The following is the order of battle of the Hellenic Army during the First Balkan War of 1912–1913.
Following the defeat in the Greco-Turkish War of 1897, starting in 1904 and especially after the Goudi coup of 1909, serious efforts were undertaken to reorganize and modernize the Army. From 1911, this task was undertaken by a French military mission. The peacetime establishment of the Hellenic Army in 1912 comprised four infantry divisions (1st at Larissa, 2nd at Athens, 3rd at Missolonghi and 4th at Nafplion) newly reformed as triangular divisions, a cavalry brigade, six Evzones battalions, four field artillery and two mountain artillery regiments, one fortress artillery battalion and various support units, including two engineer regiments and an aircraft company. From 25 March 1912, Crown Prince Constantine assumed the position of Inspector-General of the Army, becoming its de facto commander-in-chief.
Greece's territorial claims in Macedonia and Thrace clashed with those of Bulgaria, leading in the so-called 'Macedonian Struggle' between the two in 1904–1908, an armed guerrilla and propaganda conflict over influence over the local Christian populations. Nevertheless, from 1910 on, negotiations for the formation of a Greco-Bulgarian alliance, and later of a Balkan League encompassing all Christian Balkan states, began in earnest. Greece joined the League via a three-year, defensive military convention with Bulgaria on, which obliged Greece to assist Bulgaria with 120,000 men on land and its fleet at sea; Bulgaria undertook to raise 300,000 men. Nevertheless, due to the defeat of 1897, the Greek army on land was generally disregarded as a serious factor, even among the Balkan allies.
With the escalation of the crisis between the Balkan states and the Ottoman Empire, and the mobilization of Bulgarian forces on, Greece followed suit with a general mobilization at midnight of . The decree of mobilization was also carried out on Crete, which although formally an autonomous state was annexed into Greece. The existing units were brought up to full strength, and the 5th, 6th, and 7th infantry divisions were set up. The number of men from the Greek diaspora, or foreign philhellenes, who rushed to Greece to join up was such that the 6th and 7th divisions were able to have three-battalion regiments, instead of the two-battalion regiments originally envisaged.
The Greek war plan had been updated as recently as 1912, and envisaged the division of the mobilized army into three parts:
In contrast to pre-war planning, no army corps were established; instead, the divisions were directly subordinated to GHQ of the Army of Thessaly. The latter, which comprised the bulk of the Greek Army, was placed under the command of the Crown Prince and gathered around Larissa, while the Army of Epirus formed around Arta under Lieutenant General Konstantinos Sapountzakis. After the Ottoman government rejected an ultimatum of the Balkan states, Serbia and Bulgaria declared war on, with Greece following the next day.
The Army of Thessaly comprised in total 59 infantry and 4 Evzone battalions, eight cavalry companies (of the Cavalry Brigade) and six half-companies (organic to the infantry divisions), seven engineer companies (organic to the infantry divisions), 32 artillery batteries with 96 field guns, 24 mountain guns and 54 fortress guns and 70 machine-guns, and various support troops including an aviation company with four aircraft. The force totalled some 100,000 men, with front-line effectives. On these were organized in the following formations:
The theatre of operations of Epirus was of secondary strategic importance, with the Army of Epirus having initially a purely defensive mission. Initially, the Army of Epirus was outnumbered by the Ottoman forces in the area (Yanya Corps under Esad Pasha), which numbered about 20,000 men. It nevertheless pushed back the initial Ottoman assaults, took Preveza and advanced to the approaches of the Ioannina fortified zone. Two attacks on the city in December and January were repulsed by the Ottoman forces, but after extensive preparations and transfer of forces from Macedonia, the city fell following the Battle of Bizani. The Army of Epirus was commanded until by Lt Gen Konstantinos Sapountzakis, and thereafter by Crown Prince Constantine.
The Army of Epirus was composed of several independent units of roughly divisional strength. Its forces numbered eight infantry and Evzone battalions, one cavalry company and 24 guns, totalling some 10,500 men in the early days of the war. At the outbreak of the war, it comprised the following units, concentrated in and around Arta:
To these were later added two battalions of Cretans, as well as the volunteer Garibaldini legion. On, these forces formed the Epirus Division (Greek, Modern (1453-);: Μεραρχία Ηπείρου), renamed in February 1913 as the 8th Infantry Division (Greek, Modern (1453-);: VIII Μεραρχία). In early December, the Army of Epirus was reinforced with the 2nd Infantry Division, followed on 27 December by the 4th Division, by mid-January by the 6th Division and the 7th Infantry Regiment. In preparation for the Battle of Bizani, the Crown Prince brought in additional troops and artillery, while a Mixed Brigade was formed to attack the fortifications of Ioannina from the northeastern flank.
Before the Battle of Bizani, the Army of Epirus comprised the 2nd, 4th, 6th and 8th Infantry Divisions, the Mixed Brigade, a cavalry regiment and the three independent detachments of Acheron, Preveza and Himara, in total 51 infantry battalions and other units, comprising 41,400 men with 48 machine guns and 93 field and mountain guns.
These were grouped as follows, from the right to the left of the Greek front: