The Greek Civil War is fought between 1944 and 1949 in Greece between the governmental forces supported by the United Kingdom at first, and later by the USA, and the Democratic Army of Greece; the military branch of the Greek communist party.

The victory of the British—and later US-supported—government forces leads to Greece's membership in NATO and helps to define the ideological balance of power in the Aegean for the entire Cold War.The civil war consists on one side of the armed forces of the postwar non-Marxist Greek administrations, and on the other, communist-led forces, and key members of the former resistance organization (ELAS), the leadership of which is controlled by the Communist Party of Greece (KKE).The first phase of the civil war occurs in 1942-1944.

Marxist and non-Marxist resistance groups fight each other in a fratricidal conflict to establish the leadership of the Greek resistance movement.

In the second phase (1944) the ascendant communists, in military control of most of Greece, confront the returning Greek government in exile, which had been formed under Western Allied auspices in Cairo and originally included six KKE-affiliated ministers.

In the third phase (commonly called the "Third Round" by the Communists) (1946-1949), guerrilla forces controlled by KKE fight against the internationally recognized Greek Government which is formed after elections boycotted by KKE.

Although the involvement of KKE in the uprisings is universally known, the party remains legal until 1948, continuing to coordinate attacks from its Athens offices until proscription.The civil war leaves Greece with a legacy of political polarization; as a result, Greece also enters into alliance with the United States and joins NATO, while relationships with its Communist northern neighbors, both pro-Soviet and neutral, become strained.