The most classic theme out there:
Truman has a very particular understanding of who/what is "good" and who/what is "evil" in the world he is describing. The picture he paints in the Truman Doctrine of international good vs. international evil will be a lasting picture defining the Cold War.
The way the Truman Doctrine pits totalitarianism and the U.S.S.R. against democracy and the U.S. defines the ideological basis of the Cold War.
Truman is being kind of rude when he says everything about communism is bad. He clearly has a cultural bias.
Fear is a natural human instinct. It was also a natural "national" instinct/response to the growing threat of the Soviet Union. By 1947, the Soviet threat was real—they were adding new sections of the world map at an alarmingly fast rate and were already in the process of building their own atomic bomb.
This was a frightening time in world history, and Truman not only knows it, but also describes it in the Truman Doctrine.
The Truman Doctrine was a strong, declarative statement on foreign policy that stemmed from an increasingly tense and frightening situation.
The Truman Doctrine was a strong, declarative stance on foreign policy that ultimately added more fear of communism in America than the threat it was meant to contain.
You know what Uncle Ben said: "Perfect every time."
Oops—wrong Uncle Ben. Spiderman's uncle said: "With great power comes great responsibility."
The allied victory in World War II saw the emergence of the United States as a global super power defined by a strong military, economy, and international political presence. "Responsibility" is a major theme in the Truman Doctrine: Truman tries to make the case that, because we are a new world leader with abundant resources, the U.S. is directly responsible for protecting countries fighting communism / making sure democracy's sworn enemy doesn't spread.
Truman's emphasis on upholding the global responsibilities of the U.S. line up pretty well with the "responsibilities" he believed in upholding as president, such as accountability, honesty, and equality.
The idea of "taking on too many responsibilities" can be used to describe the U.S.'s foreign policy approach during the Cold War.