On July 22, 1916, a massive parade held in San Francisco, California, to celebrate Preparedness Day, in anticipation of the United States entrance into World War I, is disrupted by the explosion of a suitcase bomb, which kills 10 bystanders and wounds 40 more. It would be considered the worst terrorist act in the city's history.

By the summer of 1916, with the Great War raging in Europe and with U.S. and other neutral ships threatened by German submarine aggression, it had become clear to many in the U.S. that their country could not stand on the sidelines much longer. With this in mind, leading business figures in the city of San Francisco planned a parade in honor of American military preparedness. As the event neared, it was clear that anti-war and isolationist sentiments ran high among a significant population of the city (and the country), not only among such radical organizations as International Workers of the World (the so-called “wobblies”) but among mainstream labor leaders. These opponents of the Preparedness Day event undoubtedly shared the view voiced publicly by one critic, former U.S. Secretary of State William Jennings Bryan, who claimed that the organizers, San Francisco’s financiers and factory owners, were acting in pure self-interest, as they clearly stood to benefit from an increased production of munitions.

The Preparedness Day parade went ahead on Saturday, July 22, with a 3.5-hour-long procession of some 51,329 marchers, including 52 bands and 2,134 organizations, comprising military, civic, judicial, state and municipal divisions as well as newspaper, telephone, telegraph and streetcar unions. At 2:06 p.m., about a half-hour after the parade began, a bomb concealed in a suitcase exploded on the west side of Steuart Street, just south of Market Street, near the Ferry Building. Ten bystanders were killed by the explosion; 40 more were wounded.

Two radical labor leaders, Thomas Mooney and Warren K. Billings, were subsequently arrested and tried for the attack. In the trial that followed, complete with false witnesses and biased jury foremen, the two men were convicted, despite widespread belief that they had been framed by the prosecution. Mooney was sentenced to death; after evidence surfaced as to the corrupt nature of the prosecution, President Woodrow Wilson called on California Governor William Stephens to look further into the case. Two weeks before Mooney’s scheduled execution, Stephens commuted his sentence to life imprisonment, the same punishment Billings had received. Investigation into the case continued over the next two decades; by 1939, evidence of perjury and false testimony at the trial had so mounted that Governor Culbert Olson pardoned both men. The true identity of the Preparedness Day bomber (or bombers) remains unknown.