Final answer:
President Truman's address to Congress on March 12, 1947, was prompted by the need to provide aid to Greece and Turkey to prevent the spread of communism, marking the advent of the Truman Doctrine which shaped U.S. foreign policy during the Cold War.
Step-by-step explanation:
On March 12, 1947, President Harry S. Truman delivered a significant speech before a joint session of Congress that was prompted by a crucial international situation involving Greece and Turkey. At that time, Great Britain, financially exhausted post-World War II, could no longer support the Greek government, which was in the midst of a civil war against leftist rebels. To prevent a potential spread of communism in Greece and Turkey and the possible domino effect it might have on other nations, Truman requested Congress to approve $400 million in aid for these countries, emphasizing the need for the United States to support free peoples confronting totalitarian regimes. This Truman Doctrine, as it became known, established a new direction for U.S. foreign policy towards the active containment of communism during the Cold War era.
Truman's message to Congress framed the situation as part of the broader American-Soviet conflict. Former British Prime Minister Winston Churchill's speech in Fulton, Missouri, had previously described Europe as being divided by an 'iron curtain,' adding urgency to the American perception of the threat. This geopolitical tension and the growing pressure to counteract Soviet influence worldwide were key to Truman's request, which Congress ultimately approved, influencing U.S. foreign policy for decades.