Walter B. Gibson - The Druids and Their Legends
As the Roman Empire spread, so did Roman ideas and traditions, among them witchcraft. It did not just have to go with the Romans; sometimes it was there ahead of them, waiting, willing, and wanting them to catch up with it which, naturally, they did. Often the Romans identified the local gods in terms of their own, for the fact that the Romans were the victors proved that the gods were on their side, and that in itself was sufficient reason for the vanquished nations to accept the Roman rule. In short, Roman domination was furthered by making due allowance for the superstitions of the conquered peoples; and that included witchcraft.