In the beginning of the 14th century, the Kingdom of Hungary fell into civil war over Hungarian throne, and the central power in the Kingdom collapsed. Following this collapse, many local oligarchs assumed regal rights for themselves and ruled over large territories as de facto independent rulers, with their own private armies, administration and juristical systems.
One of these local oligarhs was Ugrin Csák, who ruled over parts of present-day regions of Szerémség (Syrmia), Szlavónia (Slavonia), Bácska (Bačka), while his residences were in Újlak (Ilok) and Pozsega (Požega). Territory over which Ugrin Csák ruled was known as Upper Syrmia, while neighbouring region of Lower Syrmia was ruled by another ruler, Stefan Dragutin of the SerbianHouse of Nemanjić. After Ugrin Csák died, in 1311, his lands were joined to the territory held by the Hungarian king Charles I (Robert). This extension of the territory under his jurisdiction, helped to Hungarian king to defeat other local oligarchs.
References
Péter Rókai, Zoltan Gyere, Tibor Pál, Sándor Kaszás, Istorija Mađara, Beograd, 2002.