On what made war “holy”

Well, in the ancient world, because we do not have the sacred/secular split with which we are so familiar nowadays, every war really in a sense was a holy war – in the sense that the gods were entirely bound up with the king and the city-state and so on. So the notion that when you went to war the gods went with you, and the gods were the ones involved in the fighting – those ideas were pretty broadly entertained in the ancient world.