Just days into his federal takeover of Washington’s police force, President Trump declared the problem solved. “D.C. was a hellhole and now it’s safe,” he said. On Monday, he said he expected the same results in Chicago, the next city on his list for a federal crackdown on crime. “We will solve Chicago within one week, maybe less, but within one week, we will have no crime in Chicago,” Trump told reporters on Monday. Mr. Trump’s bold (and misleading) pronouncements expose a key strategy behind his tough-on-crime swagger. For the president, the idea of sending federal forces into American cities...