President Trump’s plan to deport millions of people appears to be underway. Last week, federal immigration officials arrested more than 600 people at their homes and workplaces in at least 11 states, sending terror through immigrant communities.

The abruptness of the raids provoked criticism from local officials, including Mayor Bill de Blasio of New York, who vowed to “stand with” immigrant communities. But mass deportation under President Trump will also happen through a more routine policy that is in the mayor’s control: endless, unnecessary arrests for low-level offenses, which end up feeding immigrants into the federal government’s deportation machine.

It’s not enough for cities like New York to declare themselves “sanctuaries,” which simply means that the local police won’t detain noncitizens on the federal government’s behalf. If cities really want to protect immigrants, they must also end the quota-driven style of policing that makes immigrants the victims of unnecessary arrests and disproportionate punishment.

Many of these unnecessary arrests stem from the discredited idea that a draconian crackdown on the most minor offenses — littering, selling loose cigarettes, biking on the sidewalk — will prevent more serious crimes. This model of policing, known as broken windows or zero tolerance, helped to drive mass incarceration. Its next cost could be mass deportation.