Now why would D&D armies bother trying to get to a battlefield to begin with?
A few 5th level wizards could decimate the supply lines of an army at will (if you have your spellcasters protecting teamsters, then they're not on the front lines - besides, fireballs fired from 400' are kind of hard to spot in time and counterspell), burn bridges and destroy villages, while their 9th level boss could skip around starting fires in cities or ports and terrifying people with poison gas...
The real issue isn't the power balance between an army of 1000 grunts and a party of 10th level adventurers - it's the nature of the magic system.
Core D&D magic, except on a personal level, is all offense and no defense. (and even on a personal level, the attacker generally has an advantage, because the best defensive spells are too short-lived to be in use constantly)
There is nothing in the core rules to stop teleporting, scrying your enemies is ridiculously easy, there are no easy ways to protect structures or cities from fire, and there are no spells at all that can provide meaningful logistical support - the wizard, cleric and a few other people have perfect mobility and unlimited food created out of thin air, but the grunts still need to haul around their supplies the old-fashioned way, and the supplies are still produced by large numbers of defenseless villagers...
No war is going to get off the ground when it's this easy to destroy supply lines and slaughter the people who produce your food and make you the money you use to pay for your army.