A State in theory is, or has, a “monopoly on force,” but it’s more accurate to simply call it a “monopolist,” in that they think and act as if they are a “monopoly on force,” the only ones entitled to enforce rights, but can never actually attain this “monopoly.”
Statism is the idea that there should be only one agency “allowed to do force,” generally “legitimate force,” and/or the State is the only one with the “right to enforce rights,” or to do “legitimate force.” Of course, this is impossible, as everyone obviously has the right to do legitimate force.
In acting as a monopolist, a group claiming to be the State is violating everyone’s rights, by threatening any “vigilantes” enforcing their own rights and contracts, even if they followed all rules of evidence and due process.
Neither does it ever seem to be the case that the State is the only one “doing force,” legitimate or otherwise. The logistics and probability of such a situation are absurd, though I suppose not impossible.
At this point at least, the State clearly has no “monopoly on force.”
More accurately regarding the use of force: the belief is that the State is “Authority,” is speaking for “Authority,” or is wielding “Authority” — that we are morally obligated to obey their commands– and the force they do is legitimate if they say so, because they have the right to rule (authority).