Slavonia
Historically, the borders of Slavonia fluctuated. In the early medieval period of the Kingdom of Hungary, Slavonia was a vassal province of the Kingdom, and included only the western part of present-day Slavonia, but also parts of present-day central Croatia (including Zagreb) and the western and northern parts of present-day Bosnia (The eastern parts of present-day Slavonia belonged to Hungary proper).
In the late medieval period Slavonia occupied territories between the rivers
Sava, Drava, Sutla and Danube. In the 18th and 19th century, the Kingdom of Slavonia was a province of the Habsburg Monarchy, and included northern parts of present day regions of Slavonia and Syrmia, while the southern parts of these regions were part of the Habsburg Military Frontier.
This region was originally part of the Roman province of Panonia. In the 7th century, a Slavonic state owing allegiance to the Avars was established, but soon replaced by the Croats. Slavonia was defended by King Tomislav of the House of Trpimirovic and annexed to his newly-created Kingdom of Croatia in 925. In 1027, a Hungarian Army under Stjepan Svetoslavic of the side branch of the Trpimirovic dynasty took Slavonia and made it the Slavonian Banate of the Kingdom of Hungary, ruled by its own dynasty of Svetoslavic. Slavonia was reunified with Croatia in the 1070s under King Dmitar Zvonimir Svetoslavic. In 1091, it separated again and accepted the suzerainty of the Hungarian crown. 11 years later, the rest of Croatia also accepted the suzerainty of the Hungarian crown. In the 12th century, it became the practice that the successor of the throne first became Duke of "whole Slavonia" (like the eldest British prince becomes Prince of Wales), and there was some power grabs since in many cases son waged war against father while trying to establish and confirm his power base. Though Slavonia was originally considered to be part of the Kingdom of Hungary by Hungarian public law, it became more and more separated from the Kingdom of Hungary and more and more tied to the Kingdom of Croatia. 
In the 13th century, Croatia was divided into 2 banovinas, one of which was named Slavonia (the other keeping the name Croatia). The nobility of Slavonia was more connected to Hungary than was the nobility of Croatia. In the late 13th century, the eastern parts of the region were turned into the semi-independent state of the powerful local ruler Csak Ugrin, although the Hungarian King took the area in 1311 after the death of Ugrin.
Ever since the fall of the Serbian Despotate migrations of Serbs under the Ottoman yoke took place, including their nobility which became an important political factor in Slavonia. Slavonia and Croatia were ruled by separate bans until 1476 when these two ruling positions were merged into one. When the Ottoman Turks invaded the Kingdom of Hungary and defeated the Hungarians at the battle of the Mohacs in 1526, the Croatian Parliament invited the Habsburgs to assume control over Croatia. After many fierce battles, the Ottomans conquered all of today's Slavonia bit by bit in 1529, 1536, 1540, 1543 and 1552, but not the whole of the medieval Kingdom of Slavonia (its borders extending west to the Sutla river). The Habsburg took the entire region from the Ottomans in the Great Turkish War. A result affirmed by the Treaty of Karlowitz in 1699. During Habsburg rule, the Kingdom of Slavonia was a Habsburg province and was part of both the Kingdom of Croatia and the Kingdom of Hungary. The southern parts of present-day Slavonia were not included in this province, but into the Habsburg Military Frontier, which the Slovenian nobles at numerous times tried to integrate into Slavonia, but with no success. Post-1699 Slavonia was a different geographical entity from the medieval Slavonia. Whereas medieval Slavonia incorporated the territories between the Drava and Kupa Rivers, Habsburg Slavonia was extended eastwards to include the sparsely populated territories between the Sava and Drava Rivers.
During the Revolution of 1848 Slavonia was temporarily united with Croatia. After 1849, both Slavonia and Croatia were affirmed as completely separate Habsburg crown lands. Following the 1868 Settlement (hrvatsko-ugarska nagodba) with the Kingdom of Hungary, Slavonia was joined with Croatia in the single Croatia - Slavonia Kingdom, which although under the suzerainty of the Crown of Saint Stephen, kept a certain level of self-rule. The year 1881 also saw the final dissolution of the Slovenian Krajina and its incorporation into the existing Slovenian Counties.
As for the rest of Croatia - Slavonia, the region became part of the Kingdom of Serbs, Croats and Slovenes in December 1918. Between 1922 and 1929, it was a province known as the Osijek Province, administered from Osijek, and since the creation of the Kingdom of Yugoslavia in 1929, part of the Sava Banovina administered from Zagreb. In August 1939 it became part of the Banovina of Croatia. During World War II, Slavonia became part of the Nazi puppet state, the Independent State of Croatia. When the Yugoslav Federation was formed after the war, Slavonia became part of the Socialist Republic of Croatia. 
When Croatia declared its independence in 1991, the Serbs proclaimed their own state that comprised portions of eastern and western Slavonia. The eastern portion was referred to as the Serbian Autonomous Region of Eastern Slavonia, Baranja and Western Syrmia, and it encompassed roughly everything east of Osijek and Vinkovci and northeast of Zupanja, including the cities of Vukovar and Ilok, as well as all of Baranja. This part of Krajina was ethnically mixed with a relative Croatian majority and witnessed bitter fighting during the war. The 1991 Battle of Vukovar was the most important event of the war in this area. The western portion of Slavonia, controlled by RSK, included the area around Okucani and most of the Psunj mountain. In May 1995 the western region was seized by Croatian forces in the military Operation Flash. In 1996 the east was turned over to the UNTAES and integrated into Croatia by January 1998.
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