On November 11, 1919, the citizens of Centralia, Washington, gathered to watch former servicemen, local Boy Scouts, and other community groups march in the Armstice Day parade. When the marchers swung past the meeting hall of the Industrial Workers of the World, a group of veterans broke ranks, charged the hall, and were met by gunshots. Before the day was over, four of the marchers were dead and one of the Wobblies had been lynched by the mob.

Through a wealth of newly available primary source material including previously sealed court documents, FBI records released under the...