@GreivousKhanMight we combine the reasons? Perhaps the Eloians inadvertently insulted the Thangors in a misunderstanding, and one by one my people conquered the glades to both gain resources and deprive the Eloians of them?
I can see that happening, heck we could even have it the other way in the Thangors unknowingly insulted the Eloians after joking about riding one. Goldeagle's brand of comedy is probably wearing off on me. But yes, the insult could have easily sparked the start of the dispute, with skirmishes slowly snowballing the scope of the problem until it became a full-scale conflict. How does that sound?
Eventually forcing the Eloians into the desert, would mark the end of the war. A century would go by with the Eloians holding onto the grudge to some extent and probably seasonally raiding into the glades now and again. Which will shape them excellently and gives me a great number of ideas of how to structure them. This has been very helpful thank you.