23 and 24 October 1912, Kumanovo, on the road into Macedonia. The Serbian First Army meets the Ottoman Vardar army and breaks it. Skopje falls two days later. A provincial battle, unless you are the empire that has just lost a province it held for five centuries.
Why then
The Balkan states stop waiting for the great powers to settle Ottoman Europe. Serbia, Bulgaria, Greece and Montenegro form the Balkan League and attack in October, as the Ottomans are signing away Libya to Italy after a year of war. Kumanovo is the first proof the moment was real.
What they wanted
Serbia is after Macedonia and a size Austria-Hungary cannot ignore. The Ottoman command hopes to hold a frontier. Vienna is not on this field, and watches every mile.
What changed
By 1913 Serbia has nearly doubled. The allies then fight each other over the spoils, and hundreds of thousands of Muslims flee or are driven into Anatolia. In Vienna a larger Serbia starts to look like something that must be cut down to size; Sarajevo is two years away.
Still argued
Each Balkan nation tells it as a war of liberation. The Muslims who were driven out are only now entering the histories, and with them the case that it was also conquest and expulsion.