Why did the Ottoman military perform so poorly in the 1st Balkan War? While the Ottoman military wasn't equipped and trained up to the standards of the European great powers, neither were its Balkan opponents. The Ottoman military decisively defeated the Serbians in 1876, and did the same to Greece in 1893. Despite losing the Russo-Turkish War, the Ottoman military preformed relatively well despite the disparity of forces. In World War 1 the Ottomans managed to hold their own against enemies who frequently outnumbered them, and who were usually better trained, supplied, and equipped, while blockaded and fighting on three or four fronts.
However in the 1st Balkan War, the Ottoman military was decisively defeated, nearly losing Istanbul, in a few months. What happened in that war?
For one thing, they were under attack from multiple enemies, Montenegro, Serbia, Greece and Bulgaria, which divided the forces they had available. They were also dealing with insurgencies in Albania and Macedonia, and had just come off a stinging defeat at the hands of the Italians in Libya. The Balkan League was well-armed and equipped with French and Russia, even German, weaponry, which only added to the difficulties the Ottoman forces faced. Perhaps the key factor was the presence of the Greek Navy, which was decisive in shutting out the Ottoman fleet and preventing the Ottomans from resupplying and reinforcing their troops in the Balkans. The only consolation in the end was that the Bulgarians overstretched themselves, and couldn't follow up their advance on Constantinople. Aside from that, it was a harsh blow to the Empire.
Christopher Clark and Margaret MacMillan cover the wars in The Sleepwalkers and The War that Ended Peace respectively; for a good history of the wars themselves, Richard C. Hall's book is a good start (The Balkan Wars, 1912-13: Prelude to WWI)
P.S. It's also worth outing that the Ottoman Army's performance in WWI wasn't entirely stellar either; losses at Gallipoli and in the Caucasus were catastrophic.