Answer:
The idea that motivated the Truman Doctrine was "Containment." This was the strategic policy adopted by the United States during the Cold War to contain the spread of communism and the Soviet Union's influence around the world. The Truman Doctrine was announced by President Harry S. Truman in 1947, and it provided economic and military aid to Greece and Turkey to help them resist the Soviet Union's efforts to subvert them. The doctrine marked the start of the policy of containment, which formed the basis of U.S. foreign policy towards the Soviet Union for the next several decades.