In a high-magic D&D setting, the destructive spells have (at least potentially) been around almost as long as war itself.
Actually, that would vary from campaign to campaign. Much like RW military technology, the Art would improve at an increasing pace. Firearms were invented hundreds of years ago...but repeat fire weapons are relatively new. And now we have experimental weapons like Metalstorm that can fire 1 million rounds per minute.
2000 years ago in D'arcaniaa, the most powerful magics might have been Enchantments and Illusions. Fireballs might be only 300 hundred years old. Wish might only have been discovered 80 years ago.
On the other hand, in a higher-magic campaign, Og might have only discovered fire after he discovered Pyrotechnics.
So what I'm curious about is, how would military tactics have developed under those circumstances. (Or, to continue the analogy, how would real-world tactics have developed if the first warring tribes/nations already had access to howitzers, sniper rifles, and air support?)
Well, tactics are dictated by weapons, though it may take some time to learn what the best tactics
are. The Tank was introduced in WW1, but it took Hitler's Blitzkrieg to show the value of massed armored units with infantry support.
So what would have happened is that the tactics could have evolved more quickly than they did, resulting in sophisticated modern warfare tactics in an age just discovering steel...
or spellpower could have been so overwhelming that people wouldn't have figured it out for a while (mirroring RW technological pacing).
By that I mean with powerful magics being available early in human history, every skirmish becomes a potential Pearl Harbor/Dresden/Hiroshima. Real tactics might not evolve for some time because there wouldn't be a tactic better than "Strike hard, first" until someone figured out a counterspell, or tried to "judo" an enemy spellcaster...like by setting up a "Potemkin" village for him to exhaust his spells upon, then killing him, then counterstriking.
One thing is for sure, though...Castles should DEFINITELY look very different in a fantasy world than in the RW. In a world where Dragons exist, the modern hardened bunker would have arisen much sooner...and Dwarves would be getting REAAAALY wealthy taking strongpoint design comissions.