In the first decade of 20th century, the Ottoman Empire was in decline, and at the time commonly referred to as “The Sick Man of Europe”. Several small states arose in the Balkans among the Christian minorities of the dying Muslim Empire: Serbia, Greece, Bulgaria, and Montengro. To complicate matters, they were interspersed among the Ottoman’s remaining territories. In 1908, the Young Turks rebelled to modernize the Ottoman Empire in an attempt to stem the tide of Slavic and Bulgar nationalism.
During the chaos, the Austro Hungarian Empire seized Bosnia Herzegovina, just ahead of a nascent Serbia after it declared its intention to unite all ethnic Serbs. Afterward, about 1/3 of Bosnia’s Muslims relocated to the Ottoman provinces of Kosovo and Macedonia, where instead of providing a check on Serbian aggression, they immediately joined the neighboring Albanian revolt against their Ottoman co-religionists.
Stymied in the north by the Austrians, Serbia looked south and hungrily eyed the lands embroiled in the confusion caused by the Albanian revolt. They were not strong enough to take on the Ottoman’s themselves. However, they found willing but tenuous allies among Bulgaria, Greece, and Montenegro who also sought take advantage of Ottoman weakness. On 10 October 1912, the Kingdom of Montenegro was the first to declare war, ostensibly to unite Serbs but actually to acquire territory from Albania and the Ottomans.
The First Balkan War ended in 1913 with the Ottomans effectively thrown out of Europe except for Istanbul and a small slice of Thrace. However, Bulgaria was unsatisfied with their new territorial acquisitions, and felt cheated. (Historically, the land of the Bulgars was much bigger, particularly in the Middle Ages.) This perceived slight would eventually lead to the Second Balkan War which began shortly after the first concluded in 1913.
The Second Balkan War was a disaster for Bulgaria, and it lost even more territory. However, its conclusion in 1914 ended the Ottoman threat to the Balkans. As result, Serbia, with its ally Montengro, would again look north to the ethnic Serbs in the Austro-Hungarian province of Bosnia-Herzegovina. They were not strong enough to militarily overthrow the Austrians, but their battle hardened armies were strong enough to prevent Austria from invading Serbia. Unable to attack the Austro-Hungarian Empire, Serbia decided upon war of terror against the ruling Austrian Hapsburg elite. In the Bosnian capital of Sarajevo in the summer of 1914, Serb nationalists assassinated the Austrian Archduke Franz Ferdinand, the spark that ignited World War I.