The threat to such a tribe is any other group that feels they can take what that tribe has at a reasonable cost. The problem is that figuring out who can take

The threat to such a tribe is any other group that feels they can take what that tribe has at a reasonable cost. The problem is that figuring out who can take what from whom directly using force ends up with a lot of dead, and it ends up killing a lot of the farmers and artisans that ultimately are producing what you are fighting over. The argument for federation or centralization is that you formalize the systems of stealing based on measures of force that involve less death and destruction than just having a battle every time your clan thinks it's stronger than the clan down the way.