IN THE fall of 2006, crime-fighting guru David Kennedy received a phone call from the mayor of Cincinnati. The city, the mayor explained, was on fire. In 2000, Cincinnati experienced 40 homicides, but by 2005 the number had nearly doubled to 79.

The police department was chronically troubled. In 2001, police shot an unarmed black man, setting off a violent riot. In many neighborhoods, the police were seen as the enemy. There seemed no way out.

Kennedy agreed to help. The police department drew up a list of the city’s violent gang members. The city organized “call ins,” meetings where the gang bangers heard from law enforcement, representatives of the community, and others. And a message was delivered: We want you to choose another way. We will help you. But the shootings have to stop. The next time there is gunfire, we will find the shooter, and then we will take down everyone in the shooter’s gang. Promise.

The shootings quickly dropped.

This story may sound familiar. Kennedy was one of the architects of what has come to be called the “Boston Miracle,” a shockingly effective approach to calming the streets. But when Kennedy was first talking to Cincinnati’s mayor, he delivered this message: I can help you, but only if you understand that Boston didn’t end well.

This city is where Kennedy got his start. In the second half of the 1990s, the Boston Gun Project and then Operation Ceasefire were so successful at bringing down the body count that people from across the country came to see for themselves. But then, around 2000, it fell apart and the violence returned.

Kennedy wanted the violence in Cincinnati to go down, just like in Boston. But he wanted it to stay down.

Today, more than a decade after the Miracle, Kennedy and his colleagues are working to shape what started in Boston into a national movement. In a new book, “Don’t Shoot,” Kennedy writes of all that he and his colleagues have learned since Boston. He has shown that the ideas work in scores of cities. The challenge now is two-fold: figuring out how to ensure that cities which start down the path stay on the path, and, then, bringing the rest of the country along.

In Cincinnati, the killings have in fact stayed down. And Boston has since returned to the fold, under Police Commissioner Edward Davis. Kennedy has put together an organization, the National Network for Safe Communities, to serve as a clearinghouse.

Kennedy tells me that this work can cut the nation’s homicide rate in half. Read his important book – part jeremiad, part gripping crime thriller – and you will believe him.

One of the biggest obstacles Kennedy faces is the assumption that it can’t be that simple. The problem has been with us for decades, and is entwined with tectonic economic forces; there is a cultural dimension, a spiritual one.

But, at its core, it really is simple. First, when you look at gun violence, or open-air drug markets, the number of people involved is a tiny percentage of the population. The police know who they are, or can find out.

Second, if you want to change someone’s mind, there must be clear, quick, credible consequences. Any parent knows this, and a wealth of psychological research confirms it. Yet, for the most part, this is not how the justice system works: punishment is vague and far from certain.

The approach is simple, but not easy. It takes vigilant, focused police work to identify the targets. It takes tremendous, continuous cooperation – reaching across political, organizational, and cultural divides – so that a unified community can present its potential killers with a clear message about right and wrong; the help they need to change; and, if all else fails, a stark choice that they will believe.

This is how we, as a nation, can and must finally back out of the rolling destruction, by death and mass incarceration, of our cities, our society, and our moral character.