Answer:
The Truman Doctrine stated that the main goal of American foreign policy was to contain the spread of communism in the world. This goal made sense at the time because the Soviet Union had recently helped win World War II, and its economy was relatively strong, something that led many countries to follow the Soviet example of a centralized, planned economy, instead of the American example of a mostly free market economy.
The Truman Doctrine, however, also made the U.S. government paranoid, and this paranoia was the main cause behind the Red Scare. While it is true that there were some communist sympathizers in the United States, the constitution does not allow the government to infringe citizen's rights based on their political beliefs, and this disposition was often breached during the red scare.