Hey, I kinda got an idea about how the war started. How about one day a technology user was like experimenting with something new, and then it backfired and ended up accidentally killing a magic user child (or more than one idrk). And then because of that the people were outraged, and first the city, then the entire world split into two factions over such a small thing.

In fact, its something so small that many people involved in the war don't know how or why it began, but they were raised through the ages with prejudice and hatred towards one another.

And then there could be some underlying like 'evil' that wants the fighting to continue and spurs it on for whatever their evil purpose may be? Something like that.... idk if it's a good idea but yeah.