The first steps the US took in containing communism were unofficial: There were several disagreements reached during the Yalta conference (Feb 1945) and the Potsdam conference (July 17th 1945). These disagreements were ones that concerned Stalin's requests such as Soviet policy in Poland, size of reparations from Germany, and naval bases in the Straits (Dardanelles). These requests were all to increase Soviet influence over Europe, and by disagreeing to these, the US clearly made her point to contain communism. The containment of communism by agreements proved to have been the most effective compared to other methods and policies.
After receiving the Long Telegram, Truman became even the more concerned about containing communism. However, it was not until March of the next year that he made his policy clear. On March 12 Truman made a speech as a response to the UK's request of aid in dealing with the Greek civil war, where communists were taking over the country. In Truman's speech he announced that they would provide military aid to countries under attack by communist forces. After this, the communists in Greece were defeated.
However, the US had not been concerned with Europe all the more