The nation’s two-decades-long crime decline may be over. Gun violence in particular is spiraling upward in cities across America. In Baltimore, the most pressing question every morning is how many people were shot the previous night. Gun violence is up more than 60% compared with this time last year, according to Baltimore police, with 32 shootings over Memorial Day weekend. May has been the most violent month the city has seen in 15 years.

In Milwaukee, homicides were up 180% by May 17 over the same period the previous year....