What are you talking about? Maybe at very high levels a band of PCs can singlehandedly wipe out large armies. Because they're nearly demi-gods by then. But at mid-levels? A fireball is "only" a 20 ft radius. Especially if you're not running the enemy like idiots who think that army battles should be run like medieval Europe with tight formations even though there's magic rampant (not to mention CANTRIPS to massively extend communication range, the primary reason for those tight formations), you're not going to kill more than 0.01% of the thousands attacking you with each fireball. I've also always been fond of entire archery platoons readying actions to "counterspell" with "sun-blocking rain of pointy stick death." Enemies always have at least a 5% chance to hit, which means, consequently, NO, a mid level fighter cannot wade into a giant mass of soldiers and survive, unless he's obtained several-many points of DR. Sheer odds means he'll be skewered to death after a while.
And I've never seen a level 5 wizard with a wand of fireballs. Those things are expensive as hell! And if the enemies aren't all level 1, the low save DC of an item + only 5d6 damage means it could even be fairly survivable.
I don't know why people always focus on the blasting spells, they're not great ways to deal with mass troops. Cloudkill and Stinking Cloud would do a much better job over time at killing scores of people. Druid spells like
Spike Growth and
Spike Stones may not auto-death the entire enemy army. But once they realize how much crud they're wading in, and how wide the effect is, they're basically completely immobilized. If you have greater ranged attacks than the enemy side, you've effectively won the battle at that point. Even if you don't, if they don't either, they're likely to surrender. Those are the kinds of spells that MIGHT allow a mid-level party to defeat an army.
Heh, Animate Dead had no limits, huh? Maybe that's how Xykon and Redcloak have so many undead minions -- using 2E rules.
Didn't fireball in previous editions also have a much bigger area of effect? I recall in at least one edition from the comparing fireball thread that the area was so huge it was nearly impossible in anything but a very wide open space to ever use it and not hit your entire party, too. That could potentially make it a battlefield killer. Of course, the earlier you go in edition, the squishier the mage probably gets, so I don't know if he'd want to attract the attention of any survivors like that.