Two Red Scares have occurred in American history. The first, occurring after World War I, saw violent protests, political uprisings, and an extremist-leftist political ideology coming in to the thought process of Americans. The reason for these scares was due to hyper-nationalism of WWI, and the Bolshevist practices of Russian Countries.
The second Red Scare, which occurred after World War II, was due to the people thinking that foreign countries, specifically enemies of the United States, were infiltrating the US government, bringing in communist and extreme socialism.