As an outcome of World War I, the Kingdom of the Serbs, Croats, and Slovenes is formed. Croatia, Slovenia, and Bosnia and Herzegovina had been part of the fallen Austro-Hungarian empire; Serbia and Montenegro existed as an independent state (Macedonia was then part of Serbia).
Dec 1, The Kingdom of Serbs, Croats and Slovenes [later in 1929 to be called Yugoslavia] was proclaimed by Alexander Karadjordjevic, the son of King Peter of Serbia. It included the previously independent kingdoms of Serbia and Macedonia, the Hungarian-controlled regions of Croatia and Slovenia, the Austrian province of Dalmatia, Carniola and parts of Styria, Carinthia and Istria. King Alexander I renamed the Balkan state called the Kingdom of Serbs, Croats, and Slovenes to Yugoslavia in 1929