Final answer:
The Truman Doctrine demonstrated America's commitment to countering Soviet expansion and the spread of communism, particularly by aiding Greece and Turkey, which were under threat of communist uprisings.
Step-by-step explanation:
The Truman Doctrine demonstrated the United States' commitment to countering Soviet expansion during the early stages of the Cold War. The doctrine was a response to concerns about the rise of communism in Greece and Turkey. President Truman proclaimed that the US would support free peoples resisting subjugation by armed minorities or external pressures, which, in practice, meant intervening to prevent the spread of communism. By delivering substantial aid to the governments of Turkey and Greece, the Truman Doctrine signaled a clear opposition to the USSR's foreign policy goals and marked the beginning of a more interventionist American foreign policy posture.