World War II devastated the Western European economies. The United States recognized that it was in their best economic and strategic interest to help rebuild these economies, and the Marshall Plan was designed with that in mind. They spread 12 billion dollars across 18 European countries with over half going to England, France, and West Germany. Part of the U. S. motivation was to prevent these countries from veering toward communism. History has proved that the plan was successful. The United States also knew that it would benefit from an economically improving Europe—we needed trading partners with functioning economies.
The Truman Doctrine was even more specifically directed at stopping the spread of communism. Shortly after World War II Eastern European countries quickly fell into the Soviet communist bloc. The West was worried about the domino effect—the idea that the conversion of one country to communism would lead to another and to another. Right after World War II that seemed to be what was happening. So, the Truman Doctrine offered military and economic aid to countries in danger of being taken over by communist sympathizers or forces. The initial focus of the Truman Doctrine was Greece and Turkey. Eventually the policy led to the wars in Korea and Vietnam.