The Truman Doctrine was first articulated in response to events in Greece and Turkey. Both nations were suffering internal turmoil, and seemed likely to "fall," from the perspective of American leaders, to communism. In response to these events, Truman successfully urged Congress to appropriate hundreds of millions of dollars to provide aid that would help prop up anti-communist leaders in both countries, and even authorized the deployment of military advisors to aid anti-communist fighters.
Through the lens of leadership, the Truman Doctrine can be understood as an acknowledgement that the United States would assume a leading role in resisting what was seen by many as Soviet expansionism. In fact, the decision to appropriate funds for Greece was specifically a response to the fact that the British government could no longer afford to provide aid. Shortly thereafter, the United States invested billions in Western Europe under the Marshall Plan, thus solidifying their position of economic and diplomatic leadership in the postwar world.