Although the Red Scare is an American phenomenon, it is rooted in the First World War and the 1917 Russian Revolution. While the U.S. was still at war with Germany, the new Bolshevik government decided to sign a peace treaty with Germany in 1918. The treaty meant that Germans had closed the eastern front of the war and could concentrate more troops on its western front. Communist Russia was the new target of American resentment and, by extension, American radicals and leftists were regarded with suspicion and branded as "Red" (from the red flag of Communism). Wilson's administration amassed thousands of troops to northern Russia and Siberia with the official aim to safeguard these regions from Germany and Japan. In practice, however, Wilson supported the anti-Bolsheviks forces in the civil war that had erupted after the Revolution.
The Bolshevik victory and the 1919 call of the Communist International for a world revolution made left-wing sympathizers an easy target for American authorities. These militants were a perfect scapegoat for all the post-war tensions that the nation was experiencing. The fear of an imminent Communist revolution in the U.S. was sparked by a series of labor strikes that took place in 1919. Wilson's administration used the rhetoric of fear of a revolution to put an end to the strikes which had witnessed the involvement of many different professions from policemen to steel workers. It is estimated that about 4 million workers were involved in the 1919 strikes. On May 1st, Labor Day, several bombs sent in the mail to notable Americans made the threat of a revolution a palpable reality.
The American Left was actually extremely divided into small factions and experienced a great deal of in-fighting. It was thus very weak politically and unable to organize such a large scale cospiration to lead to a Revolution. Yet, Wilson's administration created a Bureau of Investigation, which, under J. Edgar Hoover's direction, arrested groups of radicals and even deported foreigners such as Emma Goldman. The Bureau also suppressed civil liberties in several cases, reaching a peak in the Palmer Raids of January 1920 when government agents broke into the homes and offices of suspects with no search warrants throughout the U.S. More than four thousand people were arrested.