[@Letter Bee]

Ok cool, so basically the idea here is the Yemarae are working on an [i]old[/i] oral history.  They literally see everyone around them as twisted aberrations or traitors because to them all of humanity was born from the carcass of Raehe, and they are the original children of Raehe.  The shamans have a substantial grip on the culture of the tribes, and they use ancient oral histories, perhaps dating back thousands of years, to reinforce the idea that the Yemarae are peaceful and just and the rest of the world has been taken from them by the wicked. 

Now, with that in mind the Yemarae may have even attacked you and not the other way around.  It wouldn't be anything so dramatic as all the tribes moving west but perhaps from time to time a chief has tried to 'take what is due' or 'cleanse the wicked' or what have you.