In general, the year 1912 was a pretty bad year for the Ottomans. We’ve already talked about that year’s Albanian Revolt, which ended with Albania having won substantial autonomy within the empire. The Italo-Turkish War ended in October 1912 with a victorious Italy in control of Libya and the Dodecanese islands (which now belong to Greece), which wasn’t great either. And just as that war was ending, a new one was starting, on October 8: the First Balkan War, fought between the Ottomans and the Balkan League (Serbia, Montenegro, Greece, and Bulgaria). This one ended with the Ottomans losing nearly all of their European territory.
As wars go, the cause of the First Balkan War was pretty simple: the Ottomans were obviously reeling, and those four Balkan states all wanted to gobble up as much of its European territory as they could. Yes, they formed a “league” to coordinate their war efforts, but each of the four had its own territorial ambitions, and they were all largely in conflict with one another (everybody was looking to annex Macedonia for themselves, for example). Those competing territorial claims made the immediate post-war settlement untenable, and led quickly to a Second Balkan War in mid-1913, with Bulgaria fighting its former allies (plus the Ottomans).
Although October 24 was still early in the war, which wouldn’t end until May 1913, it’s distinguished by the fact that the Ottomans suffered two separate defeats on the same day. Neither one of the defeats was particularly compelling, but they’re worth noting in that they both happened on the same day and highlighted the fact that the Ottomans were seriously overmatched by their former Balkan subjects by this point.
First there was the Battle of Kirk Kilisse (or Lozengrad), fought in eastern Thrace between Ottoman and Bulgarian armies. The Ottomans were significantly outnumbered (about 95,000 men to Bulgaria’s ~150,000) simply couldn’t withstand the Bulgarian Third Army’s attack and crumbled. There’s not much to say here except that the battle opened the way up for the Bulgarian Third Army to join the Bulgarian First Army and push toward Constantinople. They were met by another Ottoman army at the First Battle of Çatalca (on the outskirts of modern Istanbul) in mid-November. In that instance, much closer to home, a little less outnumbered, and having had time to prepare a strong defensive line, the Ottomans won a decisive victory–their greatest victory of the war, in fact.
The other defeat came at the Battle of Kumanovo, which is now in Macedonia, at the hands of the Serbs. Here too the Ottomans (under Field Marshal Zeki Pasha) were badly outnumbered (this is really a scenario you don’t encounter much until the 19th-20th centuries), with about 60,000 men against roughly 130,000 Serbs (including one Bulgarian division). Nor did the Ottomans possess the kind of technological edge they’d once had over the Balkan states. Despite this, Zeki Pasha insisted on attacking the Serbs rather than waiting for the Serbs to come to him. The forces were so mismatched that a heavy Ottoman attack on the left wing of the Serbian line on October 23 failed despite the fact that the main command of the Serbian army (under General Radomir Putnik) seems to have been unaware that the attack was happening (or was at least unaware of how heavy the fighting was) and never in its reserve units. The Serbian forces and their artillery were enough to drive back an initially successful Ottoman assault.
The next day, Serbian reinforcements did arrive to thwart another Ottoman attack on the Serbian left wing, and instead it was the Ottoman left wing that crumbled until a full scale assault from the Serbians. It was a crushing Serbian victory, but in truth Zeki Pasha actually lucked out; his decision to attack a far superior force on October 23 was probably not the smartest thing he could have done, and the only thing that kept his forces from being routed on that day was probably that miscommunication among the Serbs. Once that was accomplished, the Serbs turned their attention to the Ottoman center and routed it as well.
Of the two battles, Kumanovo was easily the more decisive. Its outcome forced the Ottomans to abandon any thoughts of aggression and adopt a defensive posture for the rest of the war. Still, it could have been much more decisive. Putnik and his aides appear to have spent the entire battle unaware that they were fighting a main Ottoman army. They were consistently late and slow to react, and it was only the relative strength of the Serbian army that kept the Ottomans from victory (again, the notion of a Serbian army so thoroughly outmatching an Ottoman one would have been unthinkable before the late 1800s at the earliest). After they’d won the battle, the Serbian commanders–still believing they’d only encountered a small Ottoman skirmishing force–then failed to capitalize on their victory by chasing the retreating Ottomans. Had they run the Ottomans down and destroyed their army the battle could have crippled the imperial war effort. As it was it merely set the tone for the war.