Posse comitatus is not a phrase that trips lightly off every tongue. It is typically translated from Latin as “force of the county.” Anyone who has ever watched an old Western movie will instantly recognize the first word as referring to men deputized by the sheriff to chase down some varmints who went thataway. (Rappers and their tag-alongs later gave “posse” a different context.) The full phrase is more obscure, but the concept that it embraces is enshrined in American law. The Posse Comitatus Act, passed in 1878 at the end of Reconstruction and amended but slightly over the decades, prohibits the nation’s armed forces from being used as a police force within the United States. Soldiers, the reasoning goes, exist to fight wars. Chasing local wrongdoers is a job for cops.

But many police departments today are so heavily armed with Pentagon-supplied hand-me-downs — tools of war like M-16 rifles, armored trucks, grenade launchers and more — that the principle underlying the Posse Comitatus Act can seem as if it, too, has gone thataway. Questions about whether police forces are overly militarized have been around for years. They are now being asked with new urgency because of the recent turmoil in Ferguson, Mo., where unarmed demonstrators protesting the fatal police shooting of a teenager faced off for a while against mightily armed officers in battle dress and gas masks. What the world saw were lawmen looking more like combat troops in the Mideast than peacekeepers in the Midwest.

The militarized nature of modern American policing infuses this first installment of Retro Report, a weekly video documentary series that examines major news stories from the past and explores what has happened since. The focus this week is on SWAT teams, whose numbers have soared across the country, in rugged cities and in sleepy towns. They are the principal beneficiaries of the heavy-duty military equipment that the federal government has supplied since the early 1990s, in a transfer program that has gained steam in recent years with the withdrawal of American ground forces, first from Iraq and soon from Afghanistan.

The video traces the rise of SWAT units from their earliest days in 1960s Los Angeles. There, Daryl F. Gates, who would later become chief of that city’s police force, championed a sturdily armed squad of trained officers as an essential tool of law enforcement after the deadly Watts riots of 1965. Mr. Gates fancied the name Special Weapons Attack Team. “Attack” made some elected officials wince, though. What emerged instead was Special Weapons and Tactics — same acronym but sounding somewhat less aggressive.

Los Angeles’s SWAT team tested its mettle in 1969 against a local Black Panther militia and again in 1974 during a fierce firefight with theSymbionese Liberation Army, a bizarre but dangerous band of radicals best known for having kidnapped the media heiress Patricia Hearst. Its bona fides thus established, SWAT units spread across the national landscape, romanticized in song and on television.

To these units’ defenders, the need could not be more fundamental: The world is dangerous. Some drug lords have weaponry that would be the envy of small armies; the police cannot possibly take them on with mere handguns. Terrorism lurks as an ever-present threat. And sudden menace demanding a well-armed police response can arise even in the most tranquil places. Indeed, the roster of place names identified principally with gun horrors has grown long: Newtown, Columbine, Aurora, Virginia Tech. On and on.

Not surprisingly, critics of militarized policing have a different take.

Some are troubled by what a retired District of Columbia police sergeant, Bill Donnelly, once belittled as “commando-chic regalia.” With all that armored gear and firepower, Mr. Donnelly wrote to The Washington Post in 1997, “one tends to throw caution to the wind.” Another skeptic is Peter B. Kraska, a professor at Eastern Kentucky University interviewed by Retro Report. Professor Kraska has studied this issue for decades. Originally, he said, SWAT deployment was supposed to be reserved for truly perilous situations — hostage-takings, high-powered shootouts and the like. Now, these teams execute routine warrants in “no-knock” drug raids, bursting into homes with a show of force that often far exceeds the threat to them. The number of such raids has exploded from a few thousand a year in the early 1980s to tens of thousands today. Other critics, like the American Civil Liberties Union, note a stark racial disparity, with blacks and Latinos more likely than whites to be targets.

In the process, relationships between many police departments and the public they serve are intrinsically altered. Officer Friendly has been replaced by someone looking more like G.I. Joe.

The blurring of distinctions between police and the military has troubled people like Lawrence J. Korb, a longtime analyst of national security policies, who was an assistant defense secretary in President Ronald Reagan’s first term. Mr. Korb was not happy when the Reagan administration, in the early 1980s, loosened some restrictions in the Posse Comitatus Act to enable the armed forces to get more involved in the domestic “war on drugs.” His objection was encapsulated in a 1997 interview with The Dallas Morning News. “The military is much more likely to use force of arms because that’s what they’re trained to do,” he said. “The military, to put it bluntly, is trained to vaporize, not Mirandize.”

Over the last two decades, SWAT units have become ever more heavily armed. Under the so-called 1033 Program, named for a provision of the National Defense Authorization Act, the federal government has transferred vast amounts of military equipment — machine guns and ammunition, helicopters, night-vision gear, armored cars — to local police departments. The process accelerated after the Sept. 11 attacks, under both President George W. Bush and President Obama. Inevitably, some people, including police chiefs, have asked if all this amounts to a solution in search of a problem. Take the transfer of MRAPs, the military term for mine-resistant ambush-protected armored vehicles. How many minefields are there on America’s Main Streets?

Also inevitably, mistakes are made. A wrenching example is captured in the Retro Report video, involving a 19-month-old boy who was critically injured in May when a SWAT team in Georgia fired a stun grenade into a house that was the target of a drug raid. The officers were searching for their suspect in the wrong place. Their grenade landed in the infant’s crib.

There is yet another inevitability. After all that happened in Ferguson, a backlash against militarized policing has gained force. In late August, Mr. Obama ordered a review of the equipment-transfer program. Senators said they would hold their own hearings this month. It is much too soon, though, to tell if this longstanding law enforcement strategy is truly about to go thataway.