Bulgaria, Serbia, Greece, and Montenegro sign an armistice with Turkey, ending the first Balkan War. During the two-month conflict, a military coalition between Greece, Serbia, Bulgaria, and Montenegro--known as the Balkan League--expelled Turkey from all the Ottoman Empire's former European possessions, with the exception of Constantinople (now Istanbul). In January 1913, a coup d'etat in Turkey led to a resumption of fighting, but the Balkan League was again victorious.
In 1913, the Second Balkan War began after Serbia and Greece demanded that Bulgaria cede to them portions of Macedonia. Serbia and Greece formed an alliance against Bulgaria, and Macedonia was partitioned between the victors. Nationalist tension persisted in the Balkans, and Serbia was particularly bitter about being forced to give up some of its conquests by the Austro-Hungarian Empire.
On June 28, 1914, hostility between Serbia and Austria-Hungary over Austria's possession of Bosnia-Herzegovina reached a breaking point when Serbian nationalist Gavrilo Princip assassinated Austrian Archduke Franz Ferdinand in Sarajevo. Austria-Hungary blamed the Serbian government for the attack and hoped to use the incident as justification for settling the problem of Serbian nationalism once and for all. However, as Russia supported Serbia, an Austro-Hungarian declaration of war was delayed until its leaders received assurances from German leader Kaiser Wilhelm that Germany would support their cause in the event of a Russian intervention.
On July 28, Austria-Hungary declared war on Serbia, and the tenuous peace between Europe's great powers collapsed. Within a week, Russia, Belgium, France, Great Britain, and Serbia had lined up against Austria-Hungary and Germany, and World War I had begun.