This California city defunded its police force. Killings by officers soared.

Twelve years ago, officials in Vallejo, Calif., reluctantly took a step that activists are now urging in cities across the country: They defunded their police department.

Unable to pay its bills after the 2008 financial crisis, Vallejo filed for bankruptcy and cut its police force nearly in half — to fewer than 80 officers, from a pre-recession high of more than 150. At the time, the working-class city of 122,000 north of San Francisco struggled with high rates of violent crime and simmering mistrust of its police department. It didn’t...