Account for the fear of radicalism in the United States after WWII.

In the decade or so after WWII ended, Americans came to
fear the spread of radicalism (more specifically, communism) in the US.  This fear came
about largely because of the spread of communism
overseas.

Right after WWII, the Soviet Union took over much
of Eastern Europe.  The Soviets were explicitly dedicated to spreading their ideology
and many Americans feared that the Soviets would do so by winning the hearts and minds
of people in the US.  Since the Soviet Union had been an ally in WWII, there were still
many Americans at that time who thought well of the USSR.  Other Americans feared that
these communist sympathizers would help to promote communism in the US.  This fear was
manifested in the success of Sen. Joe McCarthy's claims that he knew of many communists
in government and in the support for things like the HUAC hearings on
Hollywood.

In the years that followed, it seemed that
communism was on the move.  The communists took over China and launched an invasion of
South Korea.  Communists were fighting the French for control of Vietnam.  The Soviets
got nuclear weapons.  All of these things made it seem as if communism was succeeding in
its goal to take over the world.  Because of things like this, many Americans feared
communism and a red scare arose in the US.