One hundred years ago today, World War I was raging in Europe and many in the United States were clamoring to join the fight. In San Francisco, war advocates sponsored a Preparedness Day Parade on July 22, 1916 to drum up popular support for the War. Some 50,000 people gathered in downtown San Francisco for the parade. Shortly after the march began, an explosion rocked the celebrations – someone had set off a bomb at the corner of Stewart and Market Streets. The bombing killed 10 and injured at least 40. Two radical labor leaders were quickly arrested after the attack. These two men, Thomas Mooney and Warren Billings, were both convicted of the crime after short trials. Billings was sentenced to life in prison and Mooney to death. However, as years passed, evidence of police misconduct and perjury began to come to light. In 1939, the governor of California pardoned both men. To this day, the perpetrators of this bombing are unknown. It is the worst terrorist attack in the history of San Francisco.