Short answers, pulled from the story.
The Truman Doctrine is a U.S. foreign policy pledging American support for nations resisting authoritarian or communist pressure. President Harry S. Truman announced it to Congress on the 12th of March 1947 in an eighteen-minute address to a joint session.
Britain informed the U.S. in late 1946 that its declining economy left it unable to continue funding the Greek government's fight against KKE guerrillas. Combined with Soviet pressure on Turkey over the Turkish Straits, the situation prompted Truman to ask Congress for $400 million in aid to both countries.
In May 1947, two months after Truman's speech, a large congressional majority approved $400 million in military and economic aid to Greece and Turkey. Turkey alone received $100 million, and the U.S. Navy deployed the Midway-class aircraft carrier to the region.
George F. Kennan, an American diplomat stationed in Moscow, sent his famed "Long Telegram" in February 1946, arguing that the Soviets would only respond to force and that a long-term strategy of containment was the best approach. His framework influenced the policy underpinning the doctrine.
Yes. The Truman Doctrine was the first in a series of containment measures, followed by the Marshall Plan for European economic recovery and the creation of NATO in 1949, which provided the military alliance structure that containment required.
Historian Eric Foner writes that the doctrine set a precedent for U.S. assistance to anticommunist regimes worldwide, regardless of how undemocratic they were. Historian James T. Patterson called its sweeping rhetoric a "major step" that led to globalized commitments, with the same logic later applied to the Korean War and the Vietnam War.