During a parade in San Francisco, known as Preparedness Day, a bomb detonated on Market Street, resulting in ten deaths and around forty injuries. The event was part of a larger movement advocating for increased military readiness in light of the international geopolitical tensions of the time. Organized by various support groups, the rally aimed to promote the need for national defense as the United States grappled with the effects of World War I. As spectators gathered to show their support, the unexpected explosion transformed the festive atmosphere into chaos and confusion.

Authorities immediately began an extensive investigation to determine the origin of the bomb and the motivations behind the attack. Initial suspicions pointed towards anarchist groups that had previously threatened public order, although no individual or specific organization was definitively linked to the bombing. Several arrests were made, leading to trials that captivated the public and provided a glimpse into the fraught political climate of the era. The tragic event left a deep scar in the community, leading to discussions about security measures at public gatherings.

In the wake of the bombing, city officials and citizens alike were left questioning the safety of similar future events. The tragedy also sparked debates regarding the rise of political violence and the implications of social unrest. The bombing underscored the fragility of public safety during a time when radical ideologies were gaining traction, particularly in urban areas. Overall, the Preparedness Day bombing marked a significant moment in San Francisco's history and in the broader discourse on civil unrest and governmental response to terrorism.