PHOTOGRAPH BY MARIO ANZUONI/REUTERS

By now, you may well have seen the video of heavily armed cops in Ferguson, Missouri, apparently firing tear gas at a film crew from Al Jazeera America and sending the journalists fleeing. Coming on top of the arrest of two reporters, one from the Washington Post and the other from the Huffington Post, it's yet another stain on the reputation of the St. Louis police, which has been struggling to handle the protests that have followed the shooting of Michael Brown, an unarmed teen-ager.

My colleagues Jelani Cobb, who is in Ferguson, Amy Davidson, and Jay Caspian Kang have done excellent jobs of covering the fatal police shooting and its aftermath. The teargassing incident was so gratuitous, however, that it demands an additional comment.

In a report earlier today, Ash-Har Quraishi, the Al Jazeera reporter who was hit by the tear gas, explained that he and his colleagues weren't near the protests or the protesters when the incident took place on Wednesday night. They were about a mile away, on the other side of some police lines, and they were about to do a routine stand-up report when at least one gas canister landed next to them. "We had been in contact with police officers who were just feet away from us," Quraishi said. "We had had discussions with them; we understood this was just as far as we could get. … We didn't think there would be any problems here, so we were very surprised. We were very close to where those canisters were shot from. We yelled … yelling that we were press. But they continued to fire."

“I understand that what it looks like is not good,” Tom Jackson, the Ferguson police chief, said, speaking of the night's events. But he claimed that some protesters had thrown Molotov cocktails, bottles, and other objects at the police. In an interview with the Los Angeles Times on Wednedsay, Jon Belmar, the police chief of St. Louis County, defended the conduct of his officers over the past few days. “We’ve done everything we can to demonstrate a remarkable amount of restraint,” Belmar said. “If there was an easy way to fix this, we would have already solved the problem.”

Before dismissing Belmar's comments outright, it should be noted that dealing with angry protesters, particularly at night, can be a tough task for any police department. On Sunday night and early Monday, following a vigil for Brown, stores in Ferguson and neighboring Dellwood were looted. (So far, nine people have been charged with burglary.) If the police had stood aside and allowed this disorder to continue, they would have been criticized for losing control of the situation.

That said, though, the police reaction appears to have been heavy-handed, and, in some cases, outright thuggish. When armored personnel carriers and rubber bullets are used to break up peaceful protests, when reporters and members of the clergy are arrested and cuffed for no good reason, when cops are targeting film crews a mile from the action with something that is classified as a chemical weapon under the Geneva Convention, it's surely time to ask what has happened to policing in this country.

In a recent report that Kang mentioned in his post on Wednesday, the American Civil Liberties Union provided an answer: in the wake of 9/11 and the "War on Terror," all too many municipalities have turned their police departments into quasi-militarized forces, which, when they deem it necessary, deploy some of the armor, machinery, and weaponry of war. We've seen this process at work here in New York, where, during the 2004 Republican Convention and the Occupy Wall Street protests a couple years ago, the N.Y.P.D. deployed some fearsome firepower. We've seen heavily armed SWAT teams carrying out drug raids all across the country. And now, in the suburbs of St. Louis, a grand old city, albeit one with a long history of racial segregation and divisions, we are seeing cops who look like troops clad for battle.

Of course, the militarization of the police is not entirely new. SWAT teams date back at least to the late sixties in Los Angeles. During the eighties and nineties, many big police forces armed their officers with automatic weapons, and, partly to prosecute the war on drugs, some police departments acquired some pretty heavy weaponry. But it was 9/11 that really changed things. Under the guise of beefing up their anti-terrorist operations, police forces across the country acquired all sorts of military uniforms and hardware, sometimes using federal grants to pay for them.

Radley Balko, a reporter for the Washington Post who last year published a book titled "Rise of the Warrior Cop: The Militarization of America's Police Forces," points out that much of this new gear has ended up being used not to interdict terrorists or drug kingpins but against low-level miscreants and suspects. "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 V.F.W. halls where illegal gambling was suspected," Balko has noted. "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)."

Because most Americans don't participate in political protests, read books about militarization, or get mixed up in police raids, they don't really notice what is happening. When they turn on their televisions and see "policemen" in military fatigues and gas masks pointing assault rifles at unarmed protesters, they are shocked, and rightly so. That A.C.L.U. report concludes, "The militarization of American policing has occurred with almost no oversight, and it is time to shine a bright light on the policies, practices, and weaponry that have turned too many of our neighborhoods into war zones." Cops should be cops—not combat troops.