The Greek People's Liberation Army (Greek: Ελληνικός Λαϊκός Απελευθερωτικός Στρατός (ΕΛΑΣ), Ellinikós Laïkós Apeleftherotikós Stratós), known mostly by its acronym ELAS, was the largest and most significant military organization of the Greek resistance during World War II. It was the military arm of the National Liberation Front, a left-wing group whose main driving force was the Communist Party of Greece, during the period of the Greek resistance against the Axis occupation of Greece. It was disarmed and disbanded in February 1945, following the Dekemvriana clashes and the Varkiza Agreement.
With the German invasion and occupation of the country in April–May 1941, several hundred KKE members were able to escape and flee to the underground. Their first task was to reform the party, along with subsidiary groups like the "National Solidarity" (Εθνική Αλληλεγγύη, EA) welfare organization 28 May. After the German attack on the Soviet Union on 22 June and the break of the Molotov–Ribbentrop Pact, the newly reconstituted Communist Party found itself firmly on the anti-Axis camp , a line confirmed by the Party's 6th Plenum during 1–3 July. The Communists were committed to a "Popular Front" tactic, and tried to engage other parties from the left and the centre, including established pre-war politicians. However, the efforts proved largely fruitless. On 16 July, however, the "National Workers' Liberation Front" (Εθνικό Εργατικό Απελευθερωτικό Μέτωπο, ΕΕΑΜ) was established, bringing the country's labour union organizations together.
At the ΚΚΕ's 7th Plenum, the establishment of EAM was decided despite the refusal of mainstream politicians to participate.
ΕΑΜ was founded on 27 September 1941 by representatives of four left-wing parties: Lefteris Apostolou for the ΚΚΕ, Christos Chomenidis for the Socialist Party of Greece (SKE), Ilias Tsirimokos for the Union of People's Democracy (ELD) and Apostolos Vogiatzis for the Agricultural Party of Greece (ΑΚΕ). ΕΑΜ's charter called for the "liberation of the Nation from foreign yoke" and the "guaranteeing of the Greek people's sovereign right to determine its form of government". At the same time, while the door was left open to cooperation with other parties, the ΚΚΕ, with its large size concerning its partners, assumed a clearly-dominant position within the new movement. Furthermore, the ΚΚΕ's well-organized structure and its experience with the conditions and necessities of underground struggle were crucial to ΕΑΜ's success. Georgios Siantos was appointed as the acting leader, since Nikolaos Zachariadis, the ΚΚΕ's proper leader, was interned in Dachau concentration camp. Siantos with his affable manner and modest ways was much more popular in the party than Zachariadis.
On February 16, 1942, EAM founded the Greek People's Liberation Army (ELAS), according to the KKE's 8th Plenum decision in January 1942. ELAS initiated actions against the German and Italian forces of occupation in Greece on 7 June 1942. A communist veteran, Athanasios (Thanasis) Klaras, known as Aris Velouchiotis, with a small group of 10–15 guerrillas, entered the village of Domnista in Evrytania in June 1942 and proclaimed in front of the surprised villagers that they were about to "start the war against the forces of Axis and their local collaborators". Initially, Velouchiotis also recruited traditional local mountain-living bandits, like Dimos Karalivanos, in order to create a small group of experts in guerilla warfare.
On a night in September 1942, a small group of British SOE officers parachuted into Greece near Mt. Giona. This group, led by Brigadier Eddie Myers, had been tasked to blow up one of three bridges (Gorgopotamos, Papadia or Asopos) of the country's main railway line, and to get the two main, but competing, guerrilla groups of ELAS and EDES to cooperate.
After much deliberation, the Gorgopotamos bridge was chosen due to the difficulty of making repairs to the structure. Dimos Karalivanos, an ELAS guerrilla, was the first guerrilla the British found. At the end of October a second group of British officers were parachuted into the Greek mountains. Their leaders were Themis Marinos and Colonel Christopher Woodhouse. Their mission was to locate the guerrillas of EDES and their leader Napoleon Zervas, who were friendlier to the British Middle East Command than ELAS, and co-operate with them. The two Greek groups eventually agreed to collaborate. The British did not favour the participation of ELAS, because it was a pro-communist group, but the forces of ELAS were larger and better organised, and without their participation, the mission was more likely to fail.
On November 14, the 12 British saboteurs, the forces of ELAS (150 men) and those of EDES (60-65 men) met in the village of Viniani in Evrytania and the operation started. Ten days later, they were at Gorgopotamos. On the night of November 25, at 23:00, the guerrillas started the attack against the Italian garrison. The Italians were startled, and after little resistance, were defeated. After the defeat of the Italians, the saboteurs set the explosives. ELAS forces had placed ambushes on the routes towards the bridge, to block the approach of Italian reinforcements. The explosion occurred at 03:00. Afterwards, the guerrillas' forces returned to Viniani, to celebrate the success of the mission.
The destruction of the Gorgopotamos bridge was, along with the Norwegian heavy water sabotage in Rjukan, one of the two biggest guerrilla acts in occupied Europe. The blowing up of the bridge disrupted the German transportation of ammunition via Greece to Rommel's forces for several weeks, taking place at a time when the Afrika Korps in North Africa, retreating after the defeat of El Alamein, were in need of supplies.
The blowing up of Gorgopotamos bridge favored ELAS. Soon, many of the inhabitants of the villages of Central Greece became members of ELAS. Furthermore, people sympathised with the ELAS guerrillas because they weren't helped by the British in contrast with EDES. When 25 guerrillas deserted from ELAS, Aris Velouchiotis went to Epirus to threaten Napoleon Zervas not to come in touch with them. Later, the 25 deserters were arrested and executed in the village of Sperhiada. The winter of 1942, ELAS groups were formed in other Greek regions, like Thessaly and Macedonia. In Central Greece, Aris Velouchiotis succeeded to form a powerful semi-conventional army which could attack German and Italian forces. Aris became a legendary figure who imposed an iron discipline in ELAS. At the same time, some members of ELAS (Periklis, Tasos Leuterias, Diamantis, Nikiforos, Thiseas, Dimos Karalivanos, and Belis) have been distinguished during the battles. Aris Velouchiotis formed a group of 30–35 men, called "Mavroskoufides" (the "black caps"), who were his personal guards. During the winter of 1942–1943, new units of ELAS were composed in many regions of Greece. Some areas in the mountains of Central Greece passed from the control of Axis forces to that of ELAS.
The leadership of ELAS followed a triadic form, from its top down to platoon level: the captain (kapetánios), elected by the men and the overall leader of the unit, the military specialist (stratiotikós), usually a regular army officer, responsible for tactical planning and training, and the political leader (politikós), usually a KKE member, as EAM's representative. At its top, the General Headquarters of ELAS, these positions were filled by Aris Velouchiotis, Stefanos Sarafis and Andreas Tzimas (nom-de-guerre: Vasilis Samariniotis).
Two events of great importance took place in this period. KKE, after passing great difficulties, succeeded in reorganizing its groups destroyed by Metaxas. Many members were recruited and with the help of ELAS, which became the largest partisan army in Greece, EAM became the largest mass political organization in Greek history, claiming over 1.5 million members, enlisted in organizations that covered every neighborhood in every village. The second great event was the foundation of the United Panhellenic Organization of Youth (EPON) (Greek: Ενιαία Πανελλαδική Οργάνωση Νέων). In 1943, a small naval auxiliary navy, the Greek People's Liberation Navy (ELAN) was also founded.