OK, now for my elaborate answer:
First of all, I want to point out (maybe repeatedly) that I'm mainly interested in my literal question: To which degree can historical battles be used as a model for fantasy battles?
I can easily accept, and be convinced by, any argument stating that the degree is very high - and still enjoy any discussion about how fantasy battles are staged.
What's good for the players and the GM is good for the game world, I never mean to argue against this.
Concerning the book I brought into play, I think it was intended as D&D source book, aiming at preventing some failures of inconsistency some GMs might fall into. Like assuming a world where the players and their opponents have abundant access to magic, but the society and the authorities seem to be rather conservative, if not outright oblivious, of the possibilities of magic (in the worst case - a setting where a simple invisibility spell might fool all merchants on the market, or even guards of a treasure chamber).
This one might very well be tied to the fact that the "spell section" in the PHB makes for a "balanced" (in metagame terms) use of magic as a tactical choice of character generation/selection, but for a poor conclusive summary of what magic can do and how magic would be employed within the background setting.
Concerining my mentioning of a "sniper": I totally agree that from character ability alone, it would take a very powerful mage to emulate the long-range precision killing ability of a modern sniper - and still than, he would be restricted when it comes to repeated application, as opposed to a sniper only limited by ammuntion (which is hardly a limitation) and reloading.
But I can't stop thinking like this: What if you cast a "levitate" spell on a batch of arrows/bolts? From a naive point of view (please see below), I might assume such projectiles travel in a straight line rather than a ballistic curve, only slowed down by friction within the surrounding medium, i.e. air resistance...aiming at targets would be completely different.
Now cast a "grease" spell on them...I tend to assume the distance for such a projectile to penetrate armor would be greatly increased, since speed would be kept along greater distances.
OK, gone that far, I admit my grasp of physics is unable to cope with what a "levitate" spell actually does. Keep mass, remove weight? If it would not keep mass, a projectile would not have any momentum suited to penetrate anything. Also, a levitating character's swung axe would be rather harmless. Interesting variant, but not how I understood the spell until now.
Going further...from what I know, acceleration and gravity are indistinguable - in a closed room, you cannot tell whether the room ist accelerating (gaining speed in one direction) or subject to a gravity field. Then again, why should a "levitating" projectile be accelerated by a bowstring, but unaffected by gravity?
But even than, just a "grease" spell cast on a projectile might greatly increase its likeliness to a modern "sniper" bullet. Maybe greased projectiles would be conical rather than having a pointed tip? Not sure how being "greased" would affect penetration ability.
Maybe now the question is: Can magic be "industrialized", i.e. is mass production of magic effects possible? At least that's the question that would be very prominent within my game world...
I guess for a nation like Duthelm, it would make perfect sense to think along such lines. They might not want peasants to achieve magic ability, but they might go to great lengths to get a huge non-magic working class to contribute to magic mass production.
In modern times, a person with neither the ability to drive a car, neither any knowledge about combustion engines, could still be part of assembly line production of cars (or at least could, some decades ago)...that's how I understand Mr. Ford.
I'm not saying that fantasy worlds should go that way of integrating magic into their worlds..but such is the motivation I brought the question upe here.
I think you rightfully said that in the historical past, application of siege enginges was greatly limited by the availabyilty of operators..and I tend to assume that back then, engineers and operators of such engines where close if not identical.
If you seperate development, construction and application of war engines...well, I gues you get what I mean by modern warfare, and those are the implications I think about.
This would apply to historical siege engines as well as to fictional Magic.
But feel free to correct my thinking ;-)
