The “hard” definition of a no-go zone is an area of a city that is functionally sovereign, because law enforcement has more or less given up there. Instead, the area operates under some other system of law not sanctioned by government, with local unofficial enforcement structures. People who aren’t part of the subculture or ethnicity of the no-go zone are strongly urged not to go there and would likely be attacked if they did.
The “soft” definition is an area that has high crime, where police response time is typically slow, and where outsiders are advised not to go, at least not at night by themselves. Street gangs often dominate instead, and even sometimes offer protection services to locals. (This last phenomenon was described by Sudhir Vankatesh in his book “Gang Leader for a Day.”)
The evidence suggests hard no-go zones (as defined above) probably don’t exist, at least not in the West. Some news articles claim they exist there anyway, and then other news articles have a field day debunking the first ones, and then pro-immigration people have a field day Twittering about it and calling everyone who believed the stories paranoid and xenophobic.
Soft no-go zones certainly exist, and can be found in almost every city in the world. They have many common names: the hood, rough areas, blighted areas, ghettos, sketchy neighborhoods, etc. But “no-go” is a misnomer, since basically anyone could go there and, 99 times out of 100, not experience a confrontational incident of any kind. So they’re not really no-go zones at all. More like “don’t start a fight there” or “don’t go there and act a fool” zones.
But some aspects of hard no-go zones do exist in some of the soft no-go zones, and I suspect this is what people latch onto when they claim hard no-go zones exist in London or Paris or Stockholm or Dearborn.
For instance there are definitely areas where residents for whatever reason have more faith in, or loyalty to, their own local authority structures than the surrounding government. This was true in many black neighborhoods in the 1970s, and is part of how the Black Panthers rose to prominence. Some ultra-Orthodox Jewish neighborhoods have their own ambulances just for Jews and I’m sure the rabbis and their organizations there have a surprising amount of power.
Of course the most notorious example is Muslim enclaves that seem (to outsiders anyway) to basically be run by local clerics. Maybe cops can still patrol there, and maybe white non-Muslims can live or work there without being attacked very often, and this nullifies the “no-go” label, but operating behind the walls of those neighborhoods there is sometimes concealed a surprising amount of activity inaccessible to–or even sometimes at odds with the interests of–the wider nation. This is what we see from some of the sex slavery operations or terrorist recruitment organizations that have been uncovered over the years.
The above is the picture painted for me by many people who either live in or near those so-called no-go zones, or who seem to have read up on it and earnestly tried to determine whether no-go zones are real. My impression is that the most damning descriptions are simplistic exaggerations, but that those who scoff at the existence of no-go zones are naively discounting a lot of what’s really there.