Life in America resembles an airline passenger cabin: separate entrances, separate seating areas, separate bathrooms. The Village of Indian Hill, a wealthy suburb of Cincinnati, acclaims its rural atmosphere, its “firm administration of zoning ordinances” and its “proximity to the cultural life of a large city.” It is, in short, a parasite, taking what it values from Cincinnati while contributing as little to it as possible. In this, it is hardly unique. Hundreds of similar suburbs encrust cities across the United States.1

Even in cities where the rich and poor continue to live under the same local government, economic segregation saps political support for common, egalitarian infrastructure. Rich New Yorkers donate generously to beautify Central Park while resisting the taxation necessary to maintain parks in neighborhoods they never visit. In Washington, D.C., parents in wealthier neighborhoods contribute lavishly to parent-teacher organizations that provide extra money to public schools in their neighborhoods, but they do not vote for a similar level of funding for all city schools. Two schools in northwest Washington each raised more than half a million dollars in 2017, while several schools in southeast Washington don’t even have parent-teacher organizations. Last year, for the third time since 1970, the residents of Gwinnett County, Ga., which sits on the edge of Atlanta, refused to fund an expansion of the regional transit system into their suburban county.

The consequences of segregation are particularly stark in public education.

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• 1. Inequality is an inescapable fact of urban life. The Greek philosopher Plato, prefiguring Dr. King by a few thousand years, wrote in “The Republic” that “any city, however small, is in fact divided into two, one the city of the poor, the other of the rich.” But the crisis is a reminder that segregation is an illusion. The halves depend on one another. The rich need labor; the poor need capital. And the city needs both. Reducing segregation requires affluent Americans to share, but not necessarily to sacrifice. Building more diverse neighborhoods, and disconnecting public institutions from private wealth, will ultimately enrich the lives of all Americans — and make the cities in which they live and work a model again for the world.