The demographic history of Croatia is characterised by significant migrations, starting with the arrival of the Croats in the area. According to the work "De Administrando Imperio" written by the 10th-century Byzantine Emperor Constantine VII, the Croats arrived in the area of modern-day Croatia in the early 7th century. However, that claim is disputed, and competing hypotheses date the event between the 6th and the 9th centuries. Following the establishment of a personal union of Croatia and Hungary in 1102, and the joining of the Habsburg Empire 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 to 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. Similarly, Venetian Republic rule in Istria and in Dalmatia, following the Fifth and the Seventh Ottoman–Venetian Wars ushered gradual growth of Italian speaking population in those areas. Following the collapse of Austria-Hungary in 1918, the Hungarian population declined, especially in the areas north of the Drava river, where they represented the majority before World War I.