The Red Scare was a political and social phenomenon that overcame the United States shortly after the First World War, which ended in late 1918. During World War I, an extreme patriotism took hold in the United States. Anyone who did not demonstrate the strong patriotic beliefs and attitudes that pushed the country into the war were looked on with suspicion and dislike. This was especially true when it came to those Americans that were against the war and the U.S, involvement. But, the suspicion and distrust was also directed against German-Americans (Germany was the main enemy of the U.S. and its allies in World War), immigrants, and people who were considered to have "radical"" or extreme political beliefs that were thought to be "anti-American." These people were considered to be the Communists, Socialists, anarchists, and other groups who believed in broad changes to the U.S. government and American society. .
Communists, perhaps above all other groups, were looked on with the most suspicion and were eventually the main target of the U.S. attack on these groups. Another event that occurred during World War I, and continued in the immediate years following the war, was the Russian Revolution of 1917 and the successful formation of a Communist or Socialist state in Russia. The creation of the Soviet Union and its communist government led to a fear in the U.S. that communism could spread in America and the U.S. government would be overthrown just as the Russian government and its Tsar (the Russian King) had been replaced (Tsar Nicholas II was eventually murdered by the Communists along with his entire family).
Another factor that led to the Red scare was the formation of worker's unions that fought for better wages and working conditions for the millions of American workers (many of them immigrants or first generation Americans from Southern and Eastern Europe - Italians, Jews, Poles, Greeks, and others, who were looked at with great suspicion and dislike by many native born and more established Americans).