“You are going to be the proud owner of 25 million people,” Mr. Powell told the president. “You will own all their hopes, aspirations and problems. You’ll own it all.”

Because President Trump believes such concerns aren’t America’s business, he has been reluctant to involve U.S. troops abroad. So it’s surprising that he now appears eager to intervene in the mostly Democratic-run American cities that have been wracked by chaos, shootings and destruction in the weeks since George Floyd’s death at the hands of Minneapolis police.

“Take back your city now. If you don’t do it, I will,” the president tweeted about Seattle back in June. The other day he likened cities plagued by unrest to “war zones,” and promised that Attorney General William Barr would be announcing something this week. The danger is that, even with clear legal justification, if he sends federal officers to do a job local law enforcement should be doing, and does it against the wishes of local mayors and governors, under the Pottery Barn rule he will quickly be blamed for problems he did not create even as he relieves those who did create them of any responsibility.

Exhibit A is Portland, Ore. Like many American cities, Portland has been consumed by protests. This is an overwhelmingly white city—African-Americans make up less than 6% of the population—run by a progressive white mayor, Ted Wheeler. In the Portland context he may be the moderate: his rival in the coming election is a woman who has said, “I am Antifa.”

On Saturday night, following 52 days of increasingly violent protest, Portland police declared a riot right after demonstrators lit the local police union building on fire and tore down fences. That happened after the Department of Homeland Security dispatched federal law enforcement officers including U.S. marshals, the Federal Protective Service and U.S. Customs and Border Protection to guard the federal courthouse, which rioters had attempted to set on fire. Instantly, the Portland story has become all about President Trump and the federal officers—and not the violence and the city’s failure to stop it. Acting Homeland Security Secretary Chad Wolf says he offered Mayor Wheeler and Gov. Kate Brown assistance in law enforcement but was told to “please pack up and go home.”

Never mind that the legal case against the federal intervention is overblown. If Mr. Trump thinks it’s difficult to act in Afghanistan without the full cooperation and support of the government, he’s now finding out what it means to do so in Democratic cities, where he will be denounced and undercut at every chance. Already Twitter has seen “#Gestapo” trend, and Mayor Wheeler uses his own feed to excoriate the president’s “attack on our democracy,” his “paramilitary squads” and “the violence federal officers brought to our street.”

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.

H.L. Mencken famously said: “Democracy is the theory that the common people know what they want, and deserve to get it good and hard.” Portland is one of America’s wokest white cities, with a woke white mayor just itching to dump his failure to keep order onto Mr. Trump. The chaos now consuming American cities has arisen on the watch of progressive politicians just like Mayor Wheeler, and they don’t deserve to be so easily let off the hook.