The vast majority of world building is for the amusement of the world builder. The players will never see most of it.
I think this is more than a little unfair, but not totally wrong. Many DMs
do engage in worldbuilding purely for the fun of it, without really considering whether the players will interact with it or not. But many of them do so because of the influence of Tolkien, a juggernaut of world-building, because they know much of the joy of reading the Legendarium is that it
is so thought-out and, if not "complete," then at least very comprehensive. As was said above, worldbuilding can be like bay leaf, or like some of the more subtle forms of graphical processing. You don't necessarily notice its
presence, but you definitely notice its
absence.
I've done more than a bit of worldbuilding (as anyone who has heard me blather about
Jewel of the Desert will know!), but I try to keep it on things that are relevant. Are there other cities in the Tarrakhuna? Sure! We've never been to 90%+ of them, and have no reason to go to them, so I haven't given one thought to that, even though I'm sure they're there. In fact, I've almost given more thought to the courts of Jinnistan--Mt. Matahat, Shalast-Asmar, the City of Brass, etc.--than I have to the cities of the mortal world. If the mortal-world cities
become relevant, I can develop them.
But there are plenty of things that are of the form "a danger/resource/
thing is out there, awaiting discovery," because I know that the party's interests are such that they
will discover them, sooner or later. The truth of the northern jungles (discovered a couple years back--they had secretly been led by a couatl, one of the only celestials still on this world, but naughty word went Real Bad several centuries ago), the truth of the Elf-Forest to the south (it used to be the land of the El-Adrin, before they magically pulled themselves into a pocket dimension to escape...
something that happened ~2000 years ago, around when the Genie-Rajahs abandoned the mortal world for Jinnistan), the truth of the Bard's devilish paternal ancestor (a juicy one they're still narrowing down!), etc.
I assume the players will make characters for inside the world I build? I have never really tried to leave the world we are playing in.
While that is true, I think it is more effective to do a sort of...half-and-half, or perhaps back-and-forth approach here. You make the canvas, which the players then paint with their characters, and then you paint a scene around their characters, and then the players work their way through that scenario as best they see fit. Instead of it being a perfect thing of crystalline beauty,
Late 1e (or during 2e? I forget) TSR put out Battlesystem, which was intended to cover just this type of thing.
It's clunky as all hell, but could probably be made to work. I dreamed up my own system years ago but have yet to give it a run-out; and the one time I was a player in a D&D game that turned into army-vs-army we made up the rules kind of on the spot and they worked surprisingly well.
I've a sneaking hunch that the Birthright setting (TSR, 2e) might include some rules for mass combats as well, it's ages since I looked at it.
Battle System was in the BECMI Companion boxed set in 1984. And as a standalone AD&D boxed set in 1985.
It sounds like Battlesystem, at least for AD&D, was not what I would consider "core" rules (that is, AFAICT, only 4e took the stance of "all company-published content is core.") I did not mean, by my original statement, that
no such rules existed, just that they weren't core.
However, it sounds like Overgeeked has proven me incorrect--given BECMI
is considered one core of rules, albeit one published serially rather than all at once, it is the counter-example I asked for.
Overall, though, I still think it is fair to say that WotC D&D isn't, and
most TSR D&D wasn't, considered to be a form of wargaming. The influence is there, but it's pretty clearly not a key focus even from the beginning, and became less and less so with time.