The Palmer Raids were a series of raids conducted in November 1919 and January 1920 by the United States Department of Justice under the administration of President Woodrow Wilson to capture and arrest suspected socialists, especially anarchists and communists, and deport them from the United States. The raids particularly targeted Italian immigrants and Eastern European Jewish immigrants with alleged leftist ties, with particular focus on Italian anarchists and immigrant leftist labor activists. The raids and arrests occurred under the leadership of United States Attorney General A. Mitchell Palmer, with 6,000 people arrested across 36 cities. Though 556 foreign citizens were deported, including a number of prominent leftist leaders, Palmer's efforts were largely frustrated by officials at the U.S. Department of Labor, which had authority for deportations and objected to Palmer's methods.

The Palmer Raids occurred in the larger context of the First Red Scare, a period of reactionary fear of communists in the U.S. in the years immediately following World War I and the successful Russian Revolution. There were strikes that garnered national attention, and prompted race riots in more than 30 cities, as well as two sets of bombings in April and June 1919, including one bomb mailed to Palmer's home in response to his policy of politically motivated mass arrests and deportations.

During the First World War there was a nationwide right-wing campaign in the United States against the real and imagined divided political loyalties of immigrants and ethnic groups, who were feared to have too much loyalty for their nations of origin. In 1915, President Wilson warned against hyphenated Americans who, he charged, had "poured the poison of disloyalty into the very arteries of our national life." "Such creatures of passion, disloyalty and anarchy", Wilson continued, "must be crushed out". The Russian Revolutions of 1917 added special force to fear of labor agitators and partisans of ideologies such as anarchism and communism. The general strike in Seattle in February 1919 represented a new development in labor unrest.

The fears of Wilson and other government officials were confirmed when Galleanists—Italian immigrant followers of the anarchist Luigi Galleani—carried out a series of bombings in April and June 1919. At the end of April, some 30 Galleanist letter bombs had been mailed to a host of individuals, mostly prominent government officials and businessmen, but also law enforcement officials. Only a few reached their targets, and not all exploded when opened. Some people suffered injuries, including a housekeeper in Senator Thomas W. Hardwick's residence, who had her hands blown off. On June 2, 1919, the second wave of bombings occurred, when several much larger package bombs were detonated by Galleanists in eight American cities, including one that damaged the home of Attorney General A. Mitchell Palmer in Washington, D.C. At least one person was killed in this second attack, night watchman William Boehner, and fears were raised because it occurred in the capital. Flyers declaring war on capitalists in the name of anarchist principles accompanied each bomb.

In June 1919, Attorney General Palmer told the House Appropriations Committee that all evidence promised that radicals would "on a certain day...rise up and destroy the government at one fell swoop." He requested an increase in his budget to $2,000,000 from $1,500,000 to support his investigations of radicals, but Congress limited the increase to $100,000.

An initial raid in July 1919 against an anarchist group in Buffalo, New York, achieved little when a federal judge tossed out Palmer's case. He found in the case that the three arrested radicals, charged under a law dating from the Civil War, had proposed transforming the government by using their free speech rights and not by violence. That taught Palmer that he needed to exploit the more powerful immigration statutes that authorized the deportation of alien anarchists, violent or not. To do that, he needed to enlist the cooperation of officials at the Department of Labor. Only the Secretary of Labor could issue warrants for the arrest of alien violators of the Immigration Acts, and only he could sign deportation orders following a hearing by an immigration inspector.

On August 1, 1919, Palmer named 24-year-old J. Edgar Hoover to head a new division of the Justice Department's Bureau of Investigation, the General Intelligence Division (GID), with responsibility for investigating the programs of radical groups and identifying their members. The Boston Police Strike in early September raised concerns about possible threats to political and social stability. On October 17, the Senate passed a unanimous resolution demanding Palmer explain what actions he had or had not taken against radical aliens and why.

At 9 p.m. on November 7, 1919, a date chosen because it was the second anniversary of the Bolshevik revolution, agents of the Bureau of Investigation, together with local police, executed a series of well-publicized and violent raids against the Union of Russian Workers in 12 cities. Newspaper accounts reported some were "badly beaten" during the arrests. Many later swore they were threatened and beaten during questioning. Government agents cast a wide net, bringing in some American citizens, passers-by who admitted being Russian, some not members of the Russian Workers. Others were teachers conducting night school classes in space shared with the targeted radical group. Arrests far exceeded the number of warrants. Of 650 arrested in New York City, the government managed to deport just 43.