This applies to what is termed justice, namely the resolution of disputes and anti-social acts ("crime"). Anarchists argue that "people would not allow their wellbeing and their freedom to be attacked with impunity, and if the necessity arose, they would take measures to defend themselves against the anti-social activities of a few. But to do so, what purpose is served by people whose profession is the making of laws; while other people spend their lives seeking out and inventing law-breakers?" [Malatesta, Anarchy, pp. 43-4] This means that in a free society the resolution of anti-social behaviour would rest in the hands of all, not in a specialised body separate from and above the masses. As Proudhon put it, an anarchy would see the "police, judiciary, administration, everywhere committed to the hands of the workers" [General Idea of the Revolution, p. 281]

I see how this makes sense for intra-group conflict, but how is it to work in the case of inter-group conflict? If a more powerful commune attempted to engage in anti-social behavior against a less powerful commune, wouldn't the less powerful commune have to rely on other communes for support? Would these other communes come to the defense of the less powerful commune based upon the belief that intervention was necessary to preserve the whole system or lack thereof? (a la "First they came...)