One morning in October 2016, Lt. Col. Dave Grossman was in a Red Lion Hotel conference room in Sacramento, California, preparing to speak to a group of state troopers about what it’s like to kill.

Grossman, 60, is a former West Point psychology professor who’s spent much of his career studying killology — his term for the psychology of taking a life. Among the military and law enforcement, he’s a revered figure. His first book, On Killing, is part of the curriculum at the FBI academy and on the Marine Corps Commandant’s Professional Reading List. Its follow-up, On Combat, is probably best known for his assertion that people can be divided into three groups — sheep, wolves, and sheepdogs — and it’s the sheepdogs, “blessed with the gift of aggression,” who are responsible for protecting the sheep from the wolves. The analogy has been adopted by various military and gun-rights groups; in Clint Eastwood’s American Sniper, the father of Navy SEAL Chris Kyle gives a (fictional) dinner-table speech about sheepdogs taken directly from Grossman’s writings.

Much of Grossman’s work draws on his study of killing in combat — specifically, the psychological and physiological effects a person experiences upon taking a life. Since retiring from the Army 19 years ago, he has worked with hundreds of agencies, including the FBI, the DEA, ATF, the Secret Service, the Diplomatic Security Service, the U.S. Marshals, and U.S. Special Forces. But these days Grossman’s real bread and butter is local police. Over the past two years, he has spoken to more than 100 departments around the country. There’s probably no one in America who trains more cops; there’s almost certainly no one who trains cops who is better known.

The Sacramento event was for the California Highway Patrol, part of an annual three-day conference for “peer-support” officers, who help others after traumatic events such as shootings and mass casualties. “There are some big challenges facing law enforcement,” CHP Capt. John Arrabit said. “There’s a lot of negative press. Dave reminds us that the vast majority of the public supports law enforcement personnel. His message is: ‘What you’re doing is making a difference. It’s noble and vital. Be proud of who you are and what you do.’ ”

“Cops fight violence. What do they fight it with? Superior violence. Righteous violence.”

There were 200 or so CHP employees in attendance, from undercover investigators with grizzled, meth-dealer beards to sweet-looking dispatchers who might never hold a weapon. As they sipped free Starbucks and waited for the day to start, dozens lined up to get books signed by Grossman, who sat at a table offering merch for sale, including a children’s book ($15) and a SheepDog-branded knife ($240).

Between autographs, Grossman introduced me to a trooper named Andra Eddy. Andra’s husband, Greg, is also a trooper, assigned to a canine unit in the Bay Area. In the fall of 2012, Greg took Grossman’s class, and a few months later, he and a partner shot and killed a suspected car thief who’d aimed a gun at them. “The guy was loaded, one in the chamber, pointed at my husband — the whole nine yards,” Andra said. “Had things not lined up properly, he would have died.”

Greg, his wife told me, had spent eight years in the Army before joining the CHP and served in the Balkans in the mid-’90s. “He was deployed to really shitty circumstances, and he killed quite a few people,” she said.’ “But this was totally different. In the military you’re trained to do whatever you have to do to protect the United States. Here we are not trained to do that. We’re trained to protect and serve. We don’t train to just kill people.” In the aftermath of the shooting, Greg leaned heavily on Grossman’s On Combat. “It really helped him understand his mental state and how to deal with the anxiety he was going through,” Andra said. “Had he not read the book, he would have thought something was truly wrong with him.”

“God bless you,” Grossman said, putting a hand on her shoulder.

Andra gave him a hug. “Thank you for everything you do.”

In America’s current debate over policing, many observers have expressed concerns about the “militarization” of cops — their evolution from a traditional, defensive “guardian” model to an aggressive, “warrior”-style one. Grossman is not one of them. With increased dangers at home and the Posse Comitatus Act preventing the military from operating on U.S. soil, he says, cops need to act more like soldiers. “We are at war,” Grossman likes to tell the people he trains. “And our cops are the frontline troops in that war. You are the Delta Force. You are the Green Berets. It’s your job to put a piece of steel in your fist and kill those sons of bitches when they come to kill our kids.”

“Cops fight violence,” Grossman often says. “What do they fight it with? Superior violence. Righteous violence.” At a time when a growing number of police officials believe cops should be less eager to embrace the use of force, Grossman is teaching the opposite. Which prompts a few questions: Is an expert in “killology” the best person to be training domestic police right now? How did Grossman become so sought-after in the first place? And if our cops are really at war, as he believes, then whom, exactly, are they at war with?

