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Balkan Wars and World War I

In the First Balkan War in 1912, the Balkan League defeated the Ottoman Empire and captured its European territories, which enabled territorial expansion of the Kingdom of Serbia into regions of Raška, Kosovo, Metohija, and Vardarian Macedonia. The Second Balkan War soon ensued when Bulgaria turned on its former allies, but was defeated, resulting in the Treaty of Bucharest. In two years, Serbia enlarged its territory by 80% and its population by 50%, it also suffered high casualties on the eve of World War I, with more than 36,000 dead. Austria-Hungary became wary of the rising regional power on its borders and its potential to become an anchor for unification of Serbs and other South Slavs, and the relationship between the two countries became tense.

The assassination of Archduke Franz Ferdinand of Austria on 28 June 1914 in Sarajevo by Gavrilo Princip, a member of the Young Bosnia organisation, led to Austria-Hungary declaring war on Serbia, on 28 July 1914, setting off World War I.

Serbia won the first major battles of the war, including the Battle of Cer, and the Battle of Kolubara. Despite initial success, it was eventually overpowered by the Central Powers in 1915 and Austro-Hungarian occupation of Serbia followed. Most of its army and some people retreated to Greece and Corfu, suffering immense losses on the way. After the Central Powers' military situation on other fronts worsened, the remains of the Serb army returned east and led a final breakthrough through enemy lines on 15 September 1918, liberating Serbia and defeating Bulgaria and Austria-Hungary. 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'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 700,000, more than 16% of Serbia's prewar size, and a majority (57%) of its overall male population. Serbia suffered the biggest casualty rate in World War I.