Why were many post war americans concerned about the domestic threat posed by communism?

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For until World War II ended, the main enemy of the world was Nazi-fascism. To eliminate this common enemy, the United States and the Soviet Union united. After the conflict, however, the Soviets became a threat to American power, since it maintained an economic and political system opposite to that adopted by the United States. With the rise of localized disputes (as in Korea and Vietnam), the United States began to fear the influence of communism in the world, turning its efforts to programs of persecution and internal espionage, in order to prevent the propagation of these ideals.

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