Croats inhabited the Danube region in 7th century on the territory of the former Roman province Pannonia. At the beginning of the 9th century there was the kingdom of Pannonian Croats that rose against Frankish Empire in 818. By 1102. almost the whole Slavonia fell within Kingdom of Croatia and from then till 1527. in Hungarian-Croat state.
Slavonia spread between Sava and Drava rivers and eastern Slavonia had 4 counties: Virovitica, Pozega, Vukovar, Srijem.
In 16th century the Turks with the help of non-Slavic Vlahs of orthodox religion and Croats that converted to Islam occupied big part of Croatia and eastern Slavonia all the way to the river Cazma, up to the line between Virovitica on Drava and Sisak on Kupa and Sava. The Turkish occupation lasted till 1699. when the Christian army, together with Croatian troops liberated entire Slavonia except the most eastern part , Srijem.
In 1745. Slavonia was returned to Croatia and three out of four counties were reestablished: Virovitica, Pozega and Srijem.
The Southern part of Slavonia near the Sava river was set as Vojna krajina with the custody of Austria over it. The Turks as well as the Austrians were relocating nomads Vlahs of orthodox religion particularly in Vojna krajina. With the help of Serbian patriarchy they were turned into Serbs at the end of 19th century, although ethnically they weren't similar to the Serbs from Serbia.
When in 1881. Vojna krajina ceased to exist, its territory following the historical rights, was returned to Croatia from which had been forcefully taken away. It mainly contributed to the enlargement of Srijem county with the capital in Vukovar. It consisted of kotars Vukovar, Vinkovci, Zupanja, Ilok, Irig, Srijemska Mitrovica, Ruma, Stara Pazova and Zemun. It means that the whole Srijem belonged to the Kingdom of Croatia, which had the autonomy within Austrio-Hungarian Empire. The city of Zemun, which is today a part of Belgrade, formed a part of historical Croatian territory. It should be stated here that towns of Vukovar, Osijek, Ilok and even Knin in southern Croatia never belonged to Vojna krajina.. Also Vojna krajina never was "Serbian ground" because it always had Croatian majority.
