The First Balkan War broke out in October 1912. On 30 September/13 October 1912, the ambassadors of the three allied Balkan countries, Greece, Serbia and Bulgaria, handed an ultimatum to the Sultan's Government with claims concerning the rights of the Christian communities of the Ottoman Empire.

The ultimatum was not accepted. Military engagements began immediately. The Greek troops of Thessaly captured Elassona and on 9/10 October won the first victory in the Battle of Sarantaporo. The capture of Kozani and Grevena followed suit. At this point the allied forces had to confront the dilemma whether to advance northwards, towards Monastir, or eastwards to capture Thessaloniki. With the government's intervention and the personal intervention of Venizelos, the Commander-in-Chief, Crown Prince Constantine, consented in favour of the second option. The decisive battle took place in Yanitsa on 19/20 October. The defeat of the Turkish troops paved the way for Thessaloniki, which was captured by the Greek army on the 27th of that same month, while the Bulgarian military units were marching towards the city from the east.

In November western Macedonia was mopped up by the remnants of the Turkish troops and Florina, Kastoria and later Korytsa were liberated. The emphasis now was put on Epirus, where during the month of October mostly defensive operations have been scheduled. However, despite the small number of forces, the liberation of Preveza was achieved. After November 1912 the transfer of reinforcements from Macedonia began and the Greek army was forwarded to Ioannina. The city, however, was guarded by the Bizani fort, which held a strategic position and its defence was organized by German experts.

In addition, the winter of that year was severe and hindered operations. The siege of the city lasted for several months and the final offensive took place on 20/21 February 1913. The following day, the 22nd, the Greek army captured Ioannina.

At the same time as the land operations, considerable action was undertaken by the Greek navy in the Aegean. Greece was the only country of the three to possess a fully-equipped naval force, and so was in charge of confronting the Turkish fleet. Her flagship was the battleship Averoff and under the leadership of Admiral Pavlos Koundouriotis the Greek fleet managed to close in upon the enemy in the straits of Hellespont and Propontis. This was mainly the result of its victories in two sea battles: that of Elli on 3 December 1912 and of Limnos on 5 January 1913. The prevalence of the Greek fleet in the Aegean averted any reinforcement of the Turkish troops arriving on the fronts of Macedonia and Epirus and allowed for the safe transfer of Greek and other allied troops by ship. It also allowed the liberation of the Archipelago islands. Limnos, Thassos, Samothrace, Ayios Efstratios, Psarra, Imvros and Tenedos, that were of great importance for the development of naval operations, were liberated by October 1912 by naval forces. Later, Mytilene and Chios were captured with the landing of military forces. Last, Samos, already an autonomous principality, was declared (in October) its Union with Greece.