President Obama plans a farewell speech next week in Chicago, and perhaps he’ll notice that while he’s been in Washington his hometown has become the nation’s murder capital and largest gang war zone. Worth reflecting on is the city’s upswell in violence last year that followed political protests against law enforcement and a pullback in policing.

The Chicago Police Department reported 762 homicides in 2016, the most in two decades and more than in the cities of New York and Los Angeles combined. The 57% increase was the biggest...