The Greek Civil War was the first major confrontation of the Cold War. It was fought between 1944 and 1949 in Greece between the nationalist/non-Marxist forces of Greece (financially supported by Great Britain at first, and later by the United States) and the Democratic Army of Greece (ELAS), which was the military branch of the Communist Party of Greece (KKE).

The conflict resulted in a victory for the British — and later U.S.-supported government forces, which led to Greece receiving American funds through the Truman Doctrine and the Marshall Plan, as well as becoming a member of NATO, which helped to define the ideological balance of power in the Aegean for the entire Cold War.

The first phase of the civil war occurred in 1943–1944. Marxist and non-Marxist resistance groups fought each other in a fratricidal conflict to establish the leadership of the Greek resistance movement. In the second phase (December 1944), the ascendant communists, in military control of most of Greece, confronted the returning Greek government in exile, which had been formed under the auspices of the Western Allies in Cairo and originally included six KKE-affiliated ministers. In the third phase (called by some the "Third Round"), guerrilla forces controlled by the KKE fought against the internationally recognized Greek government which was formed after elections were boycotted by the KKE. Although the involvement of the KKE in the uprisings was universally known, the party remained legal until 1948, continuing to coordinate attacks from its Athens offices until proscription.

The war, which lasted from 1946 to 1949, was characterized by guerilla warfare between the KKE forces and Greek governmental forces mainly in the mountain ranges of northern Greece. The war ended with the NATO bombing of Mount Grammos and the final defeat of the KKE forces. The civil war left Greece with a legacy of political polarization. As a result, Greece also entered into an alliance with the United States and joined NATO, while relationships with its communist northern neighbours, both pro-Soviet and neutral, became strained.