Jody Armour, a University of Southern California law professor specializing in police use of force, said the danger is that aggressive use of military-style equipment could undo the progress many departments have made through community-based policing, in which officers work to establish bonds and solve problems with the people on their beats.
He said many Americans may not have concerned themselves with increased police use of "military trappings," since the growth has coincided with a drop in crime.
"What we have (in Ferguson) is kind of a wake-up call," Armour said. "Nobody likes to see how sausage is made."
Armour said it's tough for smaller departments to resist the offer of free weaponry. "How are you going to say no to high-powered ammunition, vehicles and the like, when the government is offering this gratis?" he said.
Unintimidating
Not all the equipment distributed to police in the Bay Area could be used in confrontations with drug suspects or demonstrators. The Alameda County Sheriff's Office obtained its 85-foot former Coast Guard cutter in 2005 as part of the Pentagon's surplus program and uses it for marine patrols at the Port of Oakland and near the airport. Sgt. J.D. Nelson, a sheriff's spokesman, conceded that the agency doesn't use the cutter often. But "when you need it, it's perfect," he said.
Other towns have also found few occasions to deploy their military equipment. In Concord, police have used their South African-made Mamba armored personnel carrier only once since obtaining it from the Pentagon in 2012 - to approach an armed man who had holed up in a house after being wounded in a confrontation with officers.
"We used Mamba to drive up close to him and take him into custody," said Concord Police Chief Guy Swanger. "The goal is to resolve situations safely, in lieu of our officers' being exposed."
He added, "For the purposes of an urban police force, or a suburban police force like Concord, it is strictly there for officer safety and hostage recue situations."
Kids love it
Mostly, he said, the military vehicle has starred at community gatherings such as the Fourth of July celebration. "We have it in parades, we have kids crawling all over it, taking pictures with their parents - we allow kids to sit up in it," the chief said.
"I think the community has embraced the use of it," Swanger said. "They know why we have it."
Ben Tisa, a use-of-force expert who retired from the FBI in 1995 and now teaches law enforcement tactics involving armored vehicles, said police once used converted bakery trucks or bank armored cars to transport SWAT teams. Now, he said, the key to effective use of the new vehicles - many hardened against explosives- is restraint.
In crowd control, "sometimes the vehicle may be a forceful presence" and "an intimidation platform to deliver" tear gas, Tisa said.
Holding in reserve
But, he said, the agency needs to understand how such a high-profile move will be perceived in a tense situation.
"It could be seen by the crowd if it is deployed right up at the line" as inflaming the situation, Tisa said. "Sometimes it is better held in reserve so it is not an up-in-your face response."
Richmond police Chief Chris Magnus agreed that keeping armored vehicles "behind the scenes and out of sight" is prudent.
"When you put everything out there, it really almost sends the message that you're looking for trouble," he said. "I get the argument that you're trying to send the message that you're not going to tolerate that behavior. But unfortunately, it also sends the message that you're looking to take some form of more aggressive action."
