The Preparedness Day parade was a show of force by San Francisco's business community and pro-war advocates, who wanted the United States to enter the European conflict. Approximately 50,000 people marched, watched by crowds lining Market Street. The bomb, packed with metal slugs, had been placed in a suitcase at the corner of Steuart and Market Streets. The explosion killed ten people and injured dozens more. In the chaotic aftermath, police and prosecutors focused their investigation on labor leaders who had opposed the parade and the war preparedness movement.

Thomas Mooney, a labor organizer and Socialist, and his associate Warren Billings were arrested and charged with the bombing. The trial was deeply controversial. Key prosecution witnesses were later discredited, and evidence of prosecutorial misconduct emerged. Mooney was convicted and sentenced to death, later commuted to life imprisonment. The case became an international cause celebre, with labor organizations worldwide denouncing the verdict as a frame-up designed to silence the labor movement. After more than two decades in prison, Mooney was pardoned by Governor Culbert Olson in 1939. The actual bomber was never identified.

The Preparedness Day bombing occurred at a moment of intense conflict between San Francisco's labor movement and its business establishment. The city's open-shop advocates saw the war preparedness campaign as an opportunity to rally public opinion against unions. The bombing, whoever was responsible, gave them ammunition. The wrongful conviction of Mooney chilled labor organizing in San Francisco for years and became a symbol of how easily the machinery of justice could be turned against political dissidents. The case is remembered today less for the bombing itself than for the trial that followed -- a cautionary tale about the dangers of prosecuting ideology rather than evidence.