The reasons for and the effectiveness of truman's foreign policy during the early years of the cold war

The reasons were to prevent the spread of Communism to areas where it was not already the form of government,and to maintain the USA position as a dominant world power. 

It was effective because Truman's policies and actions were firm,sensible,and designed to avoid a direct shooting war between USA and the Soviet Union - and because Truman was prepared to stand up to the Soviet Union internationally and find ways to achieve his foreign policy aims without using outright military force directly against the Soviet Union itself.

On February 22,1946 George F. Kennan introduced a policy of containment towards the Soviet Union. Due to his knowledge of communism and Russia's history of aggression, he felt that they would try to expand and force other countries under they're rule. With this prediction on the rise, Truman enforced a new policy called the Truman Doctrine. The Truman Doctrine stated that the United States would give political, military, and economic support to all democratic nations threatened by authoritarian powers. Along with the Marshall Plan, during the early years of the Cold War, these two policies prevented the spread of communism to countries on the brink of economic collapse. American had saved Western Europe from becoming communist.