i think spells like alter self, charm person and teleportation (not to mention self replicating undead) would have an exponentially greater effect on the battle field then fire balls and lighting bolts.

the fire balls and lightning bolts due to limited spell slots are not going to have as big an impact on the battle field that cannons and machine guns did.

but illusion, shape shifting and mind control spells, combined with most of an armies raw destructive power being focused on a few individuals means espionage, sabotage, and assassination will be what decides a war.