In his piece at the Conversation, Temitope Oriola (Associate Professor of Sociology at the University of Alberta) makes the case that the United States is at risk of an armed anti-police insurgency. While I do think his case might be a bit overstated, he definitely has a point backed up by expertise, so please give it a read. The overall message — that the police as an American institution must be reformed, otherwise violence is inevitable — is incredibly important.
My guess is that a longer piece by Dr. Oriola would better address the complexity of the US as the environment where an insurgency might occur. Various areas of the US are different enough and geographically separated enough that there can’t really be a national insurgency, but there can be multiple insurgencies in various locations that are primed for it, and those insurgencies can be supported (at least in spirit) by Americans who live far from them under very different circumstances. That might be the conditions under which meaningful police reform at the national level can occur.
In terms of the question of whether a group of armed citizens can meaningfully challenge the police, the answer is yes. Even in municipalities with an extremely high ratio of police to citizens, citizens still dramatically outnumber the police (no more than 7 officers per 1000 people). The question is how many simultaneous insurgencies are we talking about, and is that enough to deplete the state’s ability to reinforce the besieged police departments with reinforcements from the National Guard (for example). The fact that the police can so easily be overrun and that society as a whole can be so easily disrupted has been a core component of white anxiety since the time of slavery.
The US is also at risk for an armed, right-wing rural/white insurgency.