“It’s us versus them.”

That’s been a repeated refrain from protesters, police officers, experts in law enforcement behavior and some top leaders, in the wake of several incidents of unarmed people dying at the hands of police, followed by an armed man killing two NYPD officers. The tone of frustration from both sides reflects a growing sense of nationwide disenfranchisement between police departments and the communities they are tasked to protect.

Some see the current mood of hardened positions and distrust as reminiscent of a dark era of crime 20-30 years ago in cities like New York, when police barricaded themselves in squad cars or stations out of concern for their own safety. The neighborhoods, meanwhile, crumbled.

This debate has been aggravated by the deaths of Michael Brown and Eric Garner, both of whose police officer killers were not indicted by a grand jury, and the subsequent execution of two New York Police Department cops in Brooklyn, seemingly as an act of retribution.

Without a shift in the national dialogue, the NYPD, as well as other departments across the U.S., may back away from the current approach of policing with a focus on keeping communities safe and free of even petty infractions, which many believe serves as the key to ending widespread crime.

The men and women of the NYPD must feel under siege, says Donnie Gosselin, a 30-year veteran of the Boston Police Department, who served under Police Commissioner Bill Bratton before he moved from Boston to head up the NYPD.

“It’s going to be very difficult for them, moving forward, to be out there, out of the car, to walk through the neighborhoods, to engage people from all social strata,” Gosselin says.

Police in the 1950s first began employing fast-moving police cars, allowing them to race to the scene of a crime and replace the street-walking beat cops who could take as long as an hour to respond. Gosselin, now an advisor to the State Department on police reform in Central America, describes how this shift in technology heralded an era of policing when he first donned the uniform as one much more akin to how fire departments fight fires.

Officers would wait for specific reports of crimes taking place, then race to the scene – with their windows up to avoid being deafened by their own sirens – to mitigate the problem within minutes. Once order was restored, they would return to their standard positions.

“That was the dynamic of policing for the better part of the first half of my career,” says Gosselin, who retired last year. “That brings with it, unfortunately, a lack of communication on any level with the people you’re serving every day.”

“Then you go back to the station and you begin the cycle anew.”

This method of policing is believed to have brought about record high rates of crime in urban areas like New York City and Boston. It was era of drug wars, mean streets and fearful residents.

“Being a cop today is a stop-and-go nightmare,” reported U.S. News’ Gordon Witkin, Ted Gest and Dorian Friedman in a Dec. 3, 1990 feature. “It has never been easy. But all the debilitating leitmotifs of police work in past generations – the danger, the frustration, the family disruptions – have been made geometric-orders worse by the drug war in the nation's streets. There are more violent criminals, armed with more potent weaponry, showing more contempt for the men and women in blue than at any time in American history.”

“Police work on inner city streets is a domestic Vietnam,” the report continued, “a dangerous no-win struggle fought by confused, misdirected and unappreciated troops. They are racking up impressive arrest and imprisonment statistics without much changing the situation on the ground. Increasingly, police feel trapped between rising crime rates and an angry citizenry demanding immediate solutions to intractable problems.”

It went on to cite troubling statistics from the FBI: violent crime rose 5 percent in 1989, though the number of law enforcement officers rose by 7 percent. Murder in big cities in 1990 surpassed the previous record year of 1970, particularly in New York City which saw 1,077 murders in the first six months of that year.

“In these American versions of Beirut, cops are asked to be peacekeepers among street warriors, buffers between frightened black neighborhoods and young black criminals – who commit much of the violent urban crime – and social workers for families chronically in crisis.”

Top police leaders like Bratton, however, were among the early pioneers of forcing their officers to get out into the streets to engage their communities and learn about their concerns. This built trust across the so-called “blue line” and, perhaps most importantly, allowed police to determine the root of a problem instead of addressing a one-off flare up. For example, officers could find and prevent locals from selling loose cigarettes to the ire of those who had to pay for a license to do so, instead of reacting when such an incident prompts a police response.

Gosselin himself was surprised by a Boston-wide survey in the 1990s that revealed people in his precinct’s jurisdiction were mostly concerned about young people loitering on the sidewalk, not the armed robberies or domestic violence he and his fellow officers thought would be the No. 1 concern for police.

So going out in cars and breaking up idle groups of young people standing on corners upped the reputation of police in those neighborhoods.

“The idea was to get people out of the vehicles. To identify the problems before they warranted police intervention,” he says. “To start to target – it’s entirely cliche, but it rings true – this whole quality of life issue. To work with communities to identify: What are the problems within that community?”

Practitioners of this approach were not without controversy, however. Former New York City Mayor Rudolf Giuliani prioritized crackdowns on homeless people and other undesirable residents of the city. It may have improved living standards but some questioned the moves as discriminatory.

Violent crime nationwide in 2012 was down almost 13 percent from 2008, and 12 percent below the 2003 levels, according to the FBI.

In 2013, there were 332 homicides in New York City, down 20 percent from the previous year and a continuation of a sharp trend from the 1980s and 1990s.

These successes, however, have been drowned out by the string of nationally covered events this year, centered around Ferguson, Missouri and New York City.

The subsequent backlash, particularly those from within minority communities who feel police departments don’t represent their interests, has forced some officers within the NYPD to reconsider whether such risks are worth it.

“Without the support of the community, the average police officer is not going to go out of his way,” says Pete Segreti, a former NYPD officer who retired last year after 28 years, serving in a variety of assignments from beat patrols up to the Joint Terrorism Task Force. “He’s going to have a hands-off style of policing. He’s not going to go and be a proactive officer, which is what these communities need.”

Current police officers he’s spoken with are already considering that sort of retrenchment, he says.

The tense of fear created by the recent Brooklyn shooting is compounded by concerns that proactive attitudes could lead to a grand jury and subsequent accusations. There's also a belief among some in the police community that politicians no longer stand with them in unison, an issue that flared recently with charges from police unions and leaders such as former Commissioner Ray Kelly that New York Mayor Bill DeBlasio actively campaigned for election on a so-called anti-police platform.

“The backlash is going to be where officers are just going to drive around in the car, and when the radio tells you to go somewhere, that’s where they’re going to go,” Segreti says, offering an old cop joke: “‘Blue Flower Pots.’ They stand on a corner, they were out there, basically, and they just stood there. They’d take a report, and that was it.”

Top leaders, including DeBlasio and President Barack Obama, have touted the community aspects of police work, including departments’ efforts to create clubs and activities that produce community engagement.

“That’s all great, that’s all fine. But when one of these incidents happens, all this goodwill that’s been built, it’s all just wiped out,” says Steve Brandl, a professor at the University of Wisconsin at Milwaukee who specializes in police decision-making and behavior.

Police have difficulty creating goodwill because of the nature of their jobs, he says.

“They use force. That is the core function of the police. That’s why police get called, is to deal with something that isn’t supposed to be happening, and police can take care of the situation because they can tell someone what to do, or use force on them,” he says. “That is inherently controversial and problematic.”

“Given the nature of police [officers’] occupation, it’s just very difficult to do on a sustained basis.”

This harsh truth has been exacerbated by the mandate passed down from the federal government to city police forces to help protect America against further terrorist attacks. Police funding has now been dedicated to militarizing the force, and prioritizing quick-response training, causing fear among communities – like Ferguson – who don't feel they understand who police officers are truly targeting.

The Brown and Garner deaths, compounded by the shooting in Brooklyn, has created a “one-two punch” that will damage police officers’ faith in the merits of engaging the community, he says.

And it could create permanent damage.

“When you get a general sense in the community that nobody cares,” warns Gosselin, “watch out. Because that’s when the real serious crimes are about to occur.”