In December 1946, Markos Vafiadis announced the formation of a communist Democratic Army of Greece (DAG), the successor of the ELAS. The DAG never exceeded 28,000 fighters, compared with about 265,000 troops in the national army and national police force at the end of the war. The Civil War commenced in earnest during the winter of 1946-47. Vafiadis adopted a strategy of guerrilla warfare, utilizing hit-and-run tactics to harass the national army and its allied groups. DAG forces scored some notable successes, but they were unable to capture any major towns. Like almost all internecine conflicts, the Civil War was marked by brutality on both sides. Villages were destroyed and civilians killed. The atrocities of the war left lasting scars on the nation's consciousness.

By the spring of 1947, Britain no longer was able to meet Greece's escalating demands for money and supplies, so the role of external patron was assumed by the United States. With the Greek case specifically in mind, Harry S. Truman set out in March 1947 a policy of global containment of communist expansion that came to be known as the Truman Doctrine. Truman pledged United States support to all free peoples under the threat of communist takeover. Under that policy, the United States made US$400 million in aid and military assistance available to Greece. United States advisers and military personnel under General James van Fleet came to Greece to train and supply the national army and the security forces.

Although the disproportionate size of the forces had made the outcome of the Civil War inevitable, the DAG's mistakes hastened its fall. After Vafiades was ousted by KKE chief Nikos Zahariadis in mid-1947, the DAG made a disastrous shift from guerrilla tactics to conventional, set-piece battles. Outgunned and outmanned, the DAG was pushed northward into the mountains.

In mid-1949, Yugoslav leader Josip Broz Tito inflicted another costly blow by closing the supply routes through Yugoslavia as part of his policy to conciliate the West. As the situation deteriorated, forced conscription of men and women and compulsory evacuation of children eroded the DAG's popular base of support. The Civil War ended when the last DAG mountain stronghold fell at the end of August. Thus, in addition to the more than 500,000 killed in World War II, during the Civil War 80,000 more Greeks lost their lives, 700,000 more became refugees, and the national economy was left in ruins.