Short answers, pulled from the story.
The Truman Doctrine began on the 12th of March 1947, when President Harry S. Truman appeared before a joint session of Congress to declare that the United States must support free peoples resisting subjugation by armed minorities or outside pressures.
The United States adopted the Truman Doctrine because Britain informed the U.S. in late 1946 that its declining economy prevented it from continuing military and economic support for Greece and Turkey against communist threats.
Congress approved $400 million in military and economic aid for Greece and Turkey in May 1947 after Truman requested assistance following his speech on the 12th of March 1947.
Diplomat George F. Kennan influenced the Truman Doctrine through his Long Telegram which advocated containment of Soviet expansion, while Senator Arthur Vandenberg helped secure Republican support for the legislation.
The Truman Doctrine led to the formation of NATO in 1949 and is often used by historians to date the start of the Cold War beginning with Truman's address before Congress on the 12th of March 1947.