On Jan. 4 of last year, a local narcotics strike force conducted a raid on the Ogden, Utah, home of Matthew David Stewart at 8:40 p.m. The 12 officers were acting on a tip from Mr. Stewart’s former girlfriend, who said that he was growing marijuana in his basement. Mr. Stewart awoke, naked, to the sound of a battering ram taking down his door. Thinking that he was being invaded by criminals, as he later claimed, he grabbed his 9-millimeter Beretta pistol.

Photo illustration by Sean McCabe

The police say that they knocked and identified themselves, though Mr. Stewart and his neighbors said they heard no such announcement. Mr. Stewart fired 31 rounds, the police more than 250. Six of the officers were wounded, and Officer Jared Francom was killed. Mr. Stewart himself was shot twice before he was arrested. He was charged with several crimes, including the murder of Officer Francom.

The police found 16 small marijuana plants in Mr. Stewart’s basement. There was no evidence that Mr. Stewart, a U.S. military veteran with no prior criminal record, was selling marijuana. Mr. Stewart’s father said that his son suffered from post-traumatic stress disorder and may have smoked the marijuana to self-medicate.

Early this year, the Ogden city council heard complaints from dozens of citizens about the way drug warrants are served in the city. As for Mr. Stewart, his trial was scheduled for next April, and prosecutors were seeking the death penalty. But after losing a hearing last May on the legality of the search warrant, Mr. Stewart hanged himself in his jail cell.

The police tactics at issue in the Stewart case are no anomaly. Since the 1960s, in response to a range of perceived threats, law-enforcement agencies across the U.S., at every level of government, have been blurring the line between police officer and soldier. Driven by martial rhetoric and the availability of military-style equipment—from bayonets and M-16 rifles to armored personnel carriers—American police forces have often adopted a mind-set previously reserved for the battlefield. The war on drugs and, more recently, post-9/11 antiterrorism efforts have created a new figure on the U.S. scene: the warrior cop—armed to the teeth, ready to deal harshly with targeted wrongdoers, and a growing threat to familiar American liberties.

The acronym SWAT stands for Special Weapons and Tactics. Such police units are trained in methods similar to those used by the special forces in the military. They learn to break into homes with battering rams and to use incendiary devices called flashbang grenades, which are designed to blind and deafen anyone nearby. Their usual aim is to “clear” a building—that is, to remove any threats and distractions (including pets) and to subdue the occupants as quickly as possible.

Daily Republic/Associated Press – Today the U.S. has thousands of SWAT teams. A team prepares to enter a house in Vallejo, Calif., on March 20, above.

The country’s first official SWAT team started in the late 1960s in Los Angeles. By 1975, there were approximately 500 such units. Today, there are thousands. According to surveys conducted by the criminologist Peter Kraska of Eastern Kentucky University, just 13% of towns between 25,000 and 50,000 people had a SWAT team in 1983. By 2005, the figure was up to 80%.

The number of raids conducted by SWAT-like police units has grown accordingly. In the 1970s, there were just a few hundred a year; by the early 1980s, there were some 3,000 a year. In 2005 (the last year for which Dr. Kraska collected data), there were approximately 50,000 raids. Some federal agencies also now have their own SWAT teams, including NASA and the Department of the Interior.

Americans have long been wary of using the military for domestic policing. Concerns about potential abuse date back to the creation of the Constitution, when the founders worried about standing armies and the intimidation of the people at large by an overzealous executive, who might choose to follow the unhappy precedents set by Europe’s emperors and monarchs.

The idea for the first SWAT team in Los Angeles arose during the domestic strife and civil unrest of the mid-1960s. Daryl Gates, then an inspector with the Los Angeles Police Department, had grown frustrated with his department’s inability to respond effectively to incidents like the 1965 Watts riots. So his thoughts turned to the military. He was drawn in particular to Marine Special Forces and began to envision an elite group of police officers who could respond in a similar manner to dangerous domestic disturbances.

Standard-Examiner/Associated Press – When A strike force raided the home of Matthew David Stewart, one officer was killed.

Mr. Gates initially had difficulty getting his idea accepted. Los Angeles Police Chief William Parker thought the concept risked a breach in the divide between the military and law enforcement. But with the arrival of a new chief, Thomas Reddin, in 1966, Mr. Gates got the green light to start training a unit. By 1969, his SWAT team was ready for its maiden raid against a holdout cell of the Black Panthers.

At about the same time, President Richard Nixon was declaring war on drugs. Among the new, tough-minded law-enforcement measures included in this campaign was the no-knock raid—a policy that allowed drug cops to break into homes without the traditional knock and announcement. After fierce debate, Congress passed a bill authorizing no-knock raids for federal narcotics agents in 1970.

Over the next several years, stories emerged of federal agents breaking down the doors of private homes (often without a warrant) and terrorizing innocent citizens and families. Congress repealed the no-knock law in 1974, but the policy would soon make a comeback (without congressional authorization).

During the Reagan administration, SWAT-team methods converged with the drug war. By the end of the 1980s, joint task forces brought together police officers and soldiers for drug interdiction. National Guard helicopters and U-2 spy planes flew the California skies in search of marijuana plants. When suspects were identified, battle-clad troops from the National Guard, the DEA and other federal and local law enforcement agencies would swoop in to eradicate the plants and capture the people growing them.

Advocates of these tactics said that drug dealers were acquiring ever bigger weapons and the police needed to stay a step ahead in the arms race. There were indeed a few high-profile incidents in which police were outgunned, but no data exist suggesting that it was a widespread problem. A study done in 1991 by the libertarian-leaning Independence Institute found that less than one-eighth of 1% of homicides in the U.S. were committed with a military-grade weapon. Subsequent studies by the Justice Department in 1995 and the National Institute for Justice in 2004 came to similar conclusions: The overwhelming majority of serious crimes are committed with handguns, and not particularly powerful ones.

The new century brought the war on terror and, with it, new rationales and new resources for militarizing police forces. According to the Center for Investigative Reporting, the Department of Homeland Security has handed out $35 billion in grants since its creation in 2002, with much of the money going to purchase military gear such as armored personnel carriers. In 2011 alone, a Pentagon program for bolstering the capabilities of local law enforcement gave away $500 million of equipment, an all-time high.

The past decade also has seen an alarming degree of mission creep for U.S. SWAT teams. When the craze for poker kicked into high gear, a number of police departments responded by deploying SWAT teams to raid games in garages, basements and VFW halls where illegal gambling was suspected. According to news reports and conversations with poker organizations, there have been dozens of these raids, in cities such as Baltimore, Charleston, S.C., and Dallas.

In 2006, 38-year-old optometrist Sal Culosi was shot and killed by a Fairfax County, Va., SWAT officer. The investigation began when an undercover detective overheard Mr. Culosi wagering on college football games with some buddies at a bar. The department sent a SWAT team after Mr. Culosi, who had no prior criminal record or any history of violence. As the SWAT team descended, one officer fired a single bullet that pierced Mr. Culosi’s heart. The police say that the shot was an accident. Mr. Culosi’s family suspects the officer saw Mr. Culosi reaching for his cellphone and thought he had a gun.

In 2010, the police department in New Haven, Conn., sent its SWAT team to raid a bar where police believed there was underage drinking. For sheer absurdity, it is hard to beat the 2006 story about the Tibetan monks who had overstayed their visas while visiting America on a peace mission. In Iowa, the hapless holy men were apprehended by a SWAT team in full gear.

Unfortunately, the activities of aggressive, heavily armed SWAT units often result in needless bloodshed: Innocent bystanders have lost their lives and so, too, have police officers who were thought to be assailants and were fired on, as (allegedly) in the case of Matthew David Stewart.

In my own research, I have collected over 50 examples in which innocent people were killed in raids to enforce warrants for crimes that are either nonviolent or consensual (that is, crimes such as drug use or gambling, in which all parties participate voluntarily). These victims were bystanders, or the police later found no evidence of the crime for which the victim was being investigated. They include Katherine Johnston, a 92-year-old woman killed by an Atlanta narcotics team acting on a bad tip from an informant in 2006; Alberto Sepulveda, an 11-year-old accidentally shot by a California SWAT officer during a 2000 drug raid; and Eurie Stamps, killed in a 2011 raid on his home in Framingham, Mass., when an officer says his gun mistakenly discharged. Mr. Stamps wasn’t a suspect in the investigation.

What would it take to dial back such excessive police measures? The obvious place to start would be ending the federal grants that encourage police forces to acquire gear that is more appropriate for the battlefield. Beyond that, it is crucial to change the culture of militarization in American law enforcement.

Consider today’s police recruitment videos (widely available on YouTube), which often feature cops rappelling from helicopters, shooting big guns, kicking down doors and tackling suspects. Such campaigns embody an American policing culture that has become too isolated, confrontational and militaristic, and they tend to attract recruits for the wrong reasons.

If you browse online police discussion boards, or chat with younger cops today, you will often encounter some version of the phrase, “Whatever I need to do to get home safe.” It is a sentiment that suggests that every interaction with a citizen may be the officer’s last. Nor does it help when political leaders lend support to this militaristic self-image, as New York City Mayor Michael Bloomberg did in 2011 by declaring, “I have my own army in the NYPD—the seventh largest army in the world.”

The motivation of the average American cop should not focus on just making it to the end of his shift. The LAPD may have given us the first SWAT team, but its motto is still exactly the right ideal for American police officers: To protect and serve.

SWAT teams have their place, of course, but they should be saved for those relatively rare situations when police-initiated violence is the only hope to prevent the loss of life. They certainly have no place as modern-day vice squads.

Many longtime and retired law-enforcement officers have told me of their worry that the trend toward militarization is too far gone. Those who think there is still a chance at reform tend to embrace the idea of community policing, an approach that depends more on civil society than on brute force.

In this very different view of policing, cops walk beats, interact with citizens and consider themselves part of the neighborhoods they patrol—and therefore have a stake in those communities. It’s all about a baton-twirling “Officer Friendly” rather than a Taser-toting RoboCop.