By 1912, the Ottoman Empire (the Turks) was widely known as "the sick man of Europe."
The Ottomans had controlled most of the Balkans for over 500 years, but control had been slipping away little by little since the early 19th Century. Greece won her independence in 1828. Serbia, Bulgaria, and Montenegro arose as independent states after the Russo-Turkish War of 1877-78. Fierce nationalism drove these countries to seek ways to reclaim the Ottoman territories they believed were their ancestral homelands.
The Russians were involved as well, pushing Bulgaria, Montenegro and Serbia to expand their borders in the interest of "pan-Slavism." Of course, this expansion could only come at the expense of both the Ottomans and Austria-Hungary.
Austria-Hungary controlled a large multi-ethnic empire which included many Slavs and did not want Russia stirring up trouble. The British also had interests in the region, primarily in regards to control of the Dardanelles, which England saw as vital to her naval supremacy.
(Let's also not forget the Germans, the Internal Macedonian Revolutionary Organization, and Albanian independence movements, all of whom insisted on stirring the pot. Clearly, the situation was far more complicated than can be discussed here.)
Suffice to say that Bulgaria, Serbia, Greece and little Montenegro banded together to form the Balkan League, a loose affiliation whose primary goal was to expand League members' borders by taking away Ottoman territory.
Montenegro jumped the gun and started the war on October 9, 1912 by attacking Turkish forces in Albania. Serbia, Bulgaria, and Greece joined in 9 days later. The Bulgarians made strong gains towards Constantinople while the Greeks headed for Salonika and the Serbs struck out for Monastir and Scutari. The Turks sought to attack the enemy before the League forces were fully mobilized. Those attacks were ill-timed and met with disaster.
The Ottomans were soon fighting a purely defensive war on too many fronts and, fearing the loss of Constntinople, sought an armistice. The League accepted. Except for a small patch of land around Constantinople, the Ottoman Empire had been driven from the continent of Europe.
Unfortunately, the Balkan League did not fair as well in peace as it had in war. All the League members began to argue over the spoils. Bulgaria, the country whose army had arguably performed the best, was particularly upset and in 1913 launched an ill-conceived attack against the other League members, starting the Second Balkan War.
