One of President Donald Trump's many campaign promises was to "end sanctuary cities." On Wednesday, Trump took his first steps toward that end, issuing an executive order called "Enhancing Public Safety in the Interior of the United States."

Among other actions outlined in the order, cities that do not comply with federal immigration enforcement agents "are not eligible to receive Federal grants, except as deemed necessary for law enforcement purposes by the Attorney General or the Secretary." In addition, the order restored both the Immigration and Nationality Act Section 287(g) and the Secure Communities Program, which are used to compel local law enforcement agencies to enforce federal immigration law.

In response, mayors across the nation issued statements of their own, vowing to protect their cities from federal interference, including Houston's Mayor Sylvester Turner, who wrote:

I know there are a lot of families and children who are afraid and worried right now about what might happen to them.  I want them to know that Houston is, and always has been, a welcoming city, where we value and appreciate diversity.  HPD is not the Immigration and Naturalization Service.  We don’t profile, and we are not going to start profiling people to determine whether they are here illegally.  It hasn’t happened under previous mayors, and it will not happen under my administration.

Turner was joined in his response by Boston Mayor Marty Walsh, who called the executive order an attack "on Boston’s people, Boston’s strength and Boston’s values." In Seattle, Mayor Ed Murray said in a press conference Wednesday, "This city will not be bullied by this administration. We believe we have the rule of law and the courts on our side."

Meanwhile in Chicago, Mayor Rahm Emanuel said, “I want to be clear: We’re going to stay a sanctuary city. 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 is from, you are welcome in Chicago as you pursue the American Dream."

The term "sanctuary city"  up until now has had no formal or legal definition. But the executive order invokes the authority to determine which cities fit the bill. "The Secretary has the authority to designate, in his discretion and to the extent consistent with law, a jurisdiction as a sanctuary jurisdiction," reads Section 9(a), which offers no inclination of what that might mean for cities such as Houston.