Well, obviously, one factor is internal cohesion and loyalty. If two groups of about the same size and level of complexity compete for the same turf, or get into a fight for whatever reason, the winner is likely to be the one with the greater internal cohesion. Another factor is sheer size, and level of complexity. Other factors being equal. Depending on the nature of the competition, it may either eliminate the smaller group, or absorb it, or let it continue in a subordinate role.
History is full of examples.
These two factors explain much of human history. Culturally defined groups rise and fall, expand and contract, live in peace or go to war, but always the trend is toward larger and more cohesive units. This means in practice less tolerance not more, for the other group's views and interests. The end result of this selective evolution can be seen in places like Kosovo and Rwanda. Think back to our original point, the great evolutionary threshold that occurred when the Neolithic started.