But are they soldiers or police officers? It's hard to tell, and that's part of the problem.
The militarization of state and local police forces — now outfitted with the armoured vehicles, battering rams and flashbang grenades once reserved for troops — is a rising concern in the U.S. and a trend that experts suspect is seeping north of the border.
"We think it's one of the most alarming trends going on in American policing today," said Tim Lynch, director of the Project on Criminal Justice at the Cato Institute, a Washington, D.C.-based think-tank.
The unrest in the St. Louis suburb is just the latest example of how that militarization can play out in city streets, as heavily-equipped officers face down the members of their communities.
For several days since the fatal shooting of Michael Brown, 18, on Saturday, protesters and police have clashed. Vandalism and looting broke out on Sunday after a peaceful vigil. A store was set on fire.
The next night, police responded in full riot gear. The Federal Aviation Administration briefly shut down the airspace above the suburb as a precautionary measure.
Lynch says a more appropriate response to riots would be officers armed with batons and sidearms, not carrying rifles like an M-16 and donning the battle-zone-worthy camouflage clothing that have been seen in Ferguson.
"The police are reacting in a hostile manner, as if they are trying to intimidate the protesters by their military-type tactics," said Lynch. "The police are making a bad situation even worse."
Paul Szoldra, in a piece for Business Insider, asked "Why do these cops need MARPAT camo pants?", referring to the pattern designed for the U.S. Marine Corps, or MARine PATtern. When serving in Afghanistan as a U.S. Marine, Szoldra says they used big trucks and uniforms intended to project an image as occupiers, but asks when did this became OK on domestic soil?
'Neighbourhoods are not war zones'
The precipitous rise in the use of military tactics and equipment in the U.S. is startling.
Where once only a portion of cities had paramilitary units such as SWAT teams, now it's the majority.
Studies by Peter Kraska, a professor and chair of graduate studies in the school of justice studies at Eastern Kentucky University, show that between the mid-1980s and late 1990s, the percentage of cities of 50,000 or more, like St. Louis, with a paramilitary unit almost doubled to 89 per cent.
Smaller cities, those serving 25-50,000 people, saw an even greater jump – a quadrupling from 20 per cent to 80 per cent.
"These trends would mean little if these teams were relatively inactive," Kraska wrote in one of his studies in 2007. "This was not the case."
Once local police forces are trained and equipped in the ways of the military, they're keen to use it.
There's been a 1,400 per cent increase in police paramilitary deployments between 1980 and 2000, Kraska found. The majority of the deployments were for drug raids in private homes, not the intended goal of a SWAT team, which is emergency situations like hostage takings and shooter scenarios where the extra weaponry might be required.
A report released two months ago on the militarization of police by the American Civil Liberties Union criticized U.S. policing for becoming "unnecessarily and dangerously militarized."
"Neighbourhoods are not war zones, and our police officers should not be treating us like wartime enemies," said the report.
ACLU examined more than 800 SWAT deployments in 2011-2012 and found that most of those deployments, 79 per cent, were to search homes, largely in drug probes.
Botched raids cost lives, money
In fact, it's the U.S. war on drugs (the phrase itself evokes the battlefield) that led local police forces down the militarized path.
The federal government has not only doled out free or cheap surplus military gear so state and local forces can fight drug crimes, it has also handed out grants to help them buy heavy weaponry.
"It was just part of the escalation of the drug war and trying to beef up forces there," said Lynch. "We saw it even before 9/11, and then 9/11 and the wars exacerbated the trend."
Fears of terrorism fuelled even more interest in bulking up local police officers, and the wars in Iraq and Afghanistan provided surplus machinery and equipment that the federal government was keen to sell or clear out of their warehouses.
Some local police chiefs have argued that the heavy equipment was necessary as they faced criminals with increasingly advanced weaponry.
But community members and police forces themselves have both suffered fallout from the militarization of police.
Kraska counted more than 275 botched SWAT raids on private homes., while Radley Balko, author of Rise of the Warrior Cop, said he found more than 50 cases of innocent people who died. ACLU says minorities get disproportionately targeted.
Those mishaps not only fray relations with a community, but can prove costly for small police forces as they face lawsuits or are forced to disband their SWAT teams after coming under increased scrutiny.
As one small example, in early August, the Albuquerque Police Department decided to get rid of its 20,000-kilogram Mine-Resistant Ambush Protected (MRAP) vehicle – the bomb-resistant mode of transport used in Iraq and Afghanistan against insurgents.
The move followed criticism about over-militarization and a fatal police shooting of a homeless man. Still, the force hasn't given up hopes of finding a more civilian-suitable replacement.
Blurred lines between military and police
It's important to distinguish between the military and police, justice experts say, and look at why it matters that the line between the two is blurring.
"The military mission is to find the enemy force and destroy it," said Lynch. "They're not thinking about the constitutional rights of the people on the other side of the battlefield."
By comparison, a police force is expected to use minimal force to bring suspects into the court of law where the dispute can be peacefully resolved.
York University professor Margaret Beare, who studies policing, says it's "the whole notion of working with communities, not against communities" that should prevail.
The Toronto-based professor worries that the militarization of police forces that's been happening in the U.S. over the past couple of decades is bound to creep into Canada.
"We seem to want to, in policing at least, to mimic the toys, the tools, the machinery that is available to police on the other side of the border," said Beare. "It doesn't seem to matter whether our crime situation is comparable at all."
The sweeping measures taken by police officers during the G20 in Toronto four years ago, including the riot gear worn, the kettling tactics employed and the mass arrests conducted, are still the subject of lawsuits wending their way through courts.
The impact, however, can stretch beyond singular events like the G20. Once police forces are armed and trained in military tactics, some fear they tend to default to them.
"It becomes part of a mindset," said Beare. "It is accepted as part of how you go about doing police work."