Zeref keeps bring up the war between a Dragon, an Immortal and Mankind. It seems to me like he is misinterpreting the current state of the war. It seems like the real war is Acnologia vs Zeref vs Mavis. The thing with these three is that none of them seem to have any way to interact with annother.
Acnologia and Zeref both live in the physical world, so they cannot attack a ghost. And Mavis has no ability to use magic of her own.
Zeref does not seem to have dragons slayer magic, so Acnologia is immune to all his attacks.
Zeref seems to have created/allied with Obra as a way to nullify all magic attacks, which makes all of Acnologia magic unless agents Zeref.
So it would seem, all three of them need to ally with humans in order to gain an advantage. -- And their own power is pretty much irrelevant, am I alone in on this thought?