“I truly am one of the best people on the planet in a couple of areas. Whether it’s preparation for a life-or-death event or walking the sheepdog path, I really feel like I’m the preeminent authority.”

At 8 Am sharp, Grossman vaulted onto the Red Lion stage. He wore his standard uniform: black long-sleeved button-down; Levi’s with ink-stained pockets (due to the permanent markers he keeps stuffed in them when he teaches). His high-and-tight haircut was combed just right, and he prowled the stage with a marksman’s squint.

Grossman tailors his classes to his audience, so today’s — which he calls “Bulletproof Mind” — would focus largely on threats to law enforcement. He spent the first few hours laying out a frighteningly dark vision of the world, from elementary-school massacres in Israel and schoolgirl beheadings in Indonesia to our own tragedies in Orlando and San Bernardino, California. He warned of potential threats everywhere: a nuclear bomb in a boat off the coast of San Diego, Ebola-infected “suicide bio-bombers” sneaking across the Mexican border. He spent more than 10 minutes on the Beslan school massacre, a 2004 Russian terror attack in which more than 330 innocents were killed, 189 of them children. “They believe 49 terrorists were neutralized . . . and 12 escaped,” Grossman said. “You can make a very good argument that they will be the cell leaders for an attack on America.”

Next came the internal threats, specifically in America’s urban centers. “The level of day-to-day violence in our cities has never been a fraction of what it is today,” he said. And then he turned to what he describes as an even more urgent crisis: the so-called war on cops. “The number of cops murdered in the line of duty has skyrocketed,” he told the troopers. “The ­systematic murder, ambush, and execution of cops has become the norm.” He blamed “cop-haters” like civil-rights protestors and the news media, as well as “sick” TV shows such as Breaking Bad and Sons of Anarchy for ­breeding “a generation of gangbangers who sincerely believe cops are the bad guys.”

Grossman, it must be said, has a penchant for hyperbolic sound bites and stats that don’t quite stand up to fact-checking. That Russian terror attack? According to the Russian government’s official report, there were 32 terrorists, of whom 31 were killed and one was captured. The exploding murder rate? According to the FBI, the number of homicides did indeed rise dramatically in a handful of cities in 2015 — but nationwide it’s still roughly half of what it was for much of the ’70s, ’80s, and ’90s. Violent crime is also historically low.

As for the systematic murder of police, according to the FBI, 2015 was one of the safer years for cops in recent memory, with 41 officers “feloniously killed” in the line of duty — under the 10-year average of 48.8 and well below the 35-year average of 64. The 2016 numbers, which won’t be released until later this year, will almost certainly show an increase — in part because of ambush attacks on cops in Dallas and Baton Rouge, Louisiana — but the total will still be in line with the statistical norm.

Seth Stoughton, a professor of criminal law at the University of South Carolina, acknowledges that targeted assassinations of police officers, such as those that happened in Texas and Louisiana, have increased. “But the increase isn’t huge in terms of pure numbers — it’s like from five to 10,” Stoughton said. “In a population of more than 300 million, with more than 700,000 state and local officers, that doesn’t look like a war to me. It looks troubling. It looks scary. But let’s not blow it out of proportion.”

Yet Grossman sees those ambushes as a chilling sign of things to come. “When you hear about the first American cop being beheaded,” he told the CHP, “say, ‘Grossman said that was coming.’ ”

Stoughton, who spent five years as an officer with the Tallahassee Police Department, calls the emphasis on threats and fear in police training “scaremongering.” “The idea that the world is under siege from the forces of evil and you are the only thing standing in the way — that’s an awfully attractive message, and it makes an audience feel good,” he said. “But it also happens to be bullshit, and potentially destructive bullshit. It increases the risks that officers and civilians face.”

Filmmaker Craig Atkinson agrees. Atkinson is the director of Do Not Resist, a new documentary about police militarization. He spent three years filming 18 different departments, going on ride-alongs and SWAT raids, where he often heard Grossman’s name. “He was always billed to us as one of the number one trainers in all of law enforcement,” Atkinson said. Then he filmed one of Grossman’s workshops. “We were absolutely shocked to hear the messaging of violence going out,” he said.

Atkinson also questioned Grossman’s use of data. “A lot of police officers aren’t coming from a scientific background,” he said. “So when Grossman — a professor — presents something as fact, they take it as fact. But when you really drill down into any of it, it’s basically a small bit of reality blown up to justify his thinking. He’s cherry-picking ideas to illustrate his point.”

After a lunch break, Grossman spent the second half of the day on coping strategies, in the unfortunate event that the troopers were someday forced to “embrace a dirty word: kill.” He spoke about phenomena they might experience in a gunfight, such as “auditory exclusion,” in which they don’t hear their own gunshots, and the urge to defecate — what Grossman calls “taking a battle crap.” There were more dangerous physical side-effects, too, such as the loss of complex motor skills, and the possibility of seeing things that aren’t there. Grossman also provided tools for staying calm in an emergency, such as drinking a sip of water or taking deep breaths. The troopers listened intently, many nodding their heads in agreement.

In some ways it’s helpful to think of Grossman not so much as a professor but as a motivational speaker. He dispenses lots of facts and figures, perhaps not all of them accurate, but mostly he is paid to encourage cops to feel good about what they do. “You man the ramparts of civilization at a desperate and violent time,” he told the CHP. In an age when many police feel criticized and underappreciated, Grossman provides some welcome moral support.

As it neared 5 pm, Grossman finished the day by reminding the officers of their fundamental mission. “Our goal is never to kill,” he said. “Our goal is to save lives. Our strength comes from that. Our purity comes from that.” At the same time, he countered, they shouldn’t err too far in the other direction. Not pulling the trigger when you’re legally justified, he said, could be just as bad as shooting when you’re not: “Fail to take action when you should — what do they call that? Cowardice. Negligence. Dereliction of duty.”

“Don’t be afraid of getting sued — it’s just a chance for overtime,” he said. “Be afraid of getting successfully sued.”

After his talk, Grossman and I went for dinner at a nearby sports bar, where he told me about his life. He lives with his wife, Jeanne — his high school sweetheart — and their two dogs in a small town outside St. Louis, Missouri (as it happens, 45 minutes from Ferguson). He spends almost 300 days a year on the road, usually coming home one night a week for what he jokingly calls “a conjugal visit and clean underwear” before heading out again. His oldest son, Jon, runs a family-owned gunsmithing company; his youngest, Joe, helps manage the speaking business. His middle son, Eric, is an Air Force combat controller with nine combat tours and three Bronze Stars.

“The military was all I ever wanted to do,” Grossman said between sips of beer. The son of a cop from Cheyenne, Wyoming, he left school at 17 to work in the Nebraska oil fields and enlisted in the Army a year later. He graduated from Ranger School in 1978 and became an infantry platoon leader, eventually working his way up to company commander with the 7th (Light) Infantry Division at Fort Ord, California. But he never had the opportunity to serve in a Ranger regiment, and though he literally wrote the book on combat, he never got to see action himself. The first Gulf War started just after he arrived at West Point.

Grossman had 14 years in the Army when he applied to teach at West Point and got selected to teach psychology. He’d never had any training in psychology: “What West Point does is it selects people as professors, then sends them to grad school en route,” he explained. “I would have studied underwater basket weaving if it got me to West Point.” He earned his master’s in education psychology from the University of Texas at Austin and spent a year interning as a counselor at a local middle school. He taught at West Point from 1990 to 1993; the rest of his professorial experience came at Arkansas State, where he spent four years teaching military science and overseeing the ROTC program.

It was at Arkansas State that Grossman published On Killing, in 1995, to much acclaim. The Washington Post called it “an illuminating account of how soldiers learn to kill and how they live with the experience of having killed”; the New York Times called it “powerfully argued” and “full of arresting observations and insights.” The book even made fans in Hollywood: While promoting his World War II movie Fury a few years back, Brad Pitt told an interviewer, “If you want to better understand the accumulative psychic trauma incurred by our soldiers, read On Killing by Lt. Col. Dave Grossman.”

Though Grossman calls himself a behavioral scientist, he is not a researcher in the traditional academic sense. He wrote On Combat, a study on how soldiers and police officers cope with the stress associated with deadly conflict, using what he calls an “interactive feedback loop” — gathering stories from combat veterans, then presenting the information to people he trains. He’s more of a Malcolm Gladwell type, compiling anecdotes and fashioning them into a digestible narrative. As his chief qualifications, Grossman cites the “body of information I’ve crafted over the years” and his ability to “speak from the heart.” “I truly am one of the best people on the planet in a couple of areas,” he told me. “Whether it’s preparation for a life-or-death event or walking the sheepdog path, I really feel like I’m the preeminent authority.”

Since leaving the Army, Grossman frequently introduces himself as a reserve cop. (He’s a reserve deputy coroner for St. Clair County, Illinois.) “I think a lot more like a cop today than I do like a soldier,” he said. As such, he tends to take a reflexive stance against anyone he sees as harming cops. He likens protest groups such as the Black Lives Matter movement to “treason” and says “it has blood on its hands” for emboldening killers of police. He calls the media “dirtbags” and “bastards” for their coverage of Ferguson, and he accuses the Obama administration and other politicians of “pandering” to the police-reform movement. He also cites the so-called Ferguson effect — the hypothesis that cops aren’t doing their jobs for fear of being prosecuted or sued — for what he sees as the surging homicide rate. “The mayor of Chicago said cops are in the fetal position — well, you put ’em there,” Grossman said. “That’s the price we pay for cutting the legs off our cops.”

In his famous sheepdog essay, Grossman talked about how sheepdogs can sometimes accidentally scare the sheep. The sheepdog “looks a lot like the wolf,” he wrote. “He has fangs and the capacity for violence. The difference, though, is that the sheepdog must not, cannot, and will not ever harm the sheep. Any sheepdog who intentionally harms the lowliest little lamb will be punished and removed.”

The system, in other words, depends on zero tolerance and accountability. Which, it could be argued, is exactly what’s missing nearly every time a cop kills an innocent citizen without repercussions.

In 2015, 991 people were fatally shot by police officers, according to the Washington Post; 94 of them were unarmed. Yet only 18 officers were charged in fatal on-duty shootings. Of all the recent high-profile police killings, Grossman sees almost none that he believes were unjustified. Take Eric Garner, the Staten Island man who died after an illegal choke hold from the NYPD and whose last words were “I can’t breathe.” “If you can talk, you can breathe,” Grossman said. “The guy had a heart condition. The lesson is, don’t fight cops when you have a heart condition.” Or take Tamir Rice, the 12-year-old Cleveland boy who was fatally shot in a park while playing with a toy Airsoft gun. “If you had a gun pointed at you . . . ” Grossman says, sympathizing with the cop — who, for the record, did not have a gun pointed at him. “That one’s borderline. I’m not giving you that one.”

When it comes to improving policing, Grossman strongly disagrees with prevailing theories, such as that cops who shoot unarmed black citizens may be falling prey to “implicit bias.” “I don’t think there’s much of that,” he said. “The far greater bias in our society today is a bias against cops. In 10,000 TV shows and 500 movies, black people are almost never the bad guys. Name me one cop movie in the last 30 years that didn’t have a bad cop.”

Grossman does admit there’s one area of law enforcement that could be improved. “When people tell you law enforcement is broken, they’re right,” he said. “And what’s broken is sleep.” He believes when cops shoot wrongly, it’s not because they’re biased or scared or in need of better training. It’s because they’re physically and mentally exhausted. Long shifts and overtime lead to tired cops, and sleep deprivation, he said, “is the number one predictor of judgment errors, ethical problems, and use-of-force problems. If I could change one thing in the world right now to make law enforcement better,” Grossman added, “it would be mandating sleep.”

Frankly, I’m a little surprised by this. Grossman has spent two decades training law enforcement personnel and has published four books on the psychology of killing. Surely he has more insight into police shootings than this. But over and over again, in several conversations, he returned to this answer. “In every case I’ve been able to look at, we keep coming back to sleep deprivation,” Grossman said. “This is a moment that’s ripe for change — and if you want to make a difference right now, the place to aim for is sleep.”

Grossman also has written extensively about video games and the role they play in contributing to mass shootings. In his new book, Assassination Generation, he argues that first-person games like Grand Theft Auto — which he calls “a cop-killing murder simulator” — have trained millions of American children to be ruthless killers. “We have raised the most vicious generation the world has ever seen,” Grossman said in his lecture to the CHP. “Thousands of Americans see video of ISIS cutting heads off and say, ‘I want a piece of that.’ ”

Assassination Generation revisits ground Grossman originally covered in his 1998 book Stop Teaching Our Kids to Kill, in which he first laid out his ideas about media violence. Many of his claims were litigated 18 years ago, when America had this debate in the wake of Columbine. But Grossman is more adamant than ever. At one point he writes that experts who deny the link between video games and adolescent violence will someday “be viewed as the moral equivalent of Holocaust deniers.”

Near the end of the book, Grossman highlights some practical steps parents can take to battle video-game-inspired violence: more sleep, less screen time, more family meals, more time in nature. But nowhere in his list of solutions does he mention the word guns. One anecdote is particularly telling: Grossman writes about a 16-year-old in Cleveland whose parents took away his copy of Halo 3 because they thought it was too violent. His father locked the game in a lockbox, which also held a 9mm handgun. The boy stole the key, took the game and the gun, and shot both his parents in the head. Grossman blames video games for the murder; he says nothing about the pistol.

Grossman grew up around guns and nowadays fully embraces gun rights. He’s a popular speaker on the NRA circuit and last year spoke at the organization’s national meeting: “All three days. Standing-room only every day.” A big proponent of the good-guy-with-a-gun theory of crime prevention, he calls the right to concealed carry “the greatest grassroots issue of our time” and says its implementation is a matter of urgent national security. He wants America to have more gun owners, with effective background checks and rigorous training. He’d also like to see some kind of mandatory national service, as in Switzerland or Israel. “I want us to be a nation of marksmen,” Grossman said. “As long as we’re an armed nation, we’ll be a free nation. Israel has found the only possible answer: armed people everywhere.”

A week later Grossman was at Wild Card West, a boxing gym in Santa Monica. The gym’s co-owner, the movie director Peter Berg (Lone Survivor, Deepwater Horizon, Patriots Day), was trying to start a new guest-speaker series — his version of TED Talks — and Grossman was his first speaker. A few dozen guests mingled around the boxing ring, sipping beer and wine and nibbling on catered hors d’oeuvres. Berg’s frequent collaborator Mark Wahlberg was there, in a Red Sox cap and a T-shirt that showed off his biceps, as was Berg’s agent, Ari Emanuel — the brother of Chicago mayor Rahm Emanuel and one of Hollywood’s most powerful executives.

Berg climbed into the ring and told the crowd about how he first came across Grossman’s writing. While preparing to shoot Lone Survivor, he’d embedded with a SEAL team in Iraq, and all the SEALs had copies of Grossman’s books. (Standing in the back, several large, bearded men nodded in agreement.) Then Berg introduced Grossman, whom he mistakenly referred to as “Dr.” “Thanks, folks,” Grossman said to applause. “One thing right up front — appreciate the intro, Pete, but I’m not a doctor, I always try to correct that. Reasonable assumption.”

For the next hour or so, Grossman delivered an abridged version of his lecture, touching such greatest hits as “Israeli school massacres,” “you are the frontline troops,” “left-wing blogs,” and, of course, “sheepdogs.” He cited one of his favorite statistics — that 15 percent of divorces are caused by video games — and the crowd laughed before realizing he wasn’t joking. “Google it,” Grossman said. “ ’Video games, divorce’ — it comes right up.” After finishing he said he’d take a few questions, which he normally does not do. “We’ll see how it goes,” he said uncertainly.

The second question came from Ari Emanuel. “Since we’re in the political season right now,” he asked, “I was wondering what your opinion is on gun control.”

Grossman nodded. It was clear he didn’t know who Emanuel was. “Folks, I’m a behavioral scientist,” he began. “I try to stick with the science. I’ve got a lot of conservative friends who are global-warming skeptics, and it’s embarrassing. What you’ve got is called confirmation bias. The data is overwhelming. They see one study that shows some doubt, they twist the data, they twist the application. But when it comes to the gun issue, it’s almost the other way around. I’ve got left-wing antigun people, and it’s kind of embarrassing.”

He went on to say that more than 90 percent of cops favor gun rights, and that 42 states had instituted loose concealed-carry laws, “and every time, crime is down.”

But this crowd was somewhat more skeptical than Grossman’s usual audience. “There is research,” Emanuel countered. “In countries that don’t have guns, there is less violence, less murders. . . .”

“The seven most violent nations on the planet are all in Latin America and the Caribbean,” Grossman said, “and every one of them has those gun laws.”

“But you could make the other side of that argument, too,” argued Emanuel. “We can go to Israel. They don’t permit guns — only the military.”

“Oh, no.” Grossman shook his head. “That’s not accurate.”

“That is accurate,” said Emanuel.

Grossman chuckled sarcastically. “Yeah. Do some research on that one.”

“I have,” Emanuel said.

Another man raised his hand. “Where can I find the studies on those 42 states?”

“The 42 states?” said Grossman. “The NRA — nra.com. I think there’s some good info there.”

Berg stepped in. “Why don’t we take two more. . . .”

Grossman answered two more questions, one about veteran suicide (his solution: more sleep) and one from a woman asking advice about being married to a sheepdog. Emanuel cut out early, raising a skeptical eyebrow at Berg as he left. Grossman wrapped up to applause, and the audience stood to leave.

At some point in the evening, two uniformed cops from Santa Monica PD had drifted in. They were young, probably mid-to late twenties, one white, one Hispanic. They watched Grossman’s talk from the door, hanging back as if not to intrude. After Grossman worked his way to the lobby, shaking hands and saying thank-yous, he set up a table to autograph some books. The two cops were the first in line.

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