Officers with assault rifles, backed by a huge mobile grenade launcher, square up to a crowd furiously denouncing the killing of a young Black man. The scene could have been taken from archive footage of Mogadishu in 1990s Somalia or countless other battles, but this conflict is closer to home—the streets of a small U.S. town.

The images, captured in Ferguson, Missouri in 2014, open "Do Not Resist"—a disturbing documentary charting the transformation of police across the United States into forces that look like military units.

The explosive film is set to fuel an already bitter debate raging in the country over heavy-handed law enforcement following a litany of police killings of Black and brown people that has sparked protests from Ferguson to Charlotte to Chicago.

Director Craig Atkinson, whose movie is opening across the U.S. after winning Best Documentary Feature at the Tribeca Film Festival, says the U.S. law enforcement ethos has changed "from a mentality of peacekeepers to that of an occupying army."

Another eye-opening scene shows officers in black fatigues firing volley after volley of automatic rounds at cardboard targets, as if they were preparing for war rather than to "protect and serve." A khaki-clad instructor explains that security forces must prepare for all kinds of attacks, "including the Islamic State."

MRAPs, an acronym for Mine-Resistant Ambush Protected, are the armored trucks that protect troops from roadside bombs planted along the dusty roads of Iraq and Afghanistan. These heavy vehicles are now ubiquitous across the country. They are provided by the Pentagon through its 1033 Program that allows for surplus giveaways that have amounted to US$5 billion since former President Bill Clinton signed off on it in 1996.

In another shocking scene, a SWAT team arrives in an MRAP to a tree-lined street in Columbia, South Carolina, to execute a search warrant in a drug case. The officers, whose equipment looks barely distinguishable from that of an infantry division, end up badly damaging a family home in a raid that nets a small amount of loose cannabis.

Atkinson's father, a retired police officer from Detroit, Michigan, spent over a decade in one such SWAT team, the New York-based filmmaker explained, "In his time, his team intervened 29 times in 13 years. Now they are taking part in 200 raids a year," Atkinson told AFP.

Faced with a growing criticism of police killings, particularly in Black communities, Terrence Cunningham, the president of the International Association of Chiefs of Police, admitted that law enforcement officers have been "the face of oppression to far too many of our fellow citizens." He told a conference in San Diego Monday that police needed to apologize for "the actions of the past and the role that our profession has played in society's historical mistreatment of communities of color."

Peter Kraska, a criminology professor at the University of Eastern Kentucky, says there are now at least 50,000 SWAT raids a year, up from 3,000 in the 1980s. Most of the activities of these highly specialized units have little to do with the reasons for their inception, such as dealing with hostage situations, terrorist attacks and drug cartels.

According to Atkinson, one of the architects of the "warrior culture" is a hugely successful police trainer named Dave Grossman, head of a consulting firm called the Killology Research Group. "We are at war and you are the frontline troops in this war. What do you fight violence with? Superior violence," Grossman hollers at an audience of mesmerized police in one session filmed for the documentary.

The retired army lieutenant colonel has lectured throughout the U.S., according to his website, and Atkinson believes his influence has spread to every U.S. law enforcement agency.

"There are 63 million police interactions with citizens each year in the United States, and if we take that kind of mentality during a routine traffic stop, that's how there are people getting killed," he said.