Answer:
World War I, which led many to embrace strong nationalistic and anti-immigrant sympathies;
The Bolshevik Revolution in Russia, which led many to fear that immigrants, particularly from Russia, southern Europe, and eastern Europe, intended to overthrow the United States government;
The end of World War I, which caused production needs to decline and unemployment to rise. Many workers joined labor unions. Labor strikes, including the Boston Police Strike in September 1919, contributed to fears that radicals intended to spark a revolution;
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.
