The first Red Scare in the United States was a reaction to which event?

The first Red Scare in the United States was a reaction to the event of the threat posed by Communist in the U. S. The communists were often referred to as "Reds" for their allegiance to the red Soviet flag.

Your answer is: Threat posed by Communists in the U. S.

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As the Cold War between the Soviet Union and the United States intensified in the late 1940s and early 1950s, hysteria over the perceived threat posed by Communists in the U. S. became known as the Red Scare. (Communists were often referred to as "Reds" for their allegiance to the red Soviet flag.)