Slavonian Military Frontier The Slavonian Military Frontier or Slavonian Krajina was part of the Habsburg Military Frontier . It was formed out of territories the Habsburgs conquered from the Ottoman Empire and included southern parts of Slavonia and Syrmia ; today the area it covered is mostly in eastern Croatia , with its easternmost parts in northern Serbia ( mostly in Vojvodina region ) . Divisions . The Slavonian Krajina was divided between three regiments , named after towns in the area : Gradiška , Brod , and Petrovaradin ; however , the regimental seat of the Brod regiment was in Vinkovci . Other important towns in the area included Sremski Karlovci , Stara Pazova , Zemun , and Sremska Mitrovica . History . During the history , name Slavonian Krajina referred to different territories . Original Slavonian Krajina was located in what is now Central Croatia and was known as the Varaždin generalat . It was created from territories that formerly were part of medieval Banovina of Slavonia . Shortly after its creation in the 16th century , original Slavonian Krajina bordered the Ottoman Empire to the east , the Habsburg Kingdom of Croatia ( part of the Habsburg Empire ) to the west , the Croatian Krajina to the south - west , and the Habsburg Kingdom of Hungary , also part of the Habsburg Empire , to the north . After the Treaty of Karlowitz ( 1699 ) , Habsburg Monarchy expanded its territory towards east and new sections of the Military Frontier were founded along the Sava , Danube , Tisa and Mureş rivers . New parts of the frontier were divided into two generalats : the first one included territory from Gradiška to river Tisa and the second one included territories along the Tisa and Mureş rivers . Until the middle of the 18th century , the entire Slavonia region was under military administration . In 1745 , northern parts of Slavonia were placed under civil administration and were organized into the newly formed Habsburg crownland known as the Kingdom of Slavonia . The southern parts of Slavonia that remained under military administration were organized into Slavonian Military Frontier or Slavonian Krajina , while former original Slavonian Krajina was organized as Varaždin section of the Croatian Military Frontier . In the second half of the 18th century , the Slavonian Krajina bordered the Habsburg Kingdom of Croatia and the Croatian Military Frontier to the west , the Habsburg Kingdom of Slavonia and the Habsburg Kingdom of Hungary to the north , the Banat Military Frontier to the east and the Ottoman province of Bosnia and Sanjak of Smederevo to the south . The only changes of this province ' s borders and neighborhood before its integration into Croatia - Slavonia in 1882 were in 1848 – 1849 when autonomous Serbian Vojvodina included eastern parts of Slavonian Krajina and between 1849 and 1860 , during the existence of Voivodeship of Serbia and Banat of Temeschwar , a separate Habsburg crownland which became an north - eastern neighbor of Slavonian Krajina . After the creation of the Principality of Serbia in 1815 , this was the Slavonian Krajina ' s southeastern neighbor . Population and Demographics . 1776 . In 1776 , the rural population of the Slavonian military frontier was 177 , 212 . The number of Roman Catholic men was 43 , 635 , while number of Orthodox men was 33 , 970 . The number of inhabitants of cities was 11 , 353 , and that giving a total of 188 , 565 inhabitants . In Brod and Gradiška regiments Catholics outnumbered the minority Orthodox , and in Petrovaradin regiment the Orthodox were more numerous . 1820 . In 1820 , population of Slavonian Krajina included 117 , 933 Catholics and 117 , 274 Orthodox Christians . 1870 . In 1870 , Slavonian military frontier had , according to official census , 246 , 901 inhabitants . Of that number 143 , 873 were Roman Catholic and 92 , 991 were Orthodox ( living mainly in eastern Syrmia ) . Brod regiment had 82 , 540 inhabitants out of which 73 , 892 were Roman Catholic and 6 , 886 were Orthodox . Gradiška regiment had 61 , 596 inhabitants out of which 45 , 601 were Roman Catholic and 15 , 933 were Orthodox . Petrovaradin regiment had 102 , 765 inhabitants out of which 70 , 172 were Orthodox and 24 , 380 were Roman Catholic .