According to whom you ask, American intervention in the Greek civil war either led to the Truman Doctrine or was a result of it. Either way, American presidents were using the success of the Greek war to justify campaigns in Korea and Vietnam over the coming decades.

The Greek civil war was fought between the years 1944 and 1949 - the defender of their rule was an American and Great Britain backed government, and the agressor was Greek communist revolutionaries. The militant arm of the Greek Communist National Liberation Front (EAM), the People's National Army of Liberation (ELAS) had been heavily involved in the Greek resistance against Nazi Germany, and so when the Nazis withdrew in 1944 the EAM was allowed to become part of the former government, which Great Britain installed back in power.

The more radical elements of the ELAS would not agree with the British-sponsored government - after all, these people payed ideological homage to Josef Stalin (a fact that would eventually be their undoing). Also, Britain wanted to disarm the ELAS now the war was drawing to a close - the Communists opposed this move, and they declared a general strike in Athens on December 2, 1944. The day after armed members of the ELAS where involved with combat with the Athens police.

Eventually, the ELAS agreed to withdraw from Athens and turn in their weapons in fourteen days. However, the conflict would not draw to an end here, as foreign powers once against chose to intervene in Greek politics: the governments of Albania and Yugoslavia, who supported the ELAS-EAM due to territorial conflicts with Greece, invited the rebels to withdraw to the mountains near their borders to support them.

Great Britain and the United States gave military and financial aid to Greece - Britain sent 40,000 troops, and Congress permitted a $400 million joint aid package for Greece and Turkey. Between 20,000 and 30,000 guerrillas began to fight their way south towards Athens. Things looked bleak.

However, the Western forces eventually won out, and the final blow was struck to the Communists when Yugoslavia withdrew its support for them due to ideological differences - the Communist world was split into supporters of Josef Stalin and Josip Broz Tito, the Yugoslavian leader. The Greek rebels picked the wrong side. They announced their surrender on October 16, 1949 - the United States' first major postwar operation had been a success.