But any sufficiently large tribe will consist of a hierarchy of smaller tribes. How do you maintain peace between them? Affiliation with the larger tribal group helps, but only insofar as that is a more highly valued tribal marker. In practice, there is something of an alpha tribe which encompasses all the others within some sphere of influence. Government fulfills that role in convenient fashion -- it stakes out a territory where it claims jurisdiction and serves to keep order among the other tribes in that territory. Affiliation with that government is an important tribal marker itself. What identifies government as an alpha is its exclusive power to use force within its territory. Like any alpha, it doesn't tolerate challengers.
Anarcho-capitalism breaks down not at the individual level, but at the inter-tribal level. It tries to replace a single alpha tribe (and in the process, removes a broad affiliation that is useful for helping to maintain peace among tribes) with many competing alpha tribes (i.e., protection agencies) coexisting and using force to maintain order within the same territory. Such a situation is untenable and inevitably leads to territorial disputes that will end in one PA claiming sole authority in a given area. At which point, you once again have government.
Source : https://reason.com/blog/2014/05/15/iowa-student-suspended-for-bb-gun-found