Serbia fought in the Balkan Wars of 1912–13, which forced the Ottomans out of the Balkans and doubled the territory and population of the Kingdom of Serbia. In 1914, a young Bosnian Serb student named Gavrilo Princip assassinated Archduke Franz Ferdinand of Austria, which directly contributed to the outbreak of World War I. In the fighting that ensued, Serbia was invaded by Austria-Hungary. Despite being outnumbered, the Serbs defeated the Austro-Hungarians at the Battle of Cer, which marked the first Allied victory over the Central Powers in the war. Further victories at the battles of Kolubara and the Drina meant that Serbia remained unconquered as the war entered its second year. However, an invasion by the forces of Germany, Austria-Hungary and Bulgaria overwhelmed the Serbs in the winter of 1915, and a subsequent withdrawal by the Serbian Army through Albania took the lives of more than 240,000 Serbs. Serb forces spent the remaining years of the war fighting on the Salonika Front in Greece, before liberating Serbia from Austro-Hungarian occupation in November 1918. Serbia suffered the biggest casualty rate in World War I.