The demographic history of Slavonia is characterised by significant migrations, as is that of Croatia as a whole, starting with the arrival of the Croats, between the 6th and 9th centuries. Following the establishment of the personal union of Croatia and Hungary in 1102, and the joining of the Habsburg Monarchy in 1527, the Hungarian and German speaking population of Croatia began gradually increasing in number. The processes of Magyarization and Germanization varied in intensity but persisted until the beginning of the 20th century. The Ottoman conquests initiated a westward migration of parts of the Croatian population; the Burgenland Croats are direct descendants of some of those settlers. To replace the fleeing Croats the Habsburgs called on the Orthodox populations of Bosnia and Serbia to provide military service in the Croatian Military Frontier. Serb migration into this region peaked during the Great Serb Migrations of 1690 and 1737–39. Following the collapse of Austria-Hungary in 1918, the Hungarian population declined, due to emigration and ethnic bias. The changes were especially significant in the areas north of the Drava river, and Baranja County where they represented the majority before World War I.