World Weavers' Workshop

So here's my (possibly very naive) question about worldbuilding: Do isolated civilizations need a military force?

Background, I'll try to be brief, though that's not my strong-point: This story takes place on a single large continent (think China surrounded by ocean with a small (inhabited) archipelago to the east).
Both civilizations are either very early in their growth, or are rebuilding after a long ago or forgotten disaster. They share a language for trading purposes but travel between the two "nations" is rare. The islanders may visit the mainland four times a year for the traditional major market days.
No one knows whether there are others elsewhere on the planet; if there are, they're likely too far away to be a concern because there's no record/memory of anyone else...

Do I need to have some kind of military training for at least some of the people? These are villages that take their issues to the elders for resolution. They don't have tech much beyond granaries and ... I think I'm trying to make them too simple and not, at the same time. I probably at least need some local enforcers for the larger villages, yeah? (Why'd I decide to do a long ago prequel again? )

Any advice would be welcome!

           

     “Judge tenderly, if you must. There is usually a side
     you have not heard, a story you know nothing about,
     and a battle waged that you do not have to fight.”
     ~Traci Lea Larussa