“Do Not Resist” stops in Ferguson, Mo., after Michael Brown, memorialized above, was shot and killed by a police officer. Police in riot gear and armored vehicles later confronted peaceful demonstrations. (VANISH Films)

It’s no secret that American police departments have turned into mini militias, armed with all of the most destructive toys taxpayer money can buy. The documentary “Do Not Resist” examines the phenomenon through seemingly unrelated vignettes, many of which are unexpectedly quiet. For instance: After we’re transported to Ferguson, Mo., where police in riot gear tear-gas a peaceful demonstration following the death of Michael Brown, the movie captures community meetings, police training seminars and vast tracts of land filled with armored vehicles as far as the eye can see.

It would be hard to choose which of these scenes is the most indelible, but a top contender is when a SWAT team in Richland County, S.C., swarms a house to execute a search warrant. They do as much damage to the windows as they can on the way in — as a “distractionary tactic,” one explains — then handcuff everyone they find. The ensuing search uncovers a small amount of marijuana. The young black man charged with possession is a college kid doing landscaping on the side, and he asks a seemingly friendly officer to deliver the $876 in his pocket to his boss. “Tell him to get the Stihl weed eater,” he tells the police officer, who blithely decides to seize the cash instead.

A troubling portrait emerges, which is all the more interesting considering that first-time documentarian Craig Atkinson (the cinematographer from “Detropia”) is the son of a police officer who served on a SWAT team for more than a decade. Atkinson made this movie to understand how and why police work has changed so much since the “war on terror” began.

We get quite a few explanations because of incredible access to police officers, government officials and independent contractors. At one point, the camera takes us into a seminar with prolific law enforcement trainer Dave Grossman, a retired Army lieutenant colonel, who tells recruits, “Violence is your tool. . . . You are men and women of violence.” Coming across as a creepy, bloodthirsty Tony Robbins, he also explains that following a near-death encounter, they’ll enjoy the best sex they’ve had in months. His books are required reading for the FBI Academy and many police academies, the movie informs us.

Even without the guidance of narration or a single story arc, it becomes clearer and clearer that the war on terror has unwittingly spawned another war: between police officers trained to fight like soldiers and the people they’ve sworn to protect. Not every cop has it out for the little guy, of course, but the guiding attitudes at the departments featured in the film leave no room for compassion.

The movie feels sprawling at times, especially when it delves into the future of technology. The revelations are shocking: One man describes drones programmed to decide who lives and dies, or data that can forecast the malfeasance of an unborn child. But these stories don’t seem to fit with what we’ve seen before.

Still, the movie, which won the award for best documentary feature at the Tribeca Film Festival, calls attention to a trend that has led to some startling societal changes. At one point, FBI Director James B. Comey gives a speech in which he responds to the criticism of “warrior cops.” He defends the militarization of police forces, because “monsters are real.” They are — there’s no question about that. As “Do Not Resist” so effectively shows, some of them might even be in uniform.

Unrated. At AFI Silver Theatre. Contains disturbing images and strong language. 72 minutes.