Why did the post-WWI fear of radicalism arise in the United States?

The fear of radicalism that happened after WWI is
typically known as the "red scare" or sometimes the "first red scare."  During this
time, Americans were worried about the growth in radical ideas, especially among
immigrants and workers.  There were three main causes of this
fear.

First, there was the fact that Russia had become
communist and was overtly talking about spreading its revolution abroad.  This made
Americans fear that their country would be a target for
communists.

Second, there was the large influx of
immigrants, some of whom were radical, in the late 1800s and early 1900s.  Americans
felt that there were large numbers of radicals among the immigrants and that American
ideals were in danger.

Finally, there were major strikes in
many US cities in 1919.  These strikes helped to convince many Americans that the labor
movement was filled with these radical immigrants who were going to topple the US
system.

For these reasons, a fear of radicalism arose in
the US after WWI.