Tribes are wandering groups who live of what nature provides within a given area. They don't cultivate that area, so they rely on moving around so that nature can replenish while they aren't there. They fight with other tribes to protect that area, because there are only so many resources and the area won't support another tribe. Tribes are more comparable to animals in behaviour.
A community is static and they work the land, continually replenishing in order to support themselves. The neighbouring community will be doing the same so they have no reason to fight, in fact they are more likely to trade with each other.
except that most resources are also static and that not all land is equal one community nay have a dire need for water but no reasonable access, another neighbouring community may feel that while they have ready access to water that it in only enough to meet the needs of their community and they have no ability to share that does not damage their own community.
How long before conflict over the resource happens even with the best intentions of both communities.
You can replace water with any resource that is useful to the advancement and survival of a community.