Warfare is one part of the underlying fabric of the Dungeons and Dragons universe. It provides a political backdrop, where more than just the heroes smiting random creatures dwelling in random holes goes on. The world's face is changed by warfare, on occasion. So, fittingly, in 3.5 Wizards of the Coast published Heroes of Battle, which talked about this exclusively. Good book, overall.
However, it mentions two possible methods to examine warfare--either as "medieval" or "modern" inspired", with medieval being more traditional, with formations, set-pieces, and so forth where modern has all the fantastic elements being used much like we'd see on a modern battlefield--sorcerers leading squads and giants stomping and smashing their way in. Wizards teleport into garrisons, open their gates in daring commando raids. Sounds pretty sweet. The question is, however, is this "realistic" for D&D?
What I mean by "realistic" is, given the standard population and distribution, can a nation actually afford to, or build, such an army? Well, let's take a look at the basics and make a few assumptions here. I do not know, as I write this sentence, what will be concluded.
Before I do that, however, I'm going to make a guess: it would require a vastly different population distribution to even begin to pull off. To begin with, the idea of a "modern" inspired battlefield has fireballs exploding commonly, so formations are just a good way to get killed. But does that jibe with history? Not really. Historically, firepots and other catapulted weapons like that could inflict great devastation on men in the field. These weapons did exist, but they were somewhat inaccurate, so their use close in was limited. However, other factors such as the terrain and guarantee of hand-to-hand combat meant that the risk was worth taking--when someone is trying to stab you in the chest, it's nice to have only one direction to worry about that coming from. In fact, standing shoulder to shoulder with the other soldiers didn't really disappear until WWI (sort of) and really left for good in WWII. A big reason for this is that weapons that were capable of long-range, accurate, fire were not commonly available until then--and had not been really adapted to. You were most likely to inflict damage if a number of other men are standing shoulder to shoulder with you shooting in the same direction. Of course, that meant that your were a better target, but there's always tradeoffs. Now, the advent of rifling made standing shoulder to shoulder really deadly, as Pickett's Charge will attest, but until rifled weapons were widespread and commonplace, that's an unlikely scenario. Lee assumed that rifled weapons were not yet mass produced (they were, and that was why the charge failed). That's the key here--in order for these kinds of weapons to make an enormous impact, they must be commonplace enough that they can be used readily. A single man with a rifle can hit a few select targets, but a hundred together can devastate incoming forces with lesser arms. And that is the key--are magical means of sufficient quantity to be so fearsome as to wholly change the face of warfare? That would probably depend on the campaign setting--if wizards are quite common, and magic is within the grasp of the common man, as gunpowder weapons were as armored men disappeared from the field.
At first blush, barring a high-magic setting, this really seems not to be the case. A peasant makes 3 gold a month, and a scroll of even a simple spell is in the hundreds of gold, putting magic well beyond the financial means of 90% of the population. So it doesn't seem that it will be terribly common on the battlefield. But this may be a mistaken notion.
So, let's look at casting spells then. In order to cast fireball, a wizard needs to be 5th level, or a sorcerer needs to be 6th. I'm going to take a look at the number of wizards/sorcerers of that level or greater in a given settlement, and then I'm going to make a few assumptions. They're going to boil down to one conclusion: that this represents the total number of mages available for war at any given time. I do know that, of course, there will be at least 3-4 times as many in the world, but they simply are not available for drafting like this outside of the most dire of circumstances. Unless they owe duty to a lord, getting them to show up would be quite expensive, to say the least, and not necessarily likely. Drafting any skilled tradesman would be hard, as the guilds would not stand for this, and it's hard to imagine that wizards guilds would be much different. However, certain wizards would certainly owe the state their training, and so could be required to show up under certain circumstances. Sorcerers are more independent, so are less likely to be forced into service, but many might show up for whatever other reasons. I'd imagine similar for wizards. Being academic, I doubt that most wizards would have plenty of battle spells handy--most don't spend their time fighting in dungeons and otherwise risking their necks. I'd also imagine a cap to the power of a wizard showing up. I doubt that a level 20 wizard would show up to any battle, period, unless his tower was threatened, and that army would be foolish to even attempt such things. But, overall, I think that the overall numbers of wizards/sorcerers should closely resemble the city population.
So, with that determination, let's take a look at what we get:
From a large town, we have a 75% chance of a wizard being available to cast a 3rd level spell, and a 50% chance of a sorcerer. Small city, we have 2 guaranteed of both, with a 7/16 chance of one additional, which includes a 1/16 chance of two additional for wizards, and no additional for sorcerers. A large city will have 5-6 of both,and a metropolis will have 6 of each. That's not very many. Given the relative scarcity of any settlement classified as a city, even if this is just 10% of the total number of wizards/sorcerers of that tier available, there simple aren't more than a few dozen, at most, that could be called up for a battle. Even if we allow for half of all casters to be called, it's still only five or six dozen across the whole kingdom.
Then we have to consider the limitations of casting. Across all those 60 (again, tops) casters, we have maybe 140 castings of mass-combat worthy spells (I discount low-tier spells with single targets because against 10,000 or more on the field, these aren't much use). And that's all the spells available. These spells represent various choices on the part of the wizard in preparation, and on the sorcerer in initial selection. They have a vested interest in not dying, as does the army, so I doubt that they'd use all of them on offensive load, or even friendly buffing. Of that, I expect maybe half to be considered "ready" for use at any moment. So, again, tops 70 castings. Then consider the other casters on the opposing side. They'll have similar numbers, and probably will be looking to counterspell some of the more dangerous and obvious spells--only a great fool would not order some wizards to ready an action to counter incoming spells. So this further drops the usefulness.
Again, this is across all forces. Not just for battle A or whatever--every group of various size will have mages with it, depending on the size of the force. Maybe, at most, a dozen mages will accompany any contingent, and that is being very generous.--So, of all that, expect 3-12 castings of a level 4 or higher spell, total, available, for any battle. That's just not impressive.
One could counter with "well, they have wands and scrolls", but let's look at the cost of such consumables. A level 1 wand, with 50 charges, costs 750 gold, and a level 5 scroll of fireball (minimum for creation) is 375. That's a lot of money, enough to equip a number of lesser warriors or a knight in full armor. And, don't forget that these are expended. After the battle, there's less of any of those left. That armor? Yeah, that's still there, and can be recovered and refitted for a new recruit with relatively little expense for lighter armors. Heavier armors will certainly be the personal property of the owner, and go on to his heirs--and refitting them will be simpler than others.
Also consider what the loss of these means. Loss of a scroll to the enemy is disastrous--they now have another potent asset that can destroy large numbers of your forces. Loss of the armor? Well, some men are better equipped, but likely not enough to tilt the battle. Remember, the posit here is that wizards and sorcerers would tilt the battle with their power moreso than a handful of warriors (and I will not disagree with this premise, it is quite accurate as far as it goes), so granting that to the enemy is major. These supplies are easy to carry and conceal, which makes them a prime target for theft and vandalism. That means more resources to guard and secure them, which means their combat expense is much higher.
Then we have placement. They'd have to be scattered, otherwise enemy mages will simply engage in ferocious counter-battery fire, and that will be the end of the mage cadre. Wizards and sorcerers are not the hardiest of folk, and a handful of fireballs at them would effectively end them.
All told, while these would be potentially devastating, it's just hard to see them being the decisive factor outside of extraordinary circumstances--such as exceptionally powerful mages show up unexpectedly, or enemy mages are not present, etc.
Heroes of Battle does mention the cost of equipment (like I did above), and said that arming a 1st level sorcerer with a wand of magic missile and a fireball scroll would be no more expensive than a knight, and potentially much more devastating. Well, let's look at that. A knight with full-plate armor will be of at least 3rd or 4th level, and therefore have 20-50 hp. That sorcerer? Maybe 8 or 9, and that's a stretch. His AC? Maybe 14, again, tops. The knight? 20, at least. So who would you trust with expensive and dangerous assets like that? The green sorcerer, or the trusted veteran? I wouldn't give the sorcerers such weapons, at least not at that level. That's just ludicrous.
Now, with all that said and done, what do I think about modern-inspired warfare? Well, if you want it, go ahead. But you'll need large, industrial, populations to support is, just like you need large, industrial, populations to support a modern army with artillery support. It can fit in with your world as much as you want it to. Just make sure that the background fits.