After dealing with the natural features & social importance of the Posavina region in the past, presented is the importance of this region as a unique Croatian ethnic territory during the Middle Ages. With the appearance of the Ottomans & especially at the beginning of the 16th century, great ethnic changes occured, primarily due to the expulsion of Croats & arrival of new ethnic groups, mostly Orthodox Vlachs & later Muslims & ethnic Serbs. With the withdrawal of the Ottomans from the Pannonian basin to the areas south of the Sava River & the Danube, the Sava becomes the dividing line creating in its border areas two socially & politically different environments: the Slavonian Military Frontier on the Slavonian side & the Ottoman military-frontier system of kapitanates on the Bosnian side. Both systems had a special influence on the change of ethnic composition in this region. With the withdrawal of the Ottomans further toward the southeast of Europe & the Austrian occupation of Bosnia & Herzegovina the Soya River remains the border along which, especially on the Bosnian side, further changes in ethnic structure occurred. 3 Tables, 4 Figures, 23 References. Adapted from the source document.