To understand what's been happening in Ferguson, Mo., where protests and violence following a cop's shooting of an unarmed teenager summoned up a police response that looked more like a military invasion, it helps to flash back to the heyday of the Symbionese Liberation Army.

The SLA, one of the loopiest and most dangerous of the homegrown terrorist groups that flourished in the madhouse of the early 1970s, was already famous for kidnapping and "converting" Patty Hearst when its members engaged in a nationally televised shootout in Los Angeles in the spring of 1974.

The firefight, in which six terrorists died without injury to police or bystanders, helped publicize the innovations of a small group of Angeleno police officers. Eight years earlier, after the Watts riots, they began to develop the combat-ready police unit that played a central role in taking down the SLA. That unit was America's first special weapons and tactics team, or SWAT.

In an era of riots and hijackings, the SWAT model understandably spread nationwide. But as the riots died away and the threat of domestic terror receded, SWAT tactics — helicopters, heavy weaponry, the works — became increasingly integrated into normal crime-fighting, and especially into the war on drugs.

This was phase one in the militarization of America's police forces, as described in Radley Balko's essential 2013 book on the subject, "The Rise of the Warrior Cop." Phase two, in which the federal government began supplying local police with military hardware, began in the 1990s and accelerated after 9/11, under the theory that Islamic terrorists could strike anywhere, and that it might take a cop with a grenade launcher to stop them.

In the name of local preparedness, Washington has been bestowing anti-terror grants and Pentagon surplus on communities barely touched by major crime, let alone by terrorism. Tanks and aircraft, helmets and armor, guns and grenade launchers have flowed to police departments from Des Moines (home of two $180,000 bomb-disarming robots) to Keene, N.H. (population 23,000, murder rate infinitesimal and the proud custodian of an armored BearCat).

Last week, the New Republic's Alec MacGillis ran the numbers for Missouri and found that the state's Department of Public Safety received about $69 million from the Department of Homeland Security in the past five years alone. Which helps explain why the streets of a St. Louis suburb flooded so quickly with cops in gas masks and camouflage, driving armored cars and brandishing rifles like an occupying army. It's our anti-terror policies made manifest, our tax dollars at work.

And it's a path to potential disaster, for cops and citizens alike. The "S" in SWAT was there for a reason: Militarized tactics that are potentially useful in specialized circumstances — like firefights with suicidal terrorist groups — can be disastrous when employed for crowd-control purposes by rank-and-file cops.

To many critics of police militarization, of course, the helmets and heavy weaponry are just symptoms. The disease is the entire range of aggressive police tactics (from no-knock raids to stop-and-frisk), the racial disparities they help perpetuate and our society's drug laws and extraordinary incarceration rate.

Well before Ferguson, this broad critique — long pressed by a mix of libertarians like Balko and left-wingers — was gaining traction in the political mainstream. This is why sentencing reform has a growing number of Republican champions, and why Rand Paul's critique of the Ferguson police was more pointed and sweeping than President Obama's.

The argument for broad reform is appealing; it might also be overly optimistic. To be clear: I cheered Paul's comments, I support most of the reforms under consideration, I want lower incarceration rates and fewer people dying when a no-knock raid goes wrong. But there may be trade-offs here: In an era of atomization, distrust and economic stress, our punitive system may be a big part of what's keeping crime rates as low as they are now, making criminal-justice reform more complicated than a simple pro-liberty free lunch.

But the military hardware issue, the BearCats and grenade launchers and what we've seen unfold in Ferguson — that does seem easy, uncomplicated, clear. Crime rates rise and fall, but crime-fighting is a constant for police; dealing with terrorism and insurrection, however, decidedly is not. Yet for decades we've been equipping our cops as though the Symbionese Liberation Army were about to come out of retirement, as if every burst of opportunistic lawlessness could become another Watts, as though the Al-Qaida sleeper cells we feared after 9/11 were as pervasive in life as they are on "24" or "Homeland."

And this is where it's ended: With a bunch of tomfool police playing soldier, tear-gassing random civilians, arresting journalists and turning Ferguson into a watchword for policing at its worst.

Time to take their toys away.

Ross Douthat's column is distributed by the New York Times News Service.