In 1912 Turkish chauvinism and atrocities combined with Albanian insurgency to galvanize Serbia, Bulgaria, and Greece. In the first Balkan War, October 1912 to May 1913, these nations joined Montenegro to oust Turkey from the Balkans. Besides capturing western Macedonia, Kosovo, and other Serbian-populated regions, Serbian forces moved through purely Albanian-populated lands to the Adriatic. Austria-Hungary convinced the major European powers to create an independent Albania to deny Serbia an Adriatic outlet, and it forced Serbia to remove its troops from Albanian territory. The Treaty of London (1913) awarded the Serbs almost all remaining Ottoman lands in Europe, but there was immediate conflict over the division of Macedonia. With AustroHungarian approval, Bulgaria attacked its erstwhile allies in June 1913, triggering the Second Balkan War. This time Serbia, Montenegro, Greece, Romania, and Turkey defeated Bulgaria and eliminated the possibility of Bulgarian participation in a South Slav state. Its victories filled Serbia with confidence and doubled its size. But the wars also weakened the country and left it with hostile neighbors and bitter Macedonian and Albanian minorities.

Serbian victories and the Serbians' obvious contempt for Austria-Hungary brought hostility from Vienna and anti-Habsburg sentiment in all the empire's South Slavic regions, especially Bosnia and Hercegovina. Confident behind German military protection, the high command of Austria-Hungary lobbied for war to eliminate Serbia. Serbia's alliance with Slavic Russia also encouraged the growth of expansionist, nationalist secret societies in the Serbian army. The most significant of these societies was the Black Hand, a group of army officers who dominated the army and influenced the government from 1911-17.

In 1914 Archduke Franz Ferdinand, heir to the Habsburg throne and a longtime advocate of equality for the South Slavs in the empire, made an ill-prepared visit to Bosnia. On Vidovdan, Bosnian student Gavrilo Princip assassinated the archduke and the archduchess in Sarajevo. The Black Hand had armed and trained the assassin, but historians doubt that the rulers of Serbia had approved the plot. Nevertheless, on July 23 Austria-Hungary sent an ultimatum, threatening war unless Serbia allowed Vienna to join the murder investigation and suppress secret societies. Even the German kaiser felt that Serbia met the Austrian demands, but war was declared, the existing alliance structure of Europe went into force, and World War I began. The Central Powers--Germany, Austria-Hungary, and Turkey--faced the Triple Entente--France, Britain, and Russia. The Croats, Slovenes, and many Serbs in Austria-Hungary went to war against Serbia and Montenegro.

Despite overwhelming odds, Serbia twice cleared its soil of invading Austro-Hungarian armies early in the war, and late in 1914 the prime minister announced plans to unite the Serbs, Croats, and Slovenes in a South Slavic state. Italy joined the Triple Entente in 1915 and attacked Austria-Hungary, then Bulgaria joined the side of Austria-Hungary in the fall of that year. With French and Italian forces waiting in nearby Salonika (Thessalonika), German, Austro-Hungarian, and Bulgarian forces attacked Serbia in October 1915. The Serbian army, weakened by typhus, escaped through Montenegro and Albania in midwinter, suffering heavy losses. Italian units in Albania denied support, then French ships evacuated the remaining Serbian forces to Corfu.

Austria-Hungary and Bulgaria occupied Serbia and Montenegro after the retreat. After recovering, the Serbian army helped the French and British capture Bitola in September 1916. Entente armies remained inactive there until the Central Powers began to disintegrate. They then routed the Bulgarians in September 1918, swept Austro-Hungarian and German forces from Serbia, and entered Hungary. In November Austria-Hungary collapsed and the war ended. World War I destroyed one-fourth of Montenegro's population and several hundred thousand Croats and Slovenes. Serbia lost about 850,000 people, a quarter of its prewar population, and half its prewar resources.

Data as of December 1990