ent into exile to Greece and Corfu, where they recovered, regrouped and returned to the Macedonian front to lead a final breakthrough through enemy lines on 15 September 1918, liberating Serbia and defeating the Austro-Hungarian Empire and Bulgaria. Serbia, with its campaign, was a major Balkan Entente Power which contributed significantly to the Allied victory in the Balkans in November 1918, especially by helping France force Bulgaria's capitulation. Serbia was classified as a minor Entente power. Serbia was also among the main contributors to the capitulation of Austria-Hungary in Central Europe.. Serbia's casualties accounted for 8% of the total Entente military deaths; 58% (243,600) soldiers of the Serbian army perished in the war. The total number of casualties is placed around 1,000,000, more than 25% of Serbia's prewar size, and a majority (57%) of its overall male population.
As the Austro-Hungarian Empire collapsed, the territory of Syrmia united with Serbia on 24 November 1918, followed by Banat, Bačka and Baranja a day later, therefore bringing the entire Vojvodina into the Serb Kingdom. On 26 November 1918, the Podgorica Assembly deposed the House of Petrović-Njegoš, and uniting Montenegro with Serbia. On 1 December 1918, Serbian Prince Regent Alexander of Serbia proclaimed the Kingdom of the Serbs, Croats, and Slovenes under King Peter I of Serbia.
King Peter was succeed by his son, Alexander in August 1921. Serb centralists and Croat autonomists clashed in the parliament, and most governments were fragile and short-lived. Nikola Pašić, a conservative prime minister, headed or dominated most governments until his death. King Alexander changed the name of the country to Yugoslavia and changed the internal divisions from the 33 oblasts to nine new banovinas. The effect of Alexander's dictatorship was to further alienate the non-Serbs from the idea of unity. Alexander was assassinated in Marseille, during an official visit in 1934 by
