From The Federalist:

Our executive vice president at Security Studies Group, Dr. Brad Patty, wrote about the history and utility of the militia last year. This part is particularly relevant to the many unsubstantiated claims about citizens taking action. Much more likely is when citizens come under attack by terrorists, insurrectionists, rioters, arsonists, or looters.

In that case, citizens are very likely to be the only force capable of responding in defense of the common peace and lawful order, at least for a short time. In the recent crisis, however, we have seen several occasions when the police vanished from afflicted areas of cities for a whole night or longer. Citizens who are left to themselves by a failure of state and local power have every right to defend the common peace and lawful order against those who would destroy it.