This is a defining moment for New York City.

The horrific assassination of two NYPD officers -- Rafael Ramos and Wen Jian Liu -- as they sat in their patrol car on a Brooklyn street Saturday has taken the city to a dangerous new place.

As we mourn their unspeakable deaths, our hearts go out to their families. At the same time, we watch in utter dismay as a deep chasm of anger and mistrust between the NYPD and many city residents grows wider yet.

New Yorkers deserve a police force that can keep the peace and -- simultaneously -- help close the burgeoning divide between cops and citizens.

That mission is not unreasonable, and few other forces in America are as equipped as the NYPD to accomplish it.

Yet within hours of the shootings, the leader of the city's largest police union was falsely and recklessly blaming Mayor Bill de Blasio for the deaths of the two officers.

"There's blood on many hands tonight," said Patrick Lynch, head of the Patrolmen's Benevolent Association, and it "starts at City Hall in the office of the mayor."

When the funerals are over, Lynch added ominously, "those responsible" will be held accountable.

Meanwhile, as de Blasio and NYPD Commissioner William Bratton made their way through the halls of Woodhull Medical Center on Saturday, scores of rank-and-file cops turned their backs to them.

Frustration may be understandable. The department has endured weeks of anti-police protests in the wake of Eric Garner's death on Staten Island and Michael Brown's death in Ferguson, Missouri. Police officers have put up with insults, taunts and -- yes -- heinous physical attacks, which de Blasio has denounced.

But the mayor didn't create this animosity. The NYPD's fraught relationship with many minority communities was simmering long before he took office. Yet now -- in a city that is 44 percent white, 26 percent black and 27 percent Hispanic -- this is his problem to fix.

We hope, for the sake of all New Yorkers, that a rational and productive discussion of our differences can resume.

The city really has no other choice.