What caused the Red Scare in America?

The first Red Scare in the U. S. happened just after the 1917 Bolshevik Revolution and during WW1, when people were very patriotic and social agitation of left-wingers further aggravated the political, national and social tensions. After World War II, the second Red Scare happened due to a fear of communist espionage.

A "Red Scare" is the promotion of a widespread fear of a potential rise of communism or anarchism by a society or state. The First Red Scare, which occurred immediately after World War I, revolved around a perceived threat from the American labor movement, anarchist revolution and political radicalism.