In 1911, Italy was at war with the Ottoman Empire to snatch Libya. Taking advantage of the momentary distraction to its decaying southern neighbor, Russia exerted its influence over some of its southern client states to form the Balkan League in the spring of 1912. Consisting of Montenegro, Greece, Bulgaria, and Serbia, the whole point of the Balkan League was to snatch Macedonia out from under Turkey while it wasn't looking. They felt that the Ottomans were treating their ethnic kinsmen in Macedonia poorly and also just sort of wanted the territory.
The League began when Bulgaria and Serbia signed a treaty on March 13, 1912 assigning northern Macedonia to Serbia and southern Macedonia to Bulgaria after the conquest. The two also considered war against Austria-Hungary if it should make any false moves. In May of 1912, Greece and Bulgaria signed a similar agreement and Montenegro just sort of tagged along sans agreement.
Even without an agreement, Montenegro got the ball rolling by declaring war on Turkey on October 8, 1912. All of the other members apparently did the same after a ten day delay. Between the lot of them, they had 750,000 men, and they used them wisely. The Bulgarians advanced through Thrace, defeated the main body of the Ottoman forces and even laid siege to Adrianople. They got as far as the outskirts of Istanbul itself. The Serbians defeated the Turkish forces at Kumanovo in Macedonia, allowing them to capture Bitola. They then joined up with the Montenegrin forces to advance to Skopje. The Serbs captured the port of Durazzo in Albania, which was under Turkish control, and the Montenegrins laid siege to Skoder there, too. The Greek forces occupied Turkish-held Salonika in order to drive into Ioannina. When all was said and done, Macedonia, Albania, Epirus, and Thrace were all in the hands of the Balkan League.
Turkey was quite thoroughly beaten, militarily. They were forced to treat for peace before the year was out. On December 12, 1912, emissaries from all countries met in London to begin talks. Back in Turkey, though, the warlike Young Turk faction had taken control of Turkey via coup d'etat during January of the next year, meaning that the war was resumed.
Again, the Ottomans fared very poorly. The Greeks and Bulgarians completed their conquests of Ioannina and Adrianople, respectively. The Ottomans were forced to treat for peace again, this time in an even worse position. They were forced to give up nearly all of Macedonia and Albania, with Macedonia to be divided among the members of the Balkan league and Albania to be granted independence. Additionally, they were made to give up Crete to Greece, who in turn gave up the Aegean islands that it had conquered during the course of the war to British, French, and German sovereignty. The technical points of the treaty stipulated that the Ottomans give up all European territory west of a line drawn from Midia on the Black Sea to Enos on the Aegean, which, except Albania, was given to the Balkan League states. Serbia, though, was made to let go of Durazzo by Italy and Austria-Hungary, since they feared that a Serbian port would eventually become a Russian port.
After the war, the members of the Balkan League, fueled by nationalism, fell to arguing over the stuff they'd captured, but that's a story for another war.