Before Mike Duggan’s election in 2013, Detroit hadn’t had a white mayor since 1974. The city was on the skids. The cops did not come when you called them. The streetlights didn’t work. Abandoned homes proliferated as residents quit Motown in droves. The city’s population has declined by more than half since the 1950s, and Detroiters, more than 80% of whom are black, wanted a change.

Today, Detroit continues to face more than its fair share of challenges—only two cities in the U.S. had a higher murder rate last year, for example....