Why was Yugoslavia created?

Following the Treaty of Versailles in 1919, the former territories of Austria-Hungary were distributed among the newly created nations of Czechoslovakia, Yugoslavia, and Hungary. These new frontiers, however, also meant that large populations of minorities from these empires were placed within the borders of countries that were hostile towards them. In Bosnia, these were mostly Croatian and Serbian populations. In Croatia these would be mostly Serbian and German populations, and in Serbia these would be mostly Croatian, Bosnian, Hungarian, Bulgarian, Romani and Albanian populations.

The Croatian population of Yugoslavia had wanted to establish a single Yugoslav nation with no internal boundaries. The Serb population, however, wanted to create a Greater Serbian state that would incorporate lands formerly part of Austria-Hungary and the Ottoman Empire. This tension would come to a head during World War II when Croatia proclaimed independence from Yugoslavia and massacred hundreds of thousands of Serbs and Jews within its borders.