Yugoslavia was a state concept among the South Slavic intelligentsia and later popular masses from the 17th to early 20th centuries that culminated in its realization after the 1918 collapse of Austria-Hungary at the end of World War I and the formation of the Kingdom of Serbs, Croats and Slovenes. However, the kingdom was better known colloquially as Yugoslavia (or similar variants); in 1929 it was formally renamed the "Kingdom of Yugoslavia".

After the Kingdom of Serbs, Croats and Slovenes was proclaimed, the National Council elected its share of members of the Provisional National Representorship of Serbs, Croats and Slovenes. Baranja became a hideout for communist and other refugees from Miklós Horthy's White Terror. The Treaty of Trianon had assigned most of the Baranja region to Hungary, which led to massive protest and a group of people under painter Petar Dobrović to proclaim a Serb-Hungarian Baranya-Baja Republic. This Republic lasted only a few days, and on 25 August 1921 it was invaded and annexed by Hungary, in accordance with Hungarian borders defined by the Treaty of Trianon. The Treaty of Trianon assigned to Hungary some of the most northern territories under Serbian control, in which a minority of South Slavs remained. On the other hand, sizable German and Magyar minorities were left within the Kingdom's borders. Central Banat was granted to Romania, as the region was divided on ethnic grounds, so as to contain a majority of the populations, leaving a minority of Yugoslavs in Romania and a minority of Romanians in the Kingdom of SCS. The BBB region remained a historical entity in the united realm until 1922 when a new administration was adopted in accordance to the unitary system. The region was split among the Bačka (with center in Novi Sad), Belgrade and Podunavlje (with center in Smederevo) administrative areas. When the Kingdom of Yugoslavia was proclaimed in 1929, most of the region eventually became a part of the Danubian Banate, with a small part going to the City of Belgrade.