The Preparedness Day Bombing was a bombing in San Francisco, California, United States, on July 22, 1916, of a parade organised by local supporters of the Preparedness Movement which advocated American entry into World War I. During the parade a suitcase bomb was detonated, killing ten and wounding 40 in the worst terrorist attack in San Francisco's history. Two labor leaders, Thomas Mooney and Warren K Billings, were convicted in separate trials and sentenced to death, later commuted to life in prison. Later investigations found the convictions to have been based on false testimony, and the men were released in 1939 and eventually pardoned. The identity of the bombers has never been determined. Prelude By mid-1916, after viewing the carnage in Europe, the United States saw itself poised on the edge of participation in World War I. Isolationism remained strong in San Francisco, not only among radicals such as the Industrial Workers of the World ("the Wobblies"), but also among mainstream labor leaders. At the same time, with the rise of Bolshevism and labor unrest, San Francisco's business community was nervous. The Chamber of Commerce organized a Law and Order Committee, despite the diminishing influence and political clout of local trade unions. The Preparedness Day Parade was organized by the Chamber of Commerce and the anti-union conservative business establishment. The parade The huge Preparedness Day Parade of Saturday, July 22, 1916, was a target of radicals. An unsigned anti-war pamphlet issued throughout the city in mid-July read in part, "We are going to use a little direct action on the 22nd to show that militarism can't be forced on us and our children without a violent protest." Labor leader Thomas Mooney had been tipped off to threats that preceded the parade and pushed resolutions through his union, the Molders; the San Francisco Central Labor Council; and the Building Trades Council warning that provocateurs might attempt to blacken the labor movement by causing a disturbance at the parade. The parade was the largest ever held in the city. The 3.5 hour procession had 51,329 marchers, including 2,134 organizations and 52 bands. Ironically, perhaps, the starting signals were "the crash of a bomb and the shriek of a siren." Military, civic, judicial, state, and municipal divisions were followed by newspaper, telephone, telegraph and streetcar unions. Many of the following divisions came from other cities of the San Francisco Bay Area. At 2:06pm,