On March 12, 1947 before a joint session of congress President Harry S Truman recommended the program of economic and military assistance to Greece and Turkey that became known as the Truman Doctrine.
When in February 1947, Great Britain announced that they can no longer help Greece to fight against the communist rebels, President Harry Truman became worried that this will lead to expansion of communism in more countries.
Thus, President Harry S Truman and his advisers agreed on it’s now US’s turn to take action and control the situation so that communism should not take over of more countries.
In his very famous Speech, (later called as Truman Doctrine) President Harry S. Truman established that the United States would primarily supply economic and financial aid to all free nations that are under danger from inside or outside tyrannical forces.
“I believe that it must be the policy of the United States to support free peoples who are resisting attempted subjugation by armed minorities or by outside pressures.” President Harry S. Truman March 12, 1947 
Critics have called the Truman Doctrine the “first shot of the cold war”, a global license for American imperialism; an exaggerated response to an imagined communist menace that scared Americans with rhetoric, a reactionary policy that placed the United States on the side opposite freedom and social, political and economic reforms, Proof of an “arrogance of power” that helped led the United States into Vietnam. 
The main reason or in other words the hidden reason behind the Truman Doctrine was to stop expansion of Soviet Union/ communism beyond the border that it already had reached in Central and Eastern Europe.
President Truman by helping Greece and Turkey which at that time were facing problems by the communists (aided by Communist-controlled Yugoslavia, Bulgaria, and Albania) wanted to frighten the Soviet Union. A failure in Greece and Turkey would encourage Soviet Union to expand communism and would open their door toward Middle East, Asia, Africa, and Western Europe. This way Greece and Turkey at that time had became the focal point of the Cold War.
President Harry S. Truman asked for 400million in emergency aid. “We must take immediate and resolute action. 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” President Harry S. Truman March 12, 1947 
President Harry S. Truman’s Speech received a very positive reaction from all sides. A survey of the paper’s correspondents across the United States showed the majority in support
The New York times also reported that Americas “epoch of isolation” was being replaced by an epoch of American responsibility.”
President Truman gained the America’s support because; he gave very satisfying reasons for his this decision. For example in his Speech his mentioned that in the Second World War America joined the UN in order to protect democracy. “The United States contributed $341,000,000,000 toward winning World War II. This is an investment in world freedom and world peace. The assistance that I am recommending for Greece and Turkey amounts to little more than 1 tenth of 1 percent of this investment.”
The President also talked about the growth of communism around the world and how they forces their regime upon countries. “The peoples of a number of countries of the world have recently had totalitarian regimes forced upon them against their will”
Truman Doctrine made a significant change in America’s role in the World. President Harry S. Truman with the Truman Doctrine made a long term promise with the rest of the world in defeating totalitarianism and protecting democracy.
Truman Doctrine led t...