First: Undocumented immigrants are the least likely to commit violent crimes, perhaps for obvious reasons; people who claim public safety danger are seriously misinformed and should check their sources. Be sceptical of "news" sources such as DailyMail.uk that write yellow"journalism." Second: Sanctuary cities are constitutional (the Framers welcomed immigrants and encouraged them to become citizens; read Crevecoeur, e.g.) and also are protected by the Fourth Amendment. To break a contracted promise to pay for antiterrorism policework (e.g.) is just Donald Trump not paying his bill, as he has so often in his illustrious (!) private life.