Under the formal definition of “Civil War” there have been plenty of conflicts where there was no army on either side. “Bleeding Kansas” is a good example.
In any case, rising tensions would be likely to drive people with similar viewpoints together into self-supporting groups, which would further evolve into militias and eventually “armies”. This is already visible at a small scale, with the likes of Antifa, BLM, and others on the Leftist side, and various Patriot groups on the Right.
The he troubles in the border states that you have described are really what civil wars are like. No flags, soldier suits, etc. The so-called Civil War was really an invasion and conquest of a sovereign state by its neighbor. As I have said before, real civil war has no rules at all except find those people you do not like and kill them without remorse. Are we overdue for one like that? Probably.