Reigning wisdom when Mr. Giuliani took office in 1994 was that the problem-ridden city was “ungovernable.” Crime had skyrocketed in the preceding decades, with murders doubling in the 1960s and doubling once more over the next two decades before reaching 2,154 in 1991—one every four hours, roughly. Dope dealers hustled on the street alongside pushy panhandlers and prostitutes. Derelicts slept alongside graffiti-smeared buildings. An epidemic of car break-ins led owners to post “No Radio” signs in their windows, and auto alarms blared indignantly at all hours. Businesses fled: The 116 major corporate headquarters in Gotham in 1971 had dwindled to 49 by 1995. Small-business owners buzzed customers in through locked doors and, at closing time, rolled down metal security gates, luring graffiti vandals.

Mr. Giuliani, a former federal prosecutor, ignored the cliché that you could cut crime only by addressing so-called root causes, poverty and racism. He instinctively grasped the theory of social scientists James Q. Wilson and George Kelling that stopping small crimes would prevent more serious crimes, just as replacing a broken window halts more window-breaking by showing that somebody watches and acts. He had seen this theory proven in the subways in the two years before his election, when Robert Kiley, head of the Metropolitan Transportation Authority, kept subway trains and stations hosed clean of graffiti and transit police chief William Bratton started arresting turnstile jumpers and graffiti vandals. Crime in the subways began to fall. When Mr. Giuliani entered City Hall, he named Mr. Bratton the city’s police commissioner.

With the mayor’s vocal support, Mr. Bratton put broken-windows policing to work, arresting “squeegee men” who smeared dirty rags across motorists’ windshields, holding them hostage for a “contribution.” Even minor crime, he showed, had no place in Mr. Giuliani’s New York—a lesson he amplified by arresting graffiti taggers and public urinators. Police stopped, questioned and frisked those suspected of carrying weapons or casing a business, dissuading the ill-intentioned from packing guns while reducing shootings. Mr. Bratton made computer maps of crime hot spots and concentrated cops where thugs operated. City Hall’s revolutionary idea was that cops should prevent crime, not solve it after it happened.

The results were spectacular. Murders fell 20% to 1,561 the first year and a further 58% to 649 in 2001, Mr. Giuliani’s last year in office. With newly safe streets and subways, New York roared back to life. As the mayor cleared sex and smut businesses out of Times Square, he enticed Disney to restore a stately vacant theater as that area’s anchor. A visionary private effort had turned Bryant Park from a dope dealers’ den into a green oasis, and that district now thronged with tourists and office workers.

Restaurants and theaters boomed. Old businesses grew, and new ones opened, including even a tech Silicon Alley. Columbia and New York University became hot schools once parents stopped fearing urban crime. With opportunity burgeoning and the city’s rich inheritance of museums, concert halls, and landmark buildings safe to use, property values skyrocketed. Global tycoons looked to city real estate as a glamorous safe haven for money. New development proliferated, and construction workers, service staff and luxury retailers all profited. New York became once more the capital of the world.

Policy can change culture, Mr. Giuliani showed. It wasn’t the legacy of slavery that had created the disproportionately black urban underclass, he understood, but the message of victimhood and helplessness sent by framing crime as an inevitable response to oppression and welfare as deserved reparations. Criminals, not society, were to blame for crime, and welfare recipients were also citizens with agency who were responsible for their own fates.

Seizing on the 1996 welfare reform act, the Giuliani administration started an ambitious workfare program. Welfare offices became “job centers” and set welfare recipients to painting park benches, raking leaves and cleaning courtrooms. Recipients learned discipline and gained self-respect. Many moved into conventional jobs and, as in the nation as a whole, caseloads dropped dramatically.

A remarkable dividend of the new approaches to crime and welfare, one with only anecdotal rather than social-science evidence, was a dramatic thaw in race relations, as fear and resentment abated. But it didn’t last.

Beginning with the De Blasio administration in 2014, the Giuliani reforms slipped away while Amazon and then Covid and Zoom sharply challenged New York’s economic model of retail stores and office towers. As it holds on to old economic engines and searches for new ones, the city urgently needs the culture of personal agency and public safety that Mr. Giuliani fostered.

Mayor Zohran Mamdani evidently believes that legacy businesses and institutions are inexhaustible wealth-generators that, even as low-tax states beckon, will enable him to cultivate the tax-eaters and punish the taxpayers. That will prove a costly and tragic mistake.