The preliminary kingdom was formed in 1918 by the merger of the provisional State of Slovenes, Croats and Serbs (itself formed from territories of the former Austria-Hungary, encompassing Bosnia and Herzegovina and most of Croatia and Slovenia) and Banat, Bačka and Baranja (that had been part of the Kingdom of Hungary within Austria-Hungary) with the formerly independent Kingdom of Serbia. In the same year, the Kingdom of Montenegro also proclaimed its unification with Serbia, whereas the regions of Kosovo and Vardar Macedonia had become parts of Serbia prior to the unification.