A so-called sanctuary city is a location where its leader has vowed to protect residents from deportation and other forms of retaliation from the federal government.
The order Trump signed on Wednesday said he would take federal money away from sanctuary cities in an effort to keep immigrants out. Press secretary Sean Spicer explained it would "strip federal grant money from the sanctuary states and cities that harbor illegal immigrants."
Aside from the obvious factor of putting people's safety at risk, taking away federal funding from cities would be a decently significant amount of money.
In New York City, for example, federal funding amounts to $7 billion, according to DNA Info. While that's not a majority of the city's budget, it does help programs like the police's counterterrorism, public assistance and child protective services.
Major cities across the United States have declared themselves sanctuary cities, saying they will protect undocumented residents.
Despite Trump's threat to pull funding, many mayors have restated their promise to immigrants. In New York, Mayor Bill de Blasio announced the city is saving money in case it loses federal funding.
Mayor Rahm Emanuel of Chicago renewed his sanctuary city vow, saying,
We welcome people, whether you're from Poland or Pakistan, whether you're from Ireland or India or Israel and whether you're from Mexico or Moldova, where my grandfather came from, you are welcome in Chicago as you pursue the American Dream.
Mayor Ed Lee of San Francisco reiterated his commitment to sanctuary-city status.
Mayor Marty Walsh of Boston agreed, adding he would even use City Hall as a last resort to protect residents.
Many mayors, activists and experts have posited that Trump does not have the constitutional right to punish cities for their sanctuary status. The ACLU has said it will stand with sanctuary cities in court.
There are many frightening parts of Trump's latest actions for immigrants, potential immigrants and those who care about them. But there are also ways to act against the policies.
Citations: Trump Orders Mexican Border Wall to Be Built and Is Expected to Block Syrian Refugees (New York Times), What are sanctuary cities, and can they be defunded? (CNN), New York Could Lose Billions as Trump Attacks Sanctuary Cities (DNA Info), As Trump signs immigration order, Emanuel vows Chicago will stay sanctuary city (Chicago Tribune), Trump can't force 'sanctuary cities' to enforce his deportation plans (Washington Post), President Trump moves on immigration promises (Boston Globe)