It’s not lack of resources that keeps such mayors from maintaining order. It’s lack of will. As South Carolina Sen. Tim Scott pointed out after Democrats sank his crime bill, the communities suffering most from the urban unrest “have been run by Democrats for decades. Decades.”

It’s difficult to argue that these leaders have done so without the consent of the governed. Whether it was Bill de Blasio running against the police in New York or Jenny Durkan offering her own progressive agenda in Seattle, they didn’t hide from voters what they stood for. Ditto for New York Rep. Alexandria Ocasio-Cortez, who just won her congressional primary even after she helped kill a deal with Amazon that cost her constituents 25,000 good-paying jobs. In Portland, meanwhile, the city’s elected politicians have reacted to the violence in their city by starting to defund their police.

There’s no doubt the president has both the responsibility and the authority to protect federal property, which is what DHS is doing in Portland. But Mr. Trump would do well to narrow his rhetoric to make clear any federal intervention will be for this purpose and this purpose only—unless cities specifically ask for federal assistance. The hysterical reaction to the limited DHS intervention in Portland illuminates the Pottery Barn rule: The moment Mr. Trump intervenes in a troubled city, he owns it.