In the last few days the horrific stabbing death of Iryna Zarutska by oft-arrested offender Decarlos Brown, Jr. on a Charlotte, NC, commuter train finally broke into the national conversation. 

National media — aside from this newspaper — studiously ignored the heinous Aug. 22 attack, while Wikipedia’s editors did their best to airbrush it.  

Why? Because they run interference for Democrats, and the story is political nitroglycerine. 

Not only did this brutal murder of a white woman by a black man turn the Democrats’ racial narrative on its head, it underscored and boosted President Donald Trump’s messaging on urban crime. 

Brown had 14 arrests, but turnkey judges and prosecutors kept letting him go until he committed a terrifying caught-on-video crime that’s impossible to explain away — or to weaponize against Republicans.

On Monday, Trump noted the obvious conclusion: It does no good to arrest violent felons if Democratic prosecutors and judges are simply going to turn them loose on their communities. 

As Trump has been saying, dangerous people need to be locked up until they’re not dangerous anymore.

And there’s another point worth making here: the utter incoherence of Democrats’ urban policies.

If you want the kind of urban life Democrats say they want — with mass transit, dense housing and a diverse population — then public order, controlling not just violent crime but also disorderly conduct, is an absolute must.

People won’t take mass transit if they feel unsafe on it, and mass transit, by definition, forces you into close proximity with strangers. 

If you can’t trust those strangers to behave themselves, people will choose alternatives — or abandon cities entirely and move to places where the surroundings are more congenial.

In their original 1982 article on “Broken Windows” policing, criminologists George Kelling and James Q. Wilson wrote that to make urban areas successful, it’s not enough to police major crime.

You must also rein in minor offenses against public order that make public spaces feel unwelcoming. 

People are certainly afraid of violent crime, Kelling and Wilson wrote, but they’re also afraid of “being bothered by disorderly people.”

Plus, they noted, serious crime and disorderly behavior are linked. 

If disorderly conduct — panhandling, graffiti, public intoxication, vandalism — runs unchecked, criminals will conclude that this is an area where they can get away with more serious crimes. 

A neighborhood that doesn’t control graffiti or panhandling probably won’t control robbery, or drug dealing, or even murder.

In the same way, people on the street — even people who are mentally ill — take cues from the environment around them. 

In the presence of police, or even of ordinary citizens who’ll give them the stinkeye for antisocial acts, they are more likely to behave. 

If there are no police in sight, and if citizens look away and don’t meet their eyes, troublemakers sense submission, which to them means freedom to act badly.

In the first half of the 20th century, citizens did much to maintain public order. 

Threaten a woman on the subway, and male passengers would come together to toss you off.

Then we got the message not to “get involved.” 

By the modern era a brave man like Daniel Penny, who stopped a subway assailant at the risk of his own life, was prosecuted by vindictive authorities who hate the thought of citizens getting uppity and protecting themselves and others.

Well, you can have an atomized urban environment, in which everyone keeps their heads down and leaves the job of maintaining order to the police, who can’t be everywhere at all times. 

But we all know what that looks like.

It looks like New York City did before Rudy Giuliani’s “broken windows” reforms, and again after Bill de Blasio let law and order slide.

For some reason, a lot of Democratic leaders seem to like that kind of disorderly urban environment.

Maybe it’s because they’re seldom found on subways, except for photo ops while accompanied by bodyguards. 

But if you want people to live in your city instead of fleeing it, if you want people to use your mass transit instead of avoiding it, if you want a diverse citizenry that gets along instead of mutually suspicious people who separate themselves by race, then you have to maintain order.

Charlotte’s light rail system was already struggling with safety issues.

Now that they’ve seen this sickening video, convincing people to ride the city’s trains will be harder than ever. 

If DeCarlos Brown had been doing the hard time he deserved for the previous crimes he’d committed, Iryna Zarutska would have gotten home safely after working her shift.

She’d be alive today, and the mass-transit riders of Charlotte and elsewhere wouldn’t be looking over their shoulders for fear of a similar fate.  

That’s the choice. And if you want a flourishing city, it’s really no choice at all.

Glenn Harlan Reynolds is a professor of law at the University of Tennessee and founder of the InstaPundit.com blog.