There's two parts to this:

1. How do you prevent the people with arms in your community from banding together and taking over? Well, I'd imagine you'd spread defensive readiness across different sectors of your community, embedded within the populace. You would complement seasoned volunteers and appointed or elected leaders, probably with some measure of training for all adults. Ideally you would rotate your soldiers across communities too.

2. How does a society prevent a different society from gathering enough strength to conquer them? I don't know, that's an open question. Once any society has figured this out we can talk.