Well, OF COURSE the Garleans had a reason to start their conquest. Probably one they could justify to themselves, like perhaps their original land lacked the natural resources to feed their own people, but their neighboring nations did.

That doesn't justify Garlemald's current leadership treating anyone not Garlean as slave labor, cannon fodder, or a trophy to win. I'm not even touching the whole sex slave issue, but it needs to be mentioned that it's a thing that exists!

So, while I don't think Garlemald's entire pureblood population is evil to the core, nor that we couldn't find some allies who stay on the side of Garlemald, it's ABUNDANTLY clear that the people in charge of the nation think their atrocious acts against humanity are justified, even if only because that's what they've been taught their whole lives.

War is never simple.