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.