What change in U.S. policy in 1947 aimed to prevent Soviet expansion into Greece and Turkey?

The Truman Doctrine was a significant shift in U.S. foreign policy introduced in 1947 that specifically aimed to counter the threat of Soviet expansion in Europe, particularly in Greece and Turkey. The doctrine emerged from the geopolitical landscape following World War II, when the U.S. recognized the need for a proactive stance against communism. President Harry S. Truman articulated this policy in a speech where he emphasized the importance of supporting free peoples who were resisting subjugation by armed minorities or outside pressures, which directly referred to the challenges faced by Greece and Turkey at the time from communist insurgents and Soviet influence.

The Truman Doctrine established a commitment on the part of the U.S. to provide political, military, and economic assistance to countries resisting communism, marking a key moment in the onset of the Cold War. It set a precedent for American involvement in international conflicts where the threat of communism was perceived. The other options, while significant in their own rights, pertain to different aspects of U.S. policy. The European Recovery Program and the Marshall Plan focused on economic recovery in Western Europe, while the National Security Act aimed at restructuring U.S. military and intelligence apparatus to better respond to global threats. These policies, although important, did not specifically address