Mr. Bratton, again with Mr. Kelling as a consultant, introduced several innovations. One was Compstat, a computerized system to track crime that had begun with the Transit Police. The idea was to make policing smarter by deploying resources where the crimes were occurring--and hold local commanders responsible for their areas. Gradually Mr. Bratton reclaimed bus and train stations, parks and other above-ground public spaces that had succumbed to disorder. Violent crime declined even more steeply in New York than in the nation as a whole.

The trend continued under Mayor Michael Bloomberg and Police Commissioner Ray Kelly, both of whom served from 2002-13. But by 2014, when Mr. Bratton returned to New York under the progressive Mr. de Blasio, the political winds had shifted and the police were on the defensive--in part because of highly publicized accusations of police brutality, and in part because crime seems less menacing when it is so much less prevalent. "I'm not for just locking them up," Mr. Kelling says. "But you can't look at the 1990s through the lens of 2018."

History can, however, give us some perspective on the present. "I measure success by whether women feel safe walking a neighborhood's streets, children are free to play in the park, and harmony has been restored to a neighborhood," Mr. Kelling says. "And I see New York policing as being a singular paradigm shift, a once-in-a-generation event that, despite all the criticism, remains a powerful primary crime-prevention method."