Steven Alm was no courtroom novice when he started handling felony cases as a circuit judge in Hawaii. He'd already been a judge for three years, and U.S. Attorney for seven years before that. But in his very first week handling a felony docket, he noticed something that surprised him. It wasn't the nature of the felony offenses—he was prepared for that. It was the way the system dealt with probationers: offenders who had been placed under court supervision rather than being incarcerated.

Probationers were supposed to be amenable to correction. Yet Alm was reading motions that consisted of page after page of violations: 10 or more missed appointments, dirty drug tests, failure to show up for treatment. In most cases, all this misbehavior had essentially been ignored. Yet now, all of a sudden, he was being asked to send the violator to prison for five, 10 or even 20 years.

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That was five years ago. Today, Alm's initiative, known as Hawaii Opportunity Probation with Enforcement (HOPE), encompasses nearly 1,500 of the roughly 8,000 probationers on Oahu. With very little dedicated public funding, it has achieved extraordinary results: It claims an 80 percent reduction in missed appointments, an 86 percent reduction in the incidence of drug use and, based on the best current estimates, a 50 percent reduction in recidivism. A randomized evaluation of the program, slated to appear later this year, is expected to show similar results. Public-safety experts are hailing HOPE—and the ideas behind it—as something that could transform the American criminal justice system.

"There may not be another intervention that could have a more dramatic impact on crime, drug use and prison spending," says Adam Gelb, who heads the Pew Center on the States' Public Safety Performance Project. UCLA policy analyst Mark Kleiman goes even further. In his new book, When Brute Force Fails: How to Have Less Crime and Less Punishment, Kleiman argues that HOPE combined with a strategy of "focused deterrence" could reduce the nation's crime rate and prison population by 50 percent in 10 years.