"If Chicago doesn't fix the horrible 'carnage' going on, 228 shootings in 2017 with 42 killings (up 24 percent from 2016), I will send in the Feds!"

— President Trump, Jan. 24, 2017

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For now it's just another tweet from twitchy presidential thumbs, one more jab in the verbal tiff between a new Republican president and a Democratic mayor with a violence crisis spinning out of control. But before you write this off as typical Trump histrionics, consider:

Fifty-two years ago — March 20, 1965 — an angry President Lyndon Johnson reacted to Alabama's "Bloody Sunday" by declaring that he would use federal authority to protect the civil rights of the Rev. Martin Luther King Jr. and other marchers headed from Selma to Montgomery. Five days later, federal and state troops oversaw the march, capped by King's noble "How Long, Not Long" speech on the State Capitol steps.

Trump has blustered repeatedly about Chicago's rampant violence. Could he also construct a legal argument for federal intervention here? Does the menace to people who feel they can't leave their homes — the menace to children who fear walking to school — give a president the right to unleash federal resources to combat urban crime?

Good question. This much we know:

With his focus on Chicago's bloodshed, a new president is embarrassing this city's leaders. He's calling national attention to neighborhoods under siege. And he's rubbing America's face in domestic terrorism driven by superbly armed street gangs. What's the argument against considering a federal initiative? Because Chicago is so good at taking care of business? Because poor Chicagoans who live and die on city streets wouldn't welcome more help with open arms? Because other presidents never have deployed federal resources to protect the rights of innocent civilians? No, no and no.

What would that intervention be?  When most of us hear "send in the Feds!" we think "troops." But Ronald Allen, a professor of constitutional law at Northwestern University, tells us that principles of federalism keep Big Brother from running roughshod. It's one thing to, say, deploy troops to enforce a court order or to protect protesters and counterprotesters from one another at an event. "A military occupation is another matter," Allen says. "Short of an insurrection here, the U.S. can't invade Illinois."

If Trump wants to help Chicago right now, he'll stop tweeting long enough to focus on the recent U.S. Department of Justice report on serious shortcomings of policing, and police oversight, in this city. Will Trump's DOJ let the report gather dust, a relic of his predecessor's administration? Or will Trump tell his Justice Department to forge a consent decree with City Hall — essentially an enforceable pact that would help the Chicago Police Department put its best foot forward?

You've read it a hundred times: Until official Chicago rebuilds the trust of citizens who can help identify and convict criminals, many of those citizens will keep their distance.

FROM AN EDITORIAL IN THE CHICAGO TRIBUNE