After the United States entered World War I, the federal government moved to destroy the IWW as a wartime threat. On September 5, 1917, Justice Department agents simultaneously raided IWW halls and offices across the country, seizing tons of literature, correspondence, and records and arresting the union’s leadership en masse under the new Espionage Act.
The prosecutions that followed were among the largest political trials in American history. In Chicago in 1918, 101 defendants — including Big Bill Haywood — were convicted in a single mass trial and given sentences of up to twenty years; further trials followed in Wichita and Sacramento. Hundreds of Wobblies were imprisoned, many at Leavenworth, and the IWW spoke of "2,000 class-war prisoners." The raids and trials, combined with vigilante violence and the postwar Red Scare, shattered the union’s strength.