Final answer:
The Truman Doctrine supported the policy of containment by providing aid to countries threatened by communism, such as Greece and Turkey, and helped establish the U.S. approach of intervening to prevent the spread of communism, encapsulating the Cold War policy of containing Soviet influence.
Step-by-step explanation:
The Truman Doctrine was a major shift in U.S. foreign policy that supported countries threatened by communism, marking the start of the United States' policy of containment. President Truman, in his 1947 address to Congress, articulated the need for the U.S. to support free nations, such as Greece and Turkey, against communist subjugation. The policy was aimed at containing the spread of communism by providing political, military, and economic assistance to all democratic nations under threat from external or internal authoritarian forces.
By pledging $400 million in aid and military support to Greece and Turkey, the U.S. helped these countries defeat communist forces, preventing the spread of communism to neighboring regions, and thus implemented the core principle of containment that would define American foreign policy during the Cold War.
The domino theory was also implied within the Truman Doctrine, which suggests that a communist victory in one nation could quickly lead to a chain reaction of communist takeovers in neighboring states. This doctrine not only offered immediate support for Greece and Turkey but also set a precedent for U.S. intervention in situations where communist influences appeared to be gaining ground.