Luccio doubted how evil Nazi's were
I think it's important that we add the qualifier "at the time."
The original Merlin established the English monarchy. That probably involved mortal wars, but King Arthur is ahistorical.
Vlad Dracula was involved in several armed conflicts. He lost a few. That implies, but doesn't necessitate, supernatural forces on the opposing side.
I'd be supernatural elements were involved in most historical wars to one degree or another. If nothing else, blamps thrive when there's a bunch of death to cover their feeding. I'm sure it's similar for a lot of supernatural predators.
I'd also bet that any large or long lasting historical war is going to draw supernatural elements in as their interests are inevitably affected. Whites, at least with Raiths leading them, would probably push peace or a decisive victory as peace benefits their feeding habits more than chaos. It would be hard for a German wizard to not be drawn into the Thirty Years' War as conflict was so widespread. The question would be more the scale of the involvement than the fact of it.
Another possibility is that parallel supernatural wars take place during the chaos of the mortal wars. Supernatural factions could use the chaos of a war as a cover to move more openly. That alone might be enough to set off a conflict.