They know about the consequences of grand schemes of social engineering. They know what drugs and homelessness do to streets, what government interference does to blocks. They know the human costs of neutering law enforcement, and the antisemitic violence and murder that emerges.

Jews know what it means to see neighborhoods they called home for decades suddenly disintegrate. Go to the Brooklyn neighborhood of Brownsville or to the Bronx, to urban neighborhoods from Boston to Chicago, and you can still see the ruins of grand old synagogues and Jewish community centers.

Mr. Mamdani’s victory doesn’t stir anxiety of the unknown. It is reminiscent of the past. Jews can feel it in their bones, so they plan. What’s different today is that there’s an alternate route for the city, born in Crown Heights, Brooklyn.

From the early 1900s to the mid-1960s Crown Heights was an upscale, mostly Jewish neighborhood: a mix of secular Jews and observant ones, rich and poor, American-born Dodger fans and Holocaust survivors. Then things began to change.

Migration patterns, a massive social-services infrastructure that welcomed dysfunction, and a breakdown in policing destroyed many New York neighborhoods. The newcomers were black, but race wasn’t the issue. It was the gangs, drugs, violence and a pernicious new strain of antisemitism mysteriously condoned by elites. During one three-month period in 1968, 11 Jewish institutions in New York were vandalized, set on fire or firebombed. As in other parts of the city and country, many Jews began to leave Crown Heights.

The neighborhood, however, was also home to Rabbi Menachem M. Schneerson, seventh leader of the Chabad-Lubavitch Hasidic movement. The Rebbe, as he is known, charted a new course. “A phenomenon has recently spread among the Jewish people, like a plague, God forbid, of abandoning Jewish neighborhoods,” he said at a public gathering in 1969. Panic was causing people to flee, harming those left behind, especially the poor, elderly and infirm.

“People claim there’s no solution, but that’s not true,” the Rebbe said. “To start, one must stand firm and not run away.” He argued that Jews abandoning one place automatically weakened the resolve and position of communities facing similar challenges everywhere.

“I’m very concerned about Crown Heights,” Mayor John Lindsay told the Rebbe when he visited him in 1968. “It is very key to the whole community of Brooklyn.” The Rebbe saw even more: “It is a test case, not only for New York” but for the world.

Many Jews continued to leave for the greener pastures of Westchester County and Long Island, but the Hasidim stayed and planted their roots even deeper. They organized a citizen patrol, built schools, worked to head off the sale of synagogues, and created nonprofits to buy homes and resell them to community members. The Rebbe encouraged the opening of a Hasidic art gallery and a flower shop. He asked that weddings and other joyous events be held within the community to raise morale and support local kosher establishments.

It wasn’t always easy, and not everyone appreciated the Hasidim for bucking the trend. Following the 1991 riots, Al Sharpton dubbed the Crown Heights Jewish community a “Fort Apache in the black ghetto.” But they persevered, transforming the neighborhood again into a wonderful place to live, work and raise a family. In the process, they realized the Rebbe’s vision for saving the American city.

It is ironic that many of the young transplants to New York who voted for Mr. Mamdani live in such places as Crown Heights, unaware that had it not been for increased policing, government reforms they despise, and, most important, the locals who persisted during the city’s darkest days, they wouldn’t have dared enter the neighborhoods they now colonize. They know little about what came before them—and where their ideological fantasies lead.

New York is a great and important city that is worth saving. That work is only beginning—but for it to succeed, Jews and other New Yorkers must channel the Rebbe and again declare: We’re responsible for one another, and we aren’t going anywhere.