希臘內戰爆發於1944年，而持續直至1949年10月簽訂和平協議為止。發生在南歐國傢希臘境內的這場戰爭，造成五萬餘軍人死亡，將近八萬人負傷或失蹤。在參戰武裝方面，一方是受到英美支援的希臘政府軍，另一方是在二戰中英勇戰鬥，在二戰末期繳獲德軍裝備的希臘民主軍（Dimokratikos Stratos Elladas, DSE）；而後者與希臘共産黨有相當密切的關係。
希臘內戰初期，共産黨與以左派為主的希臘民主軍及其前身希臘解放軍占盡優勢，共産黨不但控製希臘大部分領土，也打敗了多股零星軍事勢力。不過隨着蘇英美勢力的介入，加上南斯拉夫鐵托政權的干涉，新成立的政府軍則日益壯大。相對的，希臘民主軍力量卻日漸分散。1946年—1949年間，英美援助的政府軍完成整備，將頽勢逐漸輓回。1949年10月，無法獲得南斯拉夫及蘇俄襄助的民主軍，終於在阿爾巴尼亞境內承認失敗，並宣佈停火。
1949年，希臘內戰雖然表面上停火，但卻嚴重造成經濟問題和政治分裂，此種情況，直至1970年代纔日趨改善。
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.