1991 as entrenched front lines of the first phase of the Croatian War of Independence . Up to August 1995 , Eastern Slavonia , Baranja and Western Syrmia was de facto part of the Republic of Serbian Krajina , while de jure still a part of Croatia , per United Nations Security Council Resolution 753 . The region did not have its own local regional administration within RSK . Geography . Territory of former Eastern Slavonia , Baranja and Western Syrmia was part of the Central European Pannonian Basin . The eastern border of region was mostly the Danube river , while approximately one third of the western border was the Drava river . The Kopački rit natural preserve located near the confluence of Drava and Danube formed a major geographical barrier - there were no road or rail connections between Baranja and the southern parts of the territory , except through Serbia . Other boundaries were not natural boundaries : the border with Hungary in the north had existed since the Kingdom of Serbs , Croats and Slovenes , the eastern border with FR Yugoslavia partly existed since the Kingdom of Slavonia ( on the Danube ) and was partly set with the formation of SFR Yugoslavia , while the border with the rest of Croatia in the west and south was formed after the fronts were settled in the first phase of the Croatian War of Independence . Eastern Slavonia is a mostly flat area , with the best type of soil where agriculture is highly developed , particularly on wheat fields , and it also has several forests as well as vineyards . The Đeletovci Oil fields are located between the villages of Đeletovci , Banovci and Nijemci . Traffic over the Brotherhood and Unity Highway ( today the A3 ) was interrupted with the formation of the ESBWS . The water transport over the Danube river continued unobstructed . The Drava river was not navigated at the time . The railway line between Zagreb and Belgrade and the transport between the Budapest and Sarajevo passing through the area were also closed . Population . The population of this area was ethnically mixed . Before the war , the total population of the area numbered 192 , 163 inhabitants , and was composed of : During the 1991 - 1995 War in Croatia , 109 , 500 Serbs lived in this area out of a total of 160 , 000 . Municipalities and Inhabited places . During the existence of the Republic of Serbian Krajina , the region was divided into five municipalities : The main cities in the area were Vukovar and Beli Manastir . Other important places included Borovo Selo , Darda , Dalj , Ilok , and Tenja . Legacy . After the RSK ' s demise in August 1995 , the region was reconstituted again as Eastern Slavonia , Baranja and Western Syrmia ( 1995 - 1998 ) . Following the November 1995 Erdut Agreement , it was subsequently controlled by the United Nations Transitional Administration for Eastern Slavonia , Baranja and Western Sirmium Today the ethnic Serb interests in the region are advocated by the Joint Council of Municipalities .