After the end of World War II, communism began to pick up speed across the world. Joseph Stalin was in power, and the Soviet Union (USSR) was rising. This worried the United States, so the government quickly adopted a policy targeted at stopping the spread of communism: containment.
During the Cold War, the USSR started increasing communist influence in Eastern Europe, Asia, Africa, and Latin America. Most of the nations who were beginning to fold into communist influences were in crisis caused by the economically ruinous WWII.
In 1947, President Harry S. Truman (interesting fact: his middle name was literally just "S") vowed that the United States would help any nation resist communism in order to prevent its spread. This became known as the Truman Doctrine. The president called for Congress to send assistance to struggling nations in order to help them rebuild their infrastructure and settle back into the now relatively peaceful world.
Greece and Turkey, for example, were both spiraling and in danger of being overrun by communism, so Truman asked Congress to send about $400 million in military and economic assistance. In his address to Congress in 1947, Truman said,
I therefore ask the Congress to provide authority for assistance to Greece and Turkey in the amount of $400,000,000 for the period ending June 30, 1948. In requesting these funds, I have taken into consideration the maximum amount of relief assistance which would be furnished to Greece out of the $350,000,000 which I recently requested that the Congress authorize for the prevention of starvation and suffering in countries devastated by the war.
The Truman Doctrine was relatively successful; after about two years, nations like Greece and Turkey were secure once again. Many historians, however, agree that containment contributed to future conflicts, such as the Vietnam War.