The term is known as the Red Scare. This fear of communist influence in the United States can be credited to the growing power of the Soviet Union after World War II.
After World War II, the United States and Soviet Union emerged as global superpowers. The goal of each country was to expand their power, influence, and ideas to other countries. These two countries also had two completely different ideas on how to run a country. This caused tension between the two, resulting in roughly four decades of tension known as the Cold War.