The Federalist points out that all those demanding the "defunding" or abolition of police forces might do well to consider the consequences, which are clearly visible in Mexico.
But let’s say these ultra-progressive municipal governments could get their wish and abolish the police in their cities entirely. What would happen? Inevitably, an armed group would emerge and impose a monopoly on the use of force.
If you want an idea of how that works, look to our southern neighbor, Mexico, where over the past decade endemically corrupt police departments in some areas have been supplanted by autodefensas, or local self-defense militias. But before you get too excited about the prospect of paramilitary autodefensas policing American cities, understand that in Mexico these groups are a mixed bag at best—and at worst they’re not much better than the corrupt local police and cartel gunmen they replaced. More importantly, their mere presence in Mexico was and is a disturbing sign of societal decay.
To understand why, a bit of background is needed. The modern autodefensas movement in Mexico arose during some of the most violent years of Mexico’s ongoing drug war. In 2013, a doctor from the cartel-ravaged state of Michoacán, José Manuel Mireles Valverde, organized one of the first self-defense militias to fight against the Knights Templar Cartel. He initially recruited ordinary men, shop keepers and farmers, to hunt down cartel henchmen and drive them out of their towns.
Initially, these ad-hoc militias met with some success, capturing or killing members of the Knights Templar, setting up roadblocks and ambushes, and expanding the number of militias operating throughout Michoacán. But as the violence in the region increased, the militias eventually caught the attention of the Mexican government, which deployed the military against both cartels and autodefensas ... By then, the line between autodefensas and cartels had begun to blur. The militias had been infiltrated by cartel members, including former members of the Knights Templar who knew the cartel was losing power.
. . .
... the autodefensas movement quickly went from being an organic uprising against a vicious cartel to a vigilante free-for-all ... As the government stepped in to control the autodefensas movement, it became increasingly clear that cartel members were joining self-defense militias, especially in Michoacán and neighboring Guerrero state. Sometimes it worked in the opposite direction. Lacking resources and weapons, self-defense militias would turn to drug cartels for financing, and would later be used by drug lords as proxy forces against their rivals.
Today, autodefensas remain active in parts of Mexico but they have largely melded into the ever-shifting patchwork of gangs, cartel off-shoots, and corrupt local police forces vying for power and territory. The fragmenting of Mexico’s criminal gangs and armed groups has helped fuel rising violence in recent years, with this year on track to break last year’s record for homicides. As far as violence and corruption go, things are worse in Mexico now than they were when Mireles formed the first autodefensa group.
That is to say, the rise of self-defense militias in Mexico, no less than the rise of cartels, is a direct result of the collapse of civil authority. Absent a functioning state, militias are no more accountable to the general public than a drug cartel—and no more capable of resisting corruption than the local or federal police.
There's more at the link.
That was pretty much my experience in large parts of sub-Saharan Africa, too. As the authority of the state and/or local government waned during periods of anarchy and civil unrest, local gangs, tribes or other groups would take advantage of the "power vacuum" to seize control of their own areas. They would levy "taxes" against the people to fund their operations (in reality, organized looting on an ongoing basis), and terrorize anyone who refused to pay, up to and including rape, torture and killing. If the "ordinary people" organized to oppose them, the opposing force would rapidly become corrupted by precisely the same temptations that had attracted their oppressors. Once entrenched, such local power players could only be dislodged by superior force - never by reasoning with them. They were making too much money (by local standards) to be willing to give it up, and nobody else in that impoverished continent had enough money to offer them a bribe big enough to stop.
Another part of that problem was that many merchants and other vendors simply refused to deliver supplies to the region(s) concerned. They became food deserts, with the only exception being foreign aid (mostly stolen by the groups in control, and sold in local markets) or subsistence agriculture. I can tell you right now, if BLM or other groups take over local city suburbs, the big stores in and near them will simply close their doors, rather than be robbed on a daily basis. That will lead to the activists (a.k.a. thugs and looters) trying to extend their activities into areas still well supplied, which will in turn provoke a violent reaction from those in the latter areas, trying to protect what they've got. Since "the best form of defense is attack", to quote a well-known saying, they'll probably take the fight to the activists in their own areas, too. In the absence of effective policing, who's going to stop either side?
That's your recipe for at least a localized civil war, right there. Don't tell me it won't happen. It will. I've seen it before, far too many times for comfort. It'll happen here, too, if we create conditions favorable for it.