In the course of 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.