I'll make the argument in a main reply.
What is the origin of the non-aggression principle? I personally rely on a construction of Ayn Rand's objective rights theory.
Basically, reason is the means by which man survives, and so reason is the means by which man must deal with other men (simplification). Not force.
Dealing with someone by reason requires parity as far as 'knowledge' is concerned. If you want to treat someone rationally, not with violence, you have to be aware of the rules they have chosen to follow when it comes to how THEY are going to react to you. What is not important to you, may be to them, and what both parties intended to be a rational interaction turns into a firefight. A simple, if imperfect, analogy is mineral rights. To some people, they exist, to others there may be no awareness of them.
Okay, so anarchists would admit that there does have to be a common law of sorts. In fact the need for a common law comes out of what I just argued.
Where the state is necessary is when you introduce the constraints of space and time to information as it concerns common law. By the time everyone decides on what consensus they'll have over which militia will go arrest a criminal, which law will judge him, and which judge will pass ruling, there could already have been a firefight.
A state is a monopoly on law and force for a given time over a given territory. It's not meant to be permanent, and should be subject to popular consent. However, its monopoly status is what gives it the ability to smooth out the constraints of space and time on information. This way, no one's dying because of a misunderstanding.
The argument for a state does end at economics. Scarcity of information does produce unintended economic consequences, but these don't inherently involve violent brawls with the irreversible result of death.
So, a state should be limited to a monopoly on violence and law concerning violence.
It's long, but I made a sort of podcast of this theory here:
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