A career as a liberal ideologue when one has no actual power or responsibility is one thing, but after the police unions gave New York City Mayor Bill de Blasio a good spanking following the Eric Garner fiasco, he learned what part of the body politic is in charge.

Protest over the killing of Freddie Gray was peaceful, but not compliant enough to suit the New York Police Department, so they arrested 143 people who just didn’t listen.

Invoking his own past as a liberal organizer, Mr. de Blasio urged reporters “not to exaggerate what happened” at the Union Square rally on Wednesday night, saying the Police Department had acted appropriately in arresting 143 people marching to protest the death of a Baltimore black man, Freddie Gray, in police custody.

Exaggerate?  How so, Mr. Mayor?

Two of those protesters were arrested after assaulting officers, one of whom was struck on the chin with a stick and injured, police officials said.

Fair enough. But that leaves 141 people unaccounted for.

Mr. de Blasio, a Democrat, convened a news conference after hearing complaints about the police response from allies like the Rev. Al Sharpton. “I’ve participated in plenty of protests, on plenty of issues,” the mayor said. “I believe deeply in how nonviolent protest has achieved social change.”

If this is beginning to sound vaguely familiar, it’s no different than, “I believe in the First Amendment, but…”

But, he added: “When the police give you instruction, you follow the instruction. It’s not debatable.”

There it is, the words of sweet capitulation to authority. You have all the rights the Constitution affords, until the cops tell you otherwise.  When they do, you comply. “It’s not debatable.”  Meet a liberal who’s been taught a lesson.  Meet a liberal who can’t bear being hated by the police, who will smack him just as quickly as they smack anyone else.  Meet a mayor who knows he’s been broken.

But Mr. de Blasio’s attitude was a far cry from late last year, when he allowed similar demonstrations to spill into the city’s highways and avenues. That episode, after a Staten Island grand jury declined to indict an officer in the death of a Eric Garner, an unarmed black man, contributed to an open law enforcement rebellion against the mayor.

What was once good is now bad.  What once received de Blasio’s support now receives his condemnation.  What happened in between to compel Bill to spin around in circles?

In a telling shift, Mr. de Blasio rebuked his interlocutors for suggesting that some of the protesters arrested on Wednesday had done little to provoke a tough police response. In December, after the Garner protests, he lashed out at a journalist who suggested that some protesters had acted violently.

Then and now.  My, how times change.

“All I heard was, ‘Listen to the police, do what the police say,’ ” said Gideon Oliver, a civil rights lawyer who represents a group that organized the rally. “If the mayor’s answer to problems of abuse of police discretion is ‘just listen to what police say,’ that’s obviously a very big problem.”

At a time when national consciousness is being raised as to the reality on the streets, that Sheriff Andy is no longer in charge, and that those same cops who keep the riff raff off Park Avenue are being “assertive” with people living above 125th Street, New York’s most progressive mayor since David Dinkins has joined the chorus of “just do as you’re told” policing.

Was there anything crazy happening on the streets that could possibly explain Bratton’s call for “aggressiveness” or de Blasio’s submission to the shield?

“They were free to protest on the sidewalk, and follow the law, and we would certainly let them express their rights,” said the supervisor, who spoke on the condition of anonymity because he was not authorized to detail department policies. “But if they broke the law, went into the street and stuff like that, we weren’t going to give them much and were going to start locking people up.”

Aside from the arrests, the supervisor said, the night went smoothly. “No major fights,” he said. “No major assaults on cops, or cops doing anything crazy to anyone else.”

They went into the street “and stuff like that”? They jaywalked?  De Blasio gave up his last ounce of dignity because they jaywalked?

A reporter pointed out that Mr. de Blasio himself had been arrested during his run for mayor, as a show of civil disobedience to protest the closing of a public hospital in Brooklyn.

The mayor scoffed and said there was no comparison to what transpired on Wednesday night.

At least this time, the mayor is right, there is no comparison. Then, he was a candidate for mayor and now he’s the mayor.  As a candidate, especially one that few thought had a chance to win, he could still remember some of his principles.  As mayor, there is no room left for principle.

But there is a more important difference that proves de Blasio right that there is no comparison.  That was before the cops taught him who’s in charge.  Mayor de Blasio took his own advice:

“When the police give you instruction, you follow the instruction. It’s not debatable.”

And the police instructed the mayor to shut up and do as he’s told.  And he did, because “it’s not debatable.”