Srijem
Srijem (Serbian: Срем, Latin: Sirmium) is a fertile region of the Pannonian plain, between the rivers of Danube and Sava.Most of Srem is located in northern Serbia and Montenegro in what is known as the Vojvodina; see Srem, Serbia and Montenegro.
The western parts of Srem/Srijem are in easternmost Croatia, though the border between this region and Slavonia to the west is unclear, though it's definitely restricted to the Vukovar-Srijem county.
The border line runs somewhere along the line Vukovar-Vinkovci-Županja. According to another interpretation, the western border follows the Bosut, Barica and Vuka rivers.