The monopoly on the use of force is quite the important part of having a state at all. If a state doesn’t have that, it descends into anarchy (the bad kind, with warlords and gangs). The US is very exceptional in this case as it has in its constitution the provision that such gangs (militia) are allowed, even desirable.
It’s basic state theory, I believe we had that in middle school, along with the division of power. I mentioned the US exception because if you went to school there, your basic state theory might have been different from mine.