How did New York City’s subways arrive at their current chaotic state? It happened because politicians can always find reasons not to do what they don’t want to do — even when they say they do.

And Bill de Blasio — because of ideology, indolence or both — doesn’t want to do the hard work necessary to keep the city’s public spaces fit for public use. Even though he regularly says otherwise.

And he’s not alone with the talk. Transit officials are promising a crackdown on below-ground disorder — even as cops roll their eyes — but that’s because it’s almost Election Day, and such chatter is both timely and cheap.

But presently the polls will open, and close, and all will return to normal in Bill de Blasio’s New York.

It’s a town where the political establishment — and City Hall especially — is allergic to, and contemptuous of, what in a more naïve age were known as the civic virtues. Think cause and effect.

Ease up on quality-of-life law enforcement — in this case platform homesteading, turnstile jumping, dope smoking and public urination — and you’re going to get more homesteaders, more jumpers, more smokers and copious public piddling.

Five years after Mike Bloomberg left the subways reasonably bum-free — and 25 years after Rudy Giuliani vastly reduced in-your-face vagrancy everywhere in the city — commuters once again must thread their way past (probably) harmless crazies; unconscious, reeking dipsomaniacs; nodding dopers and unsubtly menacing “buskers” to get to work and back each day. And don’t forget the pee puddles.

Isn’t it aromatic?

All of this was as predictable as the morning sunrise: When a great city turns its back on the hard lessons of its recent past and cedes its public spaces to the maladjusted and the menacing, those spaces soon become all but uninhabitable.

New York is not there quite yet — but the trend line is ominous, and the city’s prospects are not encouraging.

Here’s how one knows.

When Transit Authority President Andy Byford last week announced plans to address subway disorder — “bear down” on it, he said, and who would argue against the need? — two things happened:

Alas, there is an abundance of important information packed into those few words.

His frustration is clear, and his message, though muted, seems obvious: He and his colleagues are on their own when dealing with public disorder. The intricate support network forged by Giuliani and maintained by Bloom­berg has withered — more likely, it simply was abandoned — and the strategies and focused support systems that made vagrancy control work back then went with it.

That’s why the city generally, not just its subways, is packed with increasingly aggressive panhandlers, vagrant encampments both large and small, opioid shooting galleries and other reminders of the Koch/Dinkins days.

That was a time when cops also were on their own within the NYPD command structure, when there was no top-down accountability and beat officers largely were free to keep order or not, depending. And whimsy has no proper place in law enforcement.

Then came Giuliani’s signal innovation — CompStat, a computer/statistics system designed not only to hold patrolmen to account, but precinct commanders as well. It was a smashing success, as the then-plummeting crime numbers so dramatically demonstrated.

Fast forward to de Blasioville: When, following a reasonable order, a cop says, “It’s a losing battle. We’ve got to deal with the same people every day,” what he’s really saying is that the people who make sure such battles are not lost — from precinct brass down to zone sergeants — are MIA. That accountability is a dead letter. That CompStat is an empty acronym, whatever pretenses are maintained.

And that New York City’s scandalously dysfunctional public spaces are not an anomaly, but a harbinger — of the disorder that result from policies drawn up and directed from City Hall.

That’s where the real problem resides. That’s where it must be solved.

But don’t bet it will be.

Bob McManus is a contributing editor to the Manhattan Institute’s City Journal.