希腊内战爆发于1944年，而持续直至1949年10月签订和平协议为止。发生在南欧国家希腊境内的这场战争，造成五万余军人死亡，将近八万人负伤或失踪。在参战武装方面，一方是受到英美支援的希腊政府军，另一方是在二战中英勇战斗，在二战末期缴获德军装备的希腊民主军（Dimokratikos Stratos Elladas, DSE）；而后者与希腊共产党有相当密切的关系。
希腊内战初期，共产党与以左派为主的希腊民主军及其前身希腊解放军占尽优势，共产党不但控制希腊大部分领土，也打败了多股零星军事势力。不过随着苏英美势力的介入，加上南斯拉夫铁托政权的干涉，新成立的政府军则日益壮大。相对的，希腊民主军力量却日渐分散。1946年—1949年间，英美援助的政府军完成整备，将颓势逐渐挽回。1949年10月，无法获得南斯拉夫及苏俄襄助的民主军，终于在阿尔巴尼亚境内承认失败，并宣布停火。
1949年，希腊内战虽然表面上停火，但却严重造成经济问题和政治分裂，此种情况，直至1970年代才日趋改善。
The Greek Civil War (Greek: ο Eμφύλιος [Πόλεμος], "the Civil War") was fought from 1946 to 1949 between the Greek governmental army, backed by the United Kingdom, United States, and the Democratic Army of Greece, the military branch of the Greek Communist Party (KKE). It was the result of a highly polarized struggle between leftists and rightists which started from 1943 and targeted the power vacuum that the German-Italian occupation during World War II had created. One of the first conflicts of the Cold War, according to some analysts it represents the first example of a postwar Western interference in the internal politics of a foreign country, and for others, marked the first serious test of the theory of the so called Churchill-Stalin percentages agreement.
The first phase of the civil war occurred in 1942–1944, during the Occupation. With the Greek government in exile unable to influence the situation at home, various resistance groups of differing political affiliations emerged, the dominant one being the leftist National Liberation Front (EAM), controlled effectively by the Communists. Starting in autumn 1943, friction among EAM and the other resistance groups resulted in scattered clashes, which continued until the spring of 1944, when an agreement was reached forming a national unity government which included six EAM-affiliated ministers. The second phase occurred in December 1944, after the country had been liberated. EAM, in military control of most of Greece, confronted the British-backed government, and tried to wrest control of the capital, Athens. The defeat of EAM forces spelled the end of its ascendancy: ELAS was disarmed, and EAM continued as its political action as a multi-party organization. Tensions remained high however, as clashes between right and left-wing factions continued. In the third phase (1946–1949), guerrilla forces controlled by the KKE, having a political and logistic back up by the newly founded northern Socialist States (Albania, Yugoslavia, Bulgaria and USSR) fought against the internationally recognized Greek government which was formed after elections boycotted by KKE. Despite initial failures by the government forces from 1946 until 1948, increased American aid, lack of high numbers of recruits to the ranks of DSE and the side-effects of the Tito–Stalin split, led to their defeat.
The final victory of the Western-supported government forces led to Greece's membership in NATO, and helped to define the ideological balance of power in the Aegean for the entire Cold War. The civil war also left Greece with a vehemently anti-Communist security establishment, which would lead to the establishment of a military regime, and a legacy of political polarization which lasted until the 1980s.