The southern counties along the Danube, Sava, Tisa (Tisza) and Tamis (Temes) rivers, including Srem (Szerém, Sirmium) had been the richest, most developed and purely Hungarian inhabited part of the Hungarian Kingdom in the Middle Ages. The Ottoman conquest brought about a dramatic population shift between the fifteenth and seventeenth centuries, when the Hungarians were either massacred or forced to flee the area. The demographic vacuum was filled by Serbian immigration. The Serbs acquired a privileged status as frontier guards of the Habsburg realm, with full territorial, religious and cultural autonomy up to the middle of the eighteenth century.