There were 2 different parts of the Red Scare, after World War 1 and after World War 2. It was a fear of communism. It was properly known as McCarthyism after its most known supporter of The Red Scare, senator Joseph McCarthy. This ideology lead to the US getting involved in the Korean war and the Chinese Civil war.
The trial of Ethel and Julius Rosenberg, The Iron Curtain and the Soviet Unions first nuclear weapon test shocked the American public, that questioned the US national security. In Canada top secret documents concerning weapons and radar were handed over to the Soviets by a domestic spy ring. There were US spies that confessed with pride their acts of espionage. The fear tightened in 1949 by the Chinese communist winning the Chinese Civil war.
In 1939, the communist party of the United States have grown to about 50,000 members. After World War 2 congress passed the Smith Act, making it a crime to willfully teaching the ideas of overthrown the government of United States by force of violence. In 1947, president Truman signed executive order 1935 creating the "Federal's Employees Loyalty Program". This terminated those who confessed of spying for the Soviet Union. As well as some being unexpected for being un-American.