>>8629The scare started during the First World War as a response to Iron and Steel Labour Strikes that threatened the Military-Industrial Complex. The brutal casualties from the 1 Year in the European War and the later socialist Revolutions in Russia and Germany scared American Businessmen and thy played on the isolationism that the American People sought and promoted anti-communist scare stories, violently suppressing strikes and protests.
Further aggravation came from self-proclaimed anarchists' mailing bombs to prominent Americans, including United States Attorney General A. Mitchell Palmer and United States Supreme Court Associate Justice (and former Massachusetts Supreme Judicial Court Chief Justice) Oliver Wendell Holmes, Jr.
Unique events like the Boston Police Strike in September 1919 and the crushed Bonus Army, alongside the alarming candidness of General Smedley Butler, made the Military and Police forces focus on expulsing and tracking communists to prevent them from gaining footholds in the government and military.
https://en.wikipedia.org/wiki/Bonus_Army I got more but I'll have to dig.