What are two reasons why Communism was seen as such a threat to America in the 1950s?

Well for one, there was the idea that Communism was a direct threat to Democracy itself. That the rise of Communism would be the downfall of Democracy. As well, there was the fear that Communist countries would align with each other, the threat of nuclear war was already a huge scare and as such an alliance would have been seen as a threat to the US.