Ah, my mistake, I've misunderstood "Days later, the Light Ones occupying the lands of the five eastern tribes left, leaving the tribes to fend for themselves as they settled along the lakes and lived peacefully with the Lairreen.". I was under the impression they left to the East and not actually settled there.
Okay then, that would change things.
I think a massive force like the Light Ones could have been the catalyst for the formation of a tribal confederacy in case they return and to offer every tribe, great and small, the protection they wouldn't be able to get on their own. This later kept growing into what they have today, through tradition and common benefit.

And just because they don't need an authority to tell them what to think and how to feel doesn't mean they have a hive mind. They can recognize property value, they can coordinate, organize, and they're [u]not human[/u]. They won't behave like humans and they won't have the same priorities, as I'm sure your nation won't either.
The driving away of others is not particularly hard. You merely have to drive them off from the bountiful places over and over until their choice is to either leave or starve.

As an aside, I don't plan on the entire territory being nothing but steppe. I'm feeling like there should be a taiga transitioning into tundra in the North, so it's more gradual than the abrupt white on the map.