Much as I want to roll my eyes over this prehistoric race war, there's zero doubt that it happened when modern humans came into contact with Neanderthals. It's not hard to imagine at all. Plus, a modern person is clearly at the head of the Neanderthals, and one with military experience. Dehumanizing the enemy is the oldest war tactic there is, and when the enemy is a different color, well, it's pretty easy to do.