There were two periods in the 20th century that were called the Red Scare, although it can be argued that much of the 20th century was a Red Scare in America. One period was in the immediate post-World War II era but the first one was in the late 1910s and early 1920s.
The first Red Scare occurred after the Bolshevik Revolution in Russia. The revolution created a lot of paranoia in America that Communism would spread, and eventually reach the US. In fact two small communist parties formed in the United States with a total of 70,000 members. Additional fears were had because labor strikes had grown to four million workers in two years which represented 20% of all workers in America, the largest proportion in US history. President Wilson lifted price-controls that were put into place during World War I as as a result but did not revoke anti-strike regulations.
Related to the first Red Scare were the Palmer Raids which were anarchist bombings that took place between April and June of 1919 where 30 mail bombs were sent to prominent government officials and businessmen. Bombs in 8 different cities ended up exploding, however few people were injured and only a few bombs reached their intended target, including the home of Attorney General A. Mitchell Palmer. Palmer was given $500,000 to fight anarchism, Bolsheviks, and members of the Industrial Workers of the World. After the bombs, much violence occurred against socialists in Cleveland, Boston, and New York and this was known as May Day violence. In November of 1919, “radicals” ended up being deported to Russia after the Palmer Raids and most were anarchists and in January of 1920, 5,000 suspected communists were arrested across the nations. Most were seized without due process, denied attorneys, and were even deprived of basic needs like food, heat, and bathroom facilities. Additionally, 550 Russians were deported where many of them were US citizens.
The first Red Scare ended around summer of 1920 when alleged May Day strikes never occurred and AG Palmer was discredited. Additionally, conservatives of the time used the Red Scare to gut new labor unions, many unions were criticized as communist and the AFL ended losing 25% of its membership.