My idea is that its nearing the end of WW1, the central powers are falling apart and most of the allies are worn out. Then Dracula and other vampires rise up in some of the weakened countries and take them over with their own armies of monsters and undead soldiers. 

The world, drained from fighting a global war, now has to stop Dracula and his generals from taking over Europe.

Does that sound neat? Realism fights Fantasy